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  • 1. Albrectsen, B R
    et al.
    Gardfjell, H
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Orians, C M
    Murray, B
    FRitz, R S
    Slugs, willow seedlings and nutrient fertilization: intrinsic vigor inversely affects palatability2004In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 105, p. 268-278Article in journal (Refereed)
  • 2.
    Bourdeau, Paul E.
    et al.
    Michigan State Univ, Dept Fisheries & Wildlife, E Lansing, MI 48824 USA.
    Johansson, Frank
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Predator-induced morphological defences as by-products of prey behaviour: a review and prospectus2012In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 121, no 8, p. 1175-1190Article, review/survey (Refereed)
    Abstract [en]

    Predator-induced morphological defences (PIMDs) are ubiquitous. Many PIMDs may be mediated by prey behaviour rather than directly cued by predators. A survey of 92 studies indicated 40 that quantified prey behaviour, all of which document positive associations between defence production and activity reduction. Thus, PIMDs are associated with changes in prey activity, which could have caused the morphological change. We propose two possible mechanisms: 1) decreased activity reduces feeding rate, resulting in lower growth and morphological change; and 2) activity reduction conserves energy, which is reallocated for growth, subsequently changing morphology. Resource availability also causes similar morphological change to predator presence, suggesting confounding effects of resources and predators with current methodology. Future studies should estimate food ingestion, assimilation efficiency, and growth rate in the presence and absence of predators, crossing predator presence with resource levels. Not all PIMDs will be behaviourally-mediated, but consideration of causal linkages between prey behaviour and PIMDs is warranted.

  • 3.
    Carlsson-Granér, Ulla
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Thrall, Peter H
    The spatial distribution of plant populations, disease dynamics and evolution of resistance2002In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 97, no 1, p. 97-110Article in journal (Refereed)
    Abstract [en]

    Empirical studies of the interaction between the anther smut fungus Microbotryum violaceum and its host plant Lychnis alpina were combined with modelling approaches to investigate how variation in the spatial distribution of host populations influences disease dynamics and variation in resistance. Patterns of disease incidence and prevalence were surveyed in three contrasting systems of natural L. alpina populations where there is substantial variation in spatial structure, ranging from large continuous populations through to small isolated patches. Disease incidence (fraction of populations where disease was present) was highest in the continuous situation, and lowest in the most isolated populations. The reverse was true for prevalence (fraction of individuals diseased). To better understand the long-term ecological and evolutionary consequences of differences in among population spatial structure, we developed a two-dimensional spatially explicit simulation model in which host-population spacing was modelled by varying the percentage of sites suitable for the host. The general patterns of disease incidence and prevalence generated in the simulations corresponded well with the patterns observed in natural populations of L. alpina and M. violaceum; i.e. the fraction of sites with disease increased while the average disease prevalence in diseased populations decreased when host populations became more connected. One likely explanation for the differences in disease incidence and prevalence seen in natural populations is that the evolution of host resistance varies as a function of the degree of fragmentation. This is supported by simulation results that were qualitatively similar to the survey data when resistance was allowed to vary, but not when hosts were assumed to be uniformly susceptible. In the former, the frequency of resistance increased markedly as host populations became more connected.

  • 4.
    Cherif, Mehdi
    et al.
    Department of Biology, McGill University.
    Loreau, Michel
    Department of Biology, McGill University.
    Towards a more biologically realistic use of Droop's equations to model growth under multiple nutrient limitation2010In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 6, p. 897-907Article in journal (Refereed)
    Abstract [en]

    Droop's model was originally designed to describe the growth of unicellular phytoplankton species in chemostats but it is now commonly used for a variety of organisms in models of trophic interactions, ecosystem functioning, and evolution. Despite its ubiquitous use, Droop's model is still limited by several simplifying assumptions. For example, the assumption of equal theoretical maximum growth rates for all nutrients is commonly used to describe growth limited by multiple nutrients. This assumption, however, is both biologically unrealistic and potentially misleading. We propose the alternative hypothesis of equal realized maximum growth rates for all nutrients. We support our hypothesis with empirical and theoretical arguments and discuss how it may improve our understanding of the biology of growth, while avoiding some of the pitfalls of the previous assumption.

  • 5. De Long, Jonathan R.
    et al.
    Kardol, Paul
    Sundqvist, Maja K.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Dept of Forest Ecology and Management, Swedish Univ. of Agricultural Sciences, Umeå, Sweden.
    Veen, G. F. (Ciska)
    Wardle, David A.
    Plant growth response to direct and indirect temperature effects varies by vegetation type and elevation in a subarctic tundra2015In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 124, no 6, p. 772-783Article in journal (Refereed)
    Abstract [en]

    There has been growing recent use of elevational gradients as tools for assessing effects of temperature changes on vegetation properties, because these gradients enable temperature effects to be considered over larger spatial and temporal scales than is possible through conventional experiments. While many studies have explored the direct effects of temperature, the indirect effects of temperature through its long-term influence on soil abiotic or biotic properties remain essentially unexplored. We performed two climate chamber experiments using soils from a subarctic elevational gradient in Abisko, Sweden to investigate the direct effects of temperature, and indirect effects of temperature via soil legacies, on growth of two grass species. The soils were collected from each of two vegetation types (heath, dominated by dwarf shrubs, and meadow, dominated by graminoids and herbs) at each of three elevations. We found that plants responded to both the direct effect of temperature and its indirect effect via soil legacies, and that direct and indirect effects were largely decoupled. Vegetation type was a major determinant of plant responses to both the direct and indirect effects of temperature; responses to soils from increasing elevation were stronger and showed a more linear decline for meadow than for heath soils. The influence of soil biota on plant growth was independent of elevation, with a positive influence across all elevations regardless of soil origin for meadow soils but not for heath soils. Taken together, this means that responses of plant growth to soil legacy effects of temperature across the elevational gradient were driven primarily by soil abiotic, and not biotic, factors. These findings emphasize that vegetation type is a strong determinant of how temperature variation across elevational gradients impacts on plant growth, and highlight the need for considering both direct and indirect effects of temperature on plant responses to future climate change.

  • 6.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Effects of habitat structure on resource availability, diet and growth of benthivorous perch, perca-fluviatilis1993In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 67, no 3, p. 403-414Article in journal (Refereed)
    Abstract [en]

    I experimentally evaluated the impact of habitat structural complexity on the interactions between a generalist predator and a benthic macroinvertebrate prey assemblage in a freshwater pond. Benthivorous perch (Perca fluviatilis) were stocked over a range of natural densities (no fish, low, and high densities) into enclosures with or without dense submerged vegetation. The biomass of macroinvertebrate prey decreased over time in the presence of perch and was always higher in enclosures with vegetation present than in enclosures lacking vegetation. The increase in mass of perch was positively related to the abundance of macroinvertebrate prey and negatively related to perch density and the density of vegetation. In the treatments lacking vegetation, the proportion of zooplankton in the diet of perch increased, and the growth rate of perch decreased over time. In the vegetation treatments, the proportion of zooplankton in the diet was low throughout the experiment and the growth rate of perch was constant over time. As a consequence, initial increase in mass was considerably higher in the treatments lacking vegetation than in the vegetation treatments, whereas no such pattern was observed in the second half of the experiment. In the absence of vegetation, perch are apparently able to forage efficiently, but this may reduce the availability of macroinvertebrate prey to the extent that perch are forced to include less profitable zooplankton prey into their diet. In vegetated habitats, the foraging efficiency of perch is reduced, which possibly prevents over-exploitation of macroinvertebrate prey and consequently may allow for a moderate, but relatively constant, consumption of macroinvertebrates by perch. The density-dependence of growth rates in both vegetated and unvegetated habitats can only partly be explained by resource competition, which suggests the presence of an additional mechanism of density-dependence. In natural lake communities, efficient predation from benthivorous fish should keep the biomass of macroinvertebrate prey in structurally simple habitats below the high levels initially present in my experiment. In these communities, submerged vegetation may be an equally profitable habitat for juvenile perch as are open areas. Through its effects on the feeding efficiencies of juvenile perch and other benthivorous fish, submerged vegetation may affect individual growth rates and the size structure of perch populations, which may contribute to explain differences in fish community structure among lakes differing in submerged vegetation cover.

  • 7.
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Relative consumer sizes and the strengths of direct and indirect interactions in omnivorous feeding relationships1993In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 68, no 1, p. 151-157Article in journal (Refereed)
    Abstract [en]

    Omnivory (the consumption of resources from more than one trophic level) is widespread in nature and has the potential to produce a richness of indirect effects. Nevertheless, its effects on population dynamics have received very little attention. In its simplest case, omnivory involves a top consumer, an intermediate consumer, and a resource that is common to both consumers. Simple models predict that the intermediate consumer can only coexist with the top consumer if the former is more efficient in exploiting the common resource, which would imply a net positive effect of the top consumer on the equilibrium density of resources (compared to the situation where only the intermediate consumer is present). Among 22 experimental manipulations of omnivorous top consumers I found only 2 studies in which top consumers had significant positive effects on resources. This discrepancy between experimental results and model predictions is, at least partly, related to deviations of the experimental systems from model assumptions. However, considerations of relative body sizes of intermediate and top consumers suggest, that top consumers having negative net effects on the basic resource should be common in nature. I argue that in systems where intermediate consumers and basic resources are relatively similar in size, but both are much smaller than omnivorous top consumers (e.g. vertebrate omnivores feeding on benthos, soil invertebrates, terrestrial insects etc.), the direct negative effect of top consumers on basic resources should not be outweighed by indirect positive effects, and that other mechanisms (e.g. prey refuges) must be invoked to explain the persistence of intermediate consumers in many natural systems. I further argue that a better knowledge of the population dynamical consequences of omnivory and the role of relative consumer sizes is necessary to improve our understanding of the-trophic dynamics of different kinds of communities.

  • 8.
    Doi, Hideyuki
    et al.
    Institute for Chemistry and Biology of the Marine Environment, Carl-von-Ossietzky University Oldenburg.
    Cherif, Mehdi
    Department of Biology, McGill University.
    Iwabuchi, Tsubasa
    Graduate School of Life Science, Tohoku University.
    Katano, Izumi
    Institute for Chemistry and Biology of the Marine Environment, Carl-von-Ossietzky University Oldenburg.
    Stegen, James C.
    Dept of Biology, University of North Carolina.
    Striebel, Maren
    WasserCluster Lunz, University for Natural Resources and Applied Life Sciences.
    Integrating elements and energy through the metabolic dependencies of gross growth efficiency and the threshold elemental ratio2010In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 5, p. 752-765Article in journal (Refereed)
    Abstract [en]

    Metabolic theory proposes that individual growth is governed through the mass- and temperature-dependence of metabolism, and ecological stoichiometry posits that growth is maximized at consumer-specific optima of resource elemental composition. A given consumer's optimum, the threshold elemental ratio (TER), is proportional to the ratio of its maximum elemental gross growth efficiencies (GGEs). GGE is defined by the ratio of metabolism-dependent processes such that GGEs should be independent of body mass and temperature. Understanding the metabolic-dependencies of GGEs and TERs may open the path towards a theoretical framework integrating the flow of energy and chemical elements through ecosystems. However, the mass and temperature scaling of GGEs and TERs have not been broadly evaluated. Here, we use data from 95 published studies to evaluate these metabolic-dependencies for C, N and P from unicells to vertebrates. We show that maximum GGEs commonly decline as power functions of asymptotic body mass and exponential functions of temperature. The rates of change in maximum GGEs with mass and temperature are relatively slow, however, suggesting that metabolism may not causally influence maximum GGEs. We additionally derived the theoretical expectation that the TER for C:P should not vary with body mass and this was supported empirically. A strong linear relationship between carbon and nitrogen GGEs further suggests that variation in the TER for C:N should be due to variation in consumer C:N. In general we show that GGEs may scale with metabolic rate, but it is unclear if there is a causal link between metabolism and GGEs. Further integrating stoichiometry and metabolism will provide better understanding of the processes governing the flow of energy and elements from organisms to ecosystems.

  • 9.
    DUNBERG, A
    Umeå University, Faculty of Science and Technology, Department of Plant Physiology.
    Why Beech and Oak Trees Retain Leaves Until Spring: A Comment on the Contribution by Otto and Nilsson1982In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 39, no 2, p. 275-277Article in journal (Other academic)
  • 10.
    Ericson , Lars
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Burdon , JJ
    Linking field epidemiological and individual plant resistance patterns in the Betula pubescens-Melampsoridium betulinum host-pathogen interaction2009In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 118, p. 225-232Article in journal (Refereed)
    Abstract [en]

    Over a period of seven seasons (1995-2001 inclusively) four distinct epidemics of the rust pathogen Melampsoridium betulinum were recorded in stands of Betula pubescens. During those epidemics, host plants incurred varying levels of disease severity. Some individuals suffered high levels on all occasions; some low; while yet others were either intermediately affected or showed variable disease severity from season to season. Tests of pathotypic differences among 33 isolates of M. betulinum collected from six sites found a broad range of pathotypes ranging from highly virulent to highly avirulent. Similarly a random sample of 40 B. pubescens lines from one site showed a wide range of resistance phenotypes, although individuals that were either susceptible to all but one pathotype or resistant to all pathotypes were commonest. A strong relationship existed between the cumulative level of disease incurred by B. pubescens individuals in the field and the mean susceptibility of each host line as determined by their reaction to infection by each of the 33 different isolates of M. betulinum individually. Resistance in this B. pubescens population to M. betulinum is postulated to be based on a mixture of quantitative and qualitative traits, selection for which has resulted from an interplay of life history attributes of both host and pathogen.

  • 11.
    Ericson, Lars
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Wennstrom, A
    The effect of herbivory on the interaction between the clonal plant Trientalis europaea and its smut fungus Urocystis trientalis1997In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 80, no 1, p. 107-111Article in journal (Refereed)
  • 12.
    Faithfull, Carolyn
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Huss, Magnus
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Vrede, Tobias
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Bergström, Ann-Kristin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Bottom–up carbon subsidies and top–down predation pressure interact to affect aquatic food web structure2011In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 120, no 2, p. 311-320Article in journal (Refereed)
    Abstract [en]

    Human impacts such as eutrophication, overexploitation and climate change currently threaten future global food and drinking water supplies. Consequently, it is important that we understand how anthropogenic resource (bottom–up) and consumer (top–down) manipulations affect aquatic food web structure and production. Future climate changes are predicted to increase the inputs of terrestrial dissolved organic carbon to lakes. These carbon subsidies can either increase or decrease total basal production in aquatic food webs, depending on bacterial competition with phytoplankton for nutrients. This study examines the effects of carbon subsidies (bottom–up) on a pelagic community exposed to different levels of top–down predation. We conducted a large scale mesocosm experiment in an oligotrophic clear water lake in northern Sweden, using a natural plankton community exposed to three levels of glucose addition (0, 420 and 2100 mg C l–1 total added glucose) and three levels of young-of-the-year perch Perca fluviatilis density (0, 0.56 and 2 individuals m–3). Bacterioplankton production doubled with glucose addition, but phytoplankton production was unaffected, in contrast to previous studies that have manipulated carbon, nutrients or light simultaneously. This suggests that carbon addition alone is not sufficient to reduce autotrophic production, at least in an oligotrophic lake dominated by mixotrophic phytoplankton. Larval perch grazing did not produce a classical trophic cascade, but substantially altered the species composition of crustacean zooplankton and ciliate trophic levels. Glucose addition increased the biomass of rotifers, thus potentially increasing energy transfer through the heterotrophic pathway, but only when fish were absent. This study illustrates that changes in community structure due to selective feeding by top-predators can determine the influence of bottom–up carbon subsidies.

  • 13.
    Frainer, André
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Univ Tromso, Dept Arctic & Marine Biol, Tromso, Norway.
    Jabiol, Jeremy
    Gessner, Mark O.
    Bruder, Andreas
    Chauvet, Eric
    McKie, Brendan G.
    Stoichiometric imbalances between detritus and detritivores are related to shifts in ecosystem functioning2016In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 125, no 6, p. 861-871Article in journal (Refereed)
    Abstract [en]

    How are resource consumption and growth rates of litter-consuming detritivores affected by imbalances between consumer and litter C:N:P ratios? To address this question, we offered leaf litter as food to three aquatic detritivore species, which represent a gradient of increasing body N: P ratios: a crustacean, a caddisfly and a stonefly. The detritivores were placed in microcosms and submerged in a natural stream. Four contrasting leaf species were offered, both singly and in two-species mixtures, to obtain different levels of stoichiometric imbalance between the resources and their consumers. The results suggest that detritivore growth was constrained by N rather than C or P, even though 1) the N: P ratios of the consumers' body tissue was relatively low and 2) microbial leaf conditioning during the experiment reduced the N:P imbalance between detritivores and leaf litter. This surprisingly consistent N limitation may be a consequence of cumulative N-demand arising from the production of N-rich chitin in the exoskeletons of all three consumer species, which is lost during regular moults, in addition to N-demand for silk production by the caddisfly. These N requirements are not commonly quantified in stoichiometric analyses of arthropod consumers. There was no evidence for compensatory feeding, but when offered mixed-species litter varying in C:N:P ratios, detritivores consumed more of the litter species showing the highest N:P and lowest C:N ratio, accelerating the mass loss of the preferred leaf species in the litter mixture. These results show that imbalances in consumer-resource stoichiometry can have contrasting effects on coupled processes, highlighting a challenge in developing a mechanistic understanding of the role of stoichiometry in regulating ecosystem processes such as leaf litter decomposition.

  • 14. González, Angélica L.
    et al.
    Kominoski, John S.
    Danger, Michael
    Ishida, Seiji
    Iwai, Noriko
    Rubach, Anja
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Umeå University, Faculty of Science and Technology, Umeå Marine Sciences Centre (UMF).
    Can ecological stoichiometry help explain patterns of biological invasions?2010In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 5, p. 779-790Article in journal (Refereed)
    Abstract [en]

    Several mechanisms for biological invasions have been proposed, yet to date there is no common framework that can broadly explain patterns of invasion success among ecosystems with different resource availabilities. Ecological stoichiometry (ES) is the study of the balance of energy and elements in ecological interactions. This framework uses a multi-nutrient approach to mass-balance models, linking the biochemical composition of organisms to their growth and reproduction, which consequently influences ecosystem structure and functioning. We proposed a conceptual model that integrates hypotheses of biological invasions within a framework structured by fundamental principles of ES. We then performed meta-analyses to compare the growth and production performances of native and invasive organisms under low- and high-nutrient conditions in terrestrial and aquatic ecosystems. Growth and production rates of invasive organisms (plants and invertebrates) under both low- and high-nutrient availability were generally larger than those of natives. Nevertheless, native plants outperformed invasives in aquatic ecosystems under low-nutrient conditions. We suggest several distinct stoichiometry-based mechanisms to explain invasion success in low- versus high-nutrient conditions; low-nutrient conditions: higher resource-use efficiency (RUE; C:nutrient ratios), threshold elemental ratios (TERs), and trait plasticity (e.g. ability of an organism to change its nutrient requirements in response to varying nutrient environmental supply); high-nutrient conditions: higher growth rates and reproductive output related to lower tissue C:nutrient ratios, and increased trait plasticity. Interactions of mechanisms may also yield synergistic effects, whereby nutrient enrichment and enemy release have a disproportionate effect on invasion success. To that end, ES provides a framework that can help explain how chemical elements and energy constrain key physiological and ecological processes, which can ultimately determine the success of invasive organisms.

  • 15. Graae, Bente J
    et al.
    De Frenne, Pieter
    Kolb, Annette
    Brunet, Jorg
    Chabrerie, Olivier
    Verheyen, Kris
    Pepin, Nick
    Heinken, Thilo
    Zobel, Martin
    Shevtsova, Anna
    Nijs, Ivan
    Milbau, Ann
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    On the use of weather data in ecological studies along altitudinal and latitudinal gradients2012In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 121, no 1, p. 3-19Article in journal (Refereed)
    Abstract [en]

    Global warming has created a need for studies along climatic gradients to assess the effects of temperature on ecological processes. Altitudinal and latitudinal gradients are often used as such, usually in combination with air temperature data from the closest weather station recorded at 1.52 m above the ground. However, many ecological processes occur in, at, or right above the soil surface. To evaluate how representative the commonly used weather station data are for the microclimate relevant for soil surface biota, we compared weather station temperatures for an altitudinal (500900 m a.s.l.) and a latitudinal gradient (4968 degrees N) with data obtained by temperature sensors placed right below the soil surface at five sites along these gradients. The mean annual temperatures obtained from weather stations and adjusted using a lapse rate of -5.5 degrees C km-1 were between 3.8 degrees C lower and 1.6 degrees C higher than those recorded by the temperature sensors at the soil surface, depending on the position along the gradients. The monthly mean temperatures were up to 10 degrees C warmer or 5 degrees C colder at the soil surface. The within-site variation in accumulated temperature was as high as would be expected from a 300 m change in altitude or from a 4 degrees change in latitude or a climate change scenario corresponding to warming of 1.63.8 degrees C. Thus, these differences introduced by the decoupling are significant from a climate change perspective, and the results demonstrate the need for incorporating microclimatic variation when conducting studies along altitudinal or latitudinal gradients. We emphasize the need for using relevant temperature data in climate impact studies and further call for more studies describing the soil surface microclimate, which is crucial for much of the biota.

  • 16.
    Henriksson, Anna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Yu, Jun
    Umeå University, Faculty of Science and Technology, Department of Mathematics and Mathematical Statistics.
    Wardle, David A.
    Englund, Göran
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Biotic resistance in freshwater fish communities: species richness, saturation or species identity?2015In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 124, no 8, p. 1058-1064Article in journal (Refereed)
    Abstract [en]

    Some communities are susceptible to invasions and some are not. Why? Elton suggested in 1958 that the ability of the community to withstand invading species - its biotic resistance - depends on the number of resident species. Later contributors have emphasized the habitat's ability to support species, as well as the contribution of individual species to the resistance. In this study we use information from 184 introductions of Arctic char into Swedish lakes to study both abiotic and biotic aspects of the resident community's ability to resist introductions. We find that the best model included the proportion of forest cover and the proportion of agricultural land cover in the watershed in combination with the presence versus absence of northern pike. Thus, the most important biotic factor to explain the outcome of introductions of Arctic char is the presence of northern pike, a large piscivore. This means that one single species explains the outcome of the introductions better than does the species richness or the saturation level of the community.

  • 17. Hjalten, J
    et al.
    Danell, K
    Ericson, Lars
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Food selection by two vole species in relation to plant growth strategies and plant chemistry1996In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 76, no 1, p. 181-190Article in journal (Refereed)
  • 18.
    Huss, Magnus
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Byström, Pär
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Persson, Lennart
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Effects of ontogenetic scaling on resource exploitation and cohort size distributions2010In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 2, p. 384-392Article in journal (Refereed)
    Abstract [en]

    Variation in growth rates among individuals leading to the formation of broad size distributions is commonly observed in animal cohorts. Here we use laboratory derived size-scaling relationships to identify mechanisms driving changes in size distribution patterns within cohorts during early ontogeny. We introduced young-of-the-year perch (Perca fluviatilis) cohorts with different variation in body size distributions in pond enclosures. We kept the exploitative competitive environment constant by adjusting the number of introduced fish such that metabolic requirements were constant between different treatments. Based on modelling results we theoretically derived relative growth rates of differently sized fish when only taken exploitative competitive interactions into account. In agreement with predictions we found that initial variation in body size was negatively correlated with subsequent changes in body size variation in the pond experiment. Corresponding results were obtained in a field study covering 13 studied young-of-the-year perch cohorts in a small lake. Besides having a lower maximum growth capacity, initially large fish also suffered more from resource limitation in our experiment. The results suggest that exploitation competition is a major factor behind growth patterns in young fish cohorts, generally leading to size convergence. To explain the commonly observed pattern of size divergence in animal cohorts, including fish, we suggest that differential timing of diet shifts or mechanisms not related to exploitative interactions must be taken into account. For diet shifts to lead to size divergence we suggest that individuals with an initial size advantage need access to an exclusive prey which has a high growth potential. This, in turn, allows initially larger individuals to surf on a wave of growing prey while individuals only capable to feed on a depressed initial resource experience low growth rates.

  • 19.
    Huss, Magnus
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Byström, Pär
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Persson, Lennart
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Growing through predation windows:: effects on body size development in young fish2010In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, p. 1796-1804Article in journal (Refereed)
    Abstract [en]

    Th e degree to which growth in early life stages of animals is regulated via density-dependent feedbacks through preyresources is much debated. Here we have studied the infl uence of size- and density-dependent mechanisms as well as sizeselectivepredation pressure by cannibalistic perch Perca fl uviatilis on growth patterns of young-of-the-year (YOY) perchcovering several lakes and years. We found no infl uence of initial size or temperature on early body size development ofperch. In contrast, there was a negative relationship between reproductive output and the length of YOY perch at fi ve weeksof age. However, rather than an eff ect of density-dependent growth mediated via depressed resources the relationship wasdriven by positive size-selective cannibalism removing large individuals. Hence, given a positive correlation between thedensity of victims and predation pressure by cannibals, size-dependent interactions between cannibals and their victimsmay wrongly be interpreted as patterns of density-dependent growth in the victim cohort. Overall, our results support theview that density-dependent resource-limitation in early life stages is rare. Still, patterns of density-dependent growth mayemerge, but from variation in size-selective predation pressure rather than density as such. Th is illustrates the importanceof taking overall population demography and predatory interactions into account when studying growth patterns amongrecruiting individuals.

  • 20.
    Huss, Magnus
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    van Kooten, Tobias
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Persson, Lennart
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Intra-cohort cannibalism and size bimodality: a balance between hatching synchrony and resource feedbacks2010In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 12, p. 2000-2011Article in journal (Refereed)
    Abstract [en]

    Cannibalistic interactions generally depend on the size relationship between cannibals and victims. In many populations, alarge enough size variation to allow for cannibalism may not only develop among age-cohorts but also within cohorts. Westudied the implications of variation in hatching period length and initial cohort size for the emergence of cannibalism andbimodal size distributions within animal cohorts using a physiologically structured population model. We found that thedevelopment of size bimodality was critically dependent on hatching period length, victim density and the presence of afeedback via shared resources. Cannibals only gained enough energy from cannibalism to accelerate in growth when victimdensity was high relative to cannibal density at the onset of cannibalism. Furthermore, we found that the opportunity forearly hatchers to initially feed on an unexploited resource increases the likelihood both for cannibalism to occur and sizebimodality to develop. Once cannibals accelerated in growth relative to victims size bimodality, reduced victim numbersand relaxed resource competition resulted. Th us, in addition to that cannibals profi ted from cannibalism through energyextraction, their potential victims also benefi ted as the resource recovered due to cannibal thinning. To ensure recruitmentsuccess, it can be critical that a few individuals can accelerate in growth and reach a size large enough to escape sizedependentpredation and winter starvation. Hence, within-cohort cannibalism may be a potentially important mechanismto explain recruitment variation especially for cannibalistic species in temperate climates with strong seasonality. However,the scope for size bimodality to develop as a result of cannibalism may be limited by low victim densities and size andfood-dependent growth rates.

  • 21.
    Hylander, Kristoffer
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Christer
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Department of Natural and Environmental Sciences, Mittuniversitetet, Sundsvall.
    Jonsson, Bengt Gunnar
    Department of Natural and Environmental Sciences, Mittuniversitetet, Sundsvall.
    Göthner, Tove
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Differences in habitat quality explain nestedness in a land snail meta-community2005In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 108, no 2, p. 351-361Article in journal (Refereed)
    Abstract [en]

    We set up two alternative hypotheses on how environmental variables could foster nestedness; one of “nested habitats” and another of “nested habitat quality”. The former hypothesis refers to situations where the nestedness of species depends on a nestedness of discrete habitats. The latter considers situations where all species in an assemblage increase in abundance along the same environmental gradient, but differ in specialisation or tolerance. We tested whether litter-dwelling land snails (terrestrial gastropods) in boreal riparian forest exhibited a nested community structure, whether such a pattern was related to differences in environmental variables among sites, and which of the two hypotheses that best could account for the found pattern. We sampled litter from 100 m2 plots in 29 mature riparian forest sites along small streams in the boreal zone of Sweden. The number of snail species varied between 3 and 14 per site. Ranking the species-by-site matrix by PCA scores of the first ordination axis revealed a similarly significant nested pattern as when the matrix was sorted by number of species, showing that the species composition in this meta-community can be properly described as nested. Several environmental variables, most notably pH index, were correlated with the first PCA axis. All but two species had positive eigenvectors in the PCA ordination and the abundance increased considerably along the gradient for most of the species implying that the hypothesis of “nested habitats” was rejected in favour of the “nested habitat quality” hypothesis. Analyses of nestedness have seldom been performed on equal sized plots, and our study shows the importance of understanding that variation in environmental variables among sites can result in nested communities. The conservation implications are different depending on which of our two hypotheses is supported; a conservation focus on species “hotspots” is more appropriate if the communities are nested because of “nested habitat quality”.

  • 22.
    Ingvarsson, P K
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ericson, Lars
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Exploitative competition between two seed parasites on the common sedge, Carex nigra2000In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 91, no 2, p. 362-370Article in journal (Refereed)
  • 23.
    Kaarlejärvi, Elina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Concurrent biotic interactions influence plant performance at their altitudinal distribution margins2014In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 123, no 8, p. 943-952Article in journal (Refereed)
    Abstract [en]

    Recent studies have shown that biotic interactions can shape species’ distributions, but empirical data on multiple biotic interactions are scarce. Therefore, we examined effects of plant-plant and plant-herbivore interactions on plant survival, growth and reproduction at different altitudes. For these purposes we conducted a factorial neighbor removal and large herbivore exclusion experiment with six transplant species (three tall forbs with their main distribution at low altitudes and three small forbs with their main distribution at high altitudes) on Låktačohkka Mountain, northern Sweden, replicated at two altitudes (ca. 600 and 900 m a.s.l.) and consequently a 2.1 °C difference in summer air temperatures. Overall transplant survival was 93%. Two out of three tall forbs grew better at low than at high altitudes, while no significant differences in growth between altitudes were found for any of the three small forbs. Since the main difference in abiotic conditions between the altitudes was most likely in temperature (as the sites were topographically and edaphically matched as closely as possible), this result indicates that climatic warming could induce upward migration of tall low-altitude forbs. Negative plant-plant interactions prevailed at both altitudes, and we found indications that competition may set the lower altitudinal limits of some small tundra forbs. Thus, increased competition in response to climate warming may potentially shift the lower margins of high-altitude forbs’ distributions upward. Large mammalian grazers reduced the growth of tall forbs and enhanced the flowering of small forbs, and grazers could thus at least partly counteract the anticipated warming-induced distribution shifts.

  • 24.
    Karlsson, Jan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Different carbon support for community respiration and secondary production in unproductive lakes2007In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 116, no 10, p. 1691-1696Article in journal (Refereed)
    Abstract [en]

    This study investigates the allocation of allochthonous organic carbon (AlloOC) to pelagic respiration and biomass production in unproductive lakes. Metabolic process rates and stable isotopic composition (delta C-13) of crustacean zooplankton and respired CO2 were measured in the epilimnion of 13 forest lakes in northern Sweden. The delta C-13 of zooplankton was low (-31.2 to -38.0 parts per thousand) compared to that of respired CO2 (-28.4 to -30.6 parts per thousand), implying that the relative importance of AlloOC was lower for zooplankton (ca 40%) than for respiration (ca 80%). Combining delta C-13 and carbon flux data revealed that a large amount of metabolized AlloOC was lost in respiration, compared to the amount transferred to zooplankton (< 3%). Thus, despite large respiratory losses, AlloOC was still important for zooplankton growth, implying a high supply of AlloOC in comparison to phytoplankton generated organic carbon in the lakes.

  • 25.
    Karlsson, Jan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Säwström, Christin
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Benthic algae support zooplankton growth during winter in a clear-water lake2009In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 118, p. 539-544Article in journal (Refereed)
    Abstract [en]

    We used stable carbon (δ13C) and nitrogen (δ15N) isotopes to assess the importance of benthic algae for the zooplankton individual growth in winter in a shallow, clear subarctic lake. The δ13C values of calanoid (Eudiaptomus graciloides) and cyclopoid (Cyclops scutifer) zooplankton in autumn suggest a food resource of pelagic origin during the ice-free period. The zooplankton δ13C values were high in spring compared to autumn. E. graciloides did not grow over winter and the change in δ13C was attributed to a decrease in lipid content during the winter. In contrast, the increase in δ13C values of C. scutifer over the winter was explained by their growth on organic carbon generated by benthic algae. The δ15N of the C. scutifer food resource during winter was low compared to δ15N of the benthic community, suggesting that organic matter generated by benthic algae was mainly channelled to zooplankton via 15N-depleted heterotrophic bacteria. The results demonstrate that benthic algae can sustain zooplankton metabolic demands and growth during long winters, which, in turn, may promote zooplankton growth on pelagic resources during the summer. Such multi-chain omnivory challenges the view of zooplankton as mainly dependent on internal primary production and stresses the importance of benthic resources for the productivity of plankton food webs in shallow lakes.

  • 26.
    Lepori, Fabio
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Malmqvist, Björn
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Deterministic control on community assembly peaks at intermediate levels of disturbance2009In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 118, no 3, p. 471-479Article in journal (Refereed)
    Abstract [en]

    Despite long-standing research, the processes that drive community assembly remain poorly understood. We censused macroinvertebrate communities and measured flood disturbance in 17 Scandinavian mountain streams to assess the hypothesis that communities are shaped by stochastic processes under stable conditions, and increasingly by deterministic processes as disturbance becomes more severe. Each study stream was categorized as being stable (n=5), intermediate (n=7), or disturbed (n=5) depending on the severity of scouring floods. Following spring floods, the number of potential colonisers decreased with increasing disturbance, suggesting that disturbance filtered out species unable to cope with the stress involved. Communities at stable sites had the highest beta diversity, indicating that stochastic processes of community assembly were most important under the least disturbed conditions. In partial contrast with our predictions, the lowest beta diversity occurred between intermediate (not disturbed) sites, suggesting that increasing disturbance first enhances determinism but then rekindles stochasticity at severity levels beyond intermediate. Macroinvertebrate communities were shaped by deterministic processes, which recruit potential regional colonists depending on niche differences and disturbance conditions and by stochastic processes, which distribute the selected species randomly among individual localities. Although often considered opposing, stochastic and deterministic processes interact hierarchically, with relative strength modified by disturbance.

  • 27. Lindgren, Åsa
    et al.
    Klint, Johan
    Moen, Jon
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Defense mechanisms against grazing: a study of trypsin inhibitor responses to simulated grazing in the sedge Carex bigelowii2007In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 116, no 9, p. 1540-1546Article in journal (Refereed)
    Abstract [en]

    Trypsin inhibitors have been suggested to constitute an inducible defense in the sedge Carex bigelowii, and some former studies suggest that this might be a cause for the cyclic population dynamics in many alpine and arctic small mammals, for example lemmings (Lemmus lemmus). We investigated this further by using a method of simulated grazing (clipping) at different intensities, in three different habitats with varying resource availability, with different harvest times (hours after clipping), and two different stages of ramets (reproductive/vegetative) in a study from the Swedish mountain range. Our results do not indicate that C. bigelowii has an inducible defense constituted by an increase in trypsin inhibitor activity (TIA), but rather that the amount of soluble plant proteins (SPP) is lowered in wounded plants. The responses were somewhat different in the three habitats, with ramets growing in the marsh showing the highest ratio of TIA to SPP, due to low amounts of SPP. We did not find any significant effects of harvest time, or of the stage of the ramet that could support the hypothesis of an inducible defense. To conclude, we could not find any evidence for an inducible defense consisting of trypsin inhibitors in Carex bigelowii ramets, but we did find variations in the amount of SPP that may have nutritional consequences for herbivores.

  • 28. Metcalfe, Daniel B.
    et al.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Distinct impacts of different mammalian herbivore assemblages on arctic tundra CO2 exchange during the peak of the growing season2015In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 124, no 12, p. 1632-1638Article in journal (Refereed)
    Abstract [en]

    Herbivores play a key role in the carbon (C) cycle of arctic ecosystems, but these effects are currently poorly represented within models predicting land-atmosphere interactions under future climate change. Although some studies have examined the influence of various individual species of herbivores on tundra C sequestration, few studies have directly compared the effects of different herbivore assemblages. We measured peak growing season instantaneous ecosystem carbon dioxide (CO2) exchange (photosynthesis, respiration and net ecosystem exchange) on replicated plots in arctic tundra which, for 14 years, have excluded different portions of the herbivore population (grazed controls, large mammals excluded, both small and large mammals excluded). Herbivory suppressed photosynthetic CO2 uptake, but caused little change in ecosystem respiration. Despite evidence that small mammals consume a greater portion of plant biomass in these ecosystems, the effect of excluding only large herbivores was indistinguishable from that of excluding both large and small mammals. The herbivory-induced decline in photosynthesis was not entirely attributable to a decline in leaf area but also likely reflects shifts in plant community composition and/or species physiology. One shrub species - Betula nana - accounted for only around 13% of total aboveground vascular plant biomass but played a central role in controlling ecosystem CO2 uptake and release, and was suppressed by herbivory. We conclude that herbivores can have large effects on ecosystem C cycling due to shifts in plant aboveground biomass and community composition. An improved understanding of the mechanisms underlying the distinct ecosystem impacts of different herbivore groups will help to more accurately predict the net impacts of diverse herbivore communities on arctic C fluxes.

  • 29.
    Meunier, Cedric L.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Alfred-Wegener-Inst., Helmholtz-Zentrum für Polar- und Meeresforschung, Biologische Anstalt Helgoland.
    Boersma, Maarten
    Wiltshire, Karen H.
    Malzahn, Arne M.
    Zooplankton eat what they need: copepod selective feeding and potential consequences for marine systems2016In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 125, no 1, p. 50-58Article in journal (Refereed)
    Abstract [en]

    Herbivores are generally faced with a plethora of resources which differ in quality. Therefore, they should be able to select foods which most closely match their metabolic needs. Here, we tested the hypothesis that copepods of the species Acartia tonsa select prey cells based on quality differences within prey species. We assessed age-specific variation in feeding behaviour and evaluated the potential consequences of such variation for nutrient cycles. Nauplii (young) stages characterized by a low nitrogen to phosphorus (N:P) ratio in their body tissue selected for phosphorus-rich food, while older copepodite stages with higher body N: P selected for nitrogen-rich food. Further, the analysis of a 35-year data set in the southern North Sea revealed a positive correlation between the abundance of nauplii and the ratio of dissolved inorganic N:P, thus suggesting that P-availability for primary producers declines with the population densities of nauplii. Our findings demonstrate that a combination of stage-specific selective feeding and body stoichiometry has the potential to affect cycling of limiting nutrients when consumer populations change in composition.

  • 30. Moen, J
    et al.
    Gardfjell, H
    Ericson, Lars
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, L
    Shoot survival under intense grazing for two broad-leaved herbs with different chemical defense systems1996In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 75, no 3, p. 359-364Article in journal (Refereed)
  • 31.
    Oksanen, Lauri
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Sammul, Marek
    Mägi, Merike
    On the indices of plant–plant competition and their pitfalls2006In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 112, no 1, p. 149-155Article in journal (Refereed)
    Abstract [en]

    The index of relative competition intensity (RCI) has serious built-in biases, due to its asymptotic behavior when competition intensity is high and its tendency to obtain very low values when plants with neighbors intact perform better than neighbor removal plants. These biases have been partially corrected in the index of relative neighbor effect (RNE), but the existence of fixed upper and lower bounds (−1≤RNE≤+1) still creates problems and biases in communities where the average intensity of competition or facilitation is high and plant performance pronouncedly varies in space. The third commonly used index, the logarithm of response ratio (lnRR), is mathematically and statistically sound, but when computed from pair-wise comparisons between neighbor removal and control plants, this index reflects the geometric mean of the treatment effect. Moreover, linear patterns in lnRR reflect exponential patterns in the intensity of competition. As the interest of ecologists usually focuses on arithmetic means, we propose a corrected index of relative competition intensity, CRCI=arc sin (RNE). This index is fairly linear within the observed ranges of competition and facilitation, and for the range of competition intensities where RNE behaves reasonably, the two indices obtain almost identical values.

    We compared the performance of the four indices, using both imagined and real data, the latter from systems where the responses of plants to neighbor removal ranged from weak to moderate, so that RNE and CRCI were expected to behave similarly. The indices were computed both from pooled data for each community and as averages of pair-wise comparisons. lnRR and CRCI were found to behave in a consistent and bias-free manner, yielding similar results regardless of method of computation. This was, by and large, the case with RNE, too, but as the values of indices grew, the values from pair-wise comparisons became increasingly smaller than values computed from pooled data. RCI yielded grossly aberrant results in computations based on pair-wise comparisons. Therefore, the further use of RCI is unadvisable and studies where RCI has been derived from pair-wise comparisons should be excluded from meta-analyses.

  • 32.
    Olofsson, Johan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Influence of herbivory and abiotic factors on the distribution of tall forbs along a productivity gradient: a transplantation experiment2001In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 94, no 2, p. 351-357Article in journal (Refereed)
    Abstract [en]

    In arctic-alpine areas tall herb vegetation is restricted to sites with high productivity. At higher elevation, low prostate forbs and narrow-leaved graminoids dominate the vegetation in sites with a protecting snow cover during winter. In this study, I test whether herbivory or abiotic factors prevent tall forbs from growing at higher altitudes. Vegetation blocks from a tall herb meadow were transplanted to herbivore exclosures and open plots in a low-productive snowbed and a productive tall herb meadow. The tall forbs performed equally well in the exclosures on the low-productive snowbed as in the tall herb meadow, but decreased in the open plots on the low-productive snowbed. Thus, even if abiotic factors are ultimately causing many of the vegetation patterns observed in arctic-alpine plant communities, herbivory appears to be the main proximate factor responsible for the decreasing abundance of tall forbs along gradients of decreasing productivity in arctic-alpine areas.

  • 33.
    Olofsson, Johan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Moen, Jon
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Effects of herbivory on competition intensity in two arctic-alpine tundra communities with different productivity2002In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 96, no 2, p. 265-272Article in journal (Refereed)
    Abstract [en]

    The effects of long-term (I I yr) exclusion of vertebrate herbivores on competition intensity and plant community structure ere studied using manipulative field experiment,; in two arctic-alpine plant communities with contrasting productivity : an unproductive snowbed and a considerably more productive tall herb meadow. In the snowbed. the exclusion of herbivorous mammals resulted in a significant increase in the biomasses of vascular plants and cryptogams, whereas no corresponding response Lis observed on the tall herb meadow. The intensity of competition. measured with a neighbour removal experiment, did not differ significantly between three of the four habitat x treatment combinations - snowbed exposures. meadow exclosures and open meadow plots but as significantly loader on open snowbed plots, Our results thud, suggest that the low competition intensity in the unproductive snow bed is caused by herbivorous mammals, which tend to depress plant biomass in relatively unproductive habitats When herbivorous mammals have been excluded for a sufficiently long time to allots the build-up of plant biomass even in Unproductive habitat,. between-habitat differences in competition intensity disappear.

  • 34.
    Olofsson, Johan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Moen, Jon
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    On the balance between positive and negative plant interactions in harsh environments1999In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 86, no 3, p. 539-543Article in journal (Refereed)
    Abstract [en]

    Positive interactions between plants typically occur where the presence of a species ameliorates the abiotic environment for another. However, there is also a potential for resource competition to act at the same time, which creates a situation where the net outcome is a balance between positive and negative interactions. We present data from a nine-year study in two extreme high alpine habitats that was designed to test whether the effects of established Ranunculus glacialis individuals on germination and growth of Oxyria, digyna are primarily positive or negative at the altitudinal limit of vascular plants. We show net effects ranging from neutral to negative, but no positive effects were detected. We also argue that close associations between plants in these harsh environments may both ameliorate and deteriorate the abiotic environment, and that experimental manipulations are necessary to tell the difference.

  • 35.
    Olofsson, Johan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Oksanen, Lauri
    Role of litter decomposition for the increased primary production in areas heavily grazed by reindeer: a litterbag experiment2002In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 96, no 3, p. 507-515Article in journal (Refereed)
    Abstract [en]

    Heavy grazing and trampling by reindeer increase nutrient cycling and primary production in areas where grasslands hake replaced shrub and moss tundra. One way in which herbivores can affect nutrient cycling is through changing the litter decomposition processes. We studied the effect of herbivory on litter decomposition rate by reciprocal transplantation of litter between lightly grazed and heavily grazed areas. using the litterbag technique, We used litter from two of the most common species on the lightly grazed side, Betula nana and Empetrum nigrum, and two of the most common species on the heavily grazed side. Carex bigelowii and Deschampsia flexuosa, We found that herbivory improved litter quality by favouring species with easily decomposed litter. However. herbivore also improved litter quality by increasing the nitrogen content and lowering the C/N ratio of each species. Decomposition rates even correlated with the abundance of the plant category in question, Shrub litter decomposed faster in the lightly grazed area where shrubs were common, and graminoid litter decomposed faster in the heavily grazed area where graminoids were common. These results indicate that the decomposer micro-organisms are adapted to the most common litter types. This studs shoals that detailed information about the effect of herbivore on litter quality is important to understand differences between the short-term and long-term effects of herbivory on nutrient cycling and primary production.

  • 36.
    Olofsson, Johan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Strengbom, Joachim
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Response of galling invertebrates on Salix lanata to reindeer herbivory2000In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 91, no 3, p. 493-498Article in journal (Refereed)
    Abstract [en]

    Browsing and defoliation often increase the densities of insect herbivores on woody plants. Densities of herbivorous invertebrates were estimated in a long-term grazing manipulation experiment. More then 30-yr-old fences allow us to compare densities of invertebrate herbivores on Salix lanata in areas heavily grazed and areas lightly grazed by reindeer. The number of gall-forming insects (Pontania glabrifons) and gall-forming mitts were higher on the heavily grazed shrubs than on lightly grazed shrubs. In contrast to most short-term studies, the heavily grazed S. lanata had shorter current annual shoots. No difference in leaf size, leaf nitrogen content, or C:N ratio between grazing intensities were detected. However, the enhanced natural delta N-15 value indicates that heavily grazed shrubs get a higher proportion of their N directly from reindeer faeces. Lear weight per unit area and relative fluctuating asymmetry of leaf shape increased in heavily grazed S. lanata. Enhanced relative fluctuating asymmetry might indicate higher susceptibility to herbivores. Long-term grazing seems to increase the density of invertebrate herbivory in the same way as short-term grazing, even if the plant responses differ substantially.

  • 37.
    O'Sullivan, Daniel
    et al.
    Inst. of Integrative and Comparative Biology, Univ. of Leeds, Leeds, UK.
    Benton, Tim G.
    Inst. of Integrative and Comparative Biology, Univ. of Leeds, Leeds, UK.
    Cameron, Tom C.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Inst. of Integrative and Comparative Biology, Univ. of Leeds, Leeds, UK.
    Inter-patch movement in an experimental system: the effects of life history and the environment2014In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 123, no 5, p. 623-629Article in journal (Refereed)
    Abstract [en]

    An important process for the persistence of populations subjected to habitat loss and fragmentation is the dispersal of individuals between habitat patches. Dispersal involves emigration from a habitat patch, movement between patches through the surrounding landscape, and immigration into a new suitable habitat patch. Both landscape and physical condition of the disperser are known to influence dispersal ability, although disentangling these effects can often be difficult in the wild. In one of the first studies of its kind, we used an invertebrate model system to investigate how dispersal success is affected by the interaction between the habitat condition, as determined by food availability, and life history characteristics (which are also influenced by food availability). Dispersal of juvenile and adult mites (male and female) from either high food or low food natal patches were tested separately in connected three patch systems where the intervening habitat patches were suitable (food supplied) or unsuitable (no food supplied). We found that dispersal success was reduced when low food habitat patches were coupled to colonising patches via unsuitable intervening patches. Larger body size was shown to be a good predictor of dispersal success, particularly when the intervening landscape is unsuitable. Our results suggest that there is an interaction between habitat fragmentation and habitat suitability in determining dispersal success: if patches degrade in suitability and this affects the ability to disperse successfully then the effective connectance across landscapes may be lowered. Understanding these consequences will be important in informing our understanding of how species, and the communities in which they are embedded, may potentially respond to habitat fragmentation.

  • 38. Persson, L
    et al.
    Johansson, L
    Andersson, G
    Diehl, Sebastian
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Hamrin, SF
    Density dependent interactions in lake ecosystems: whole lake perturbation experiments1993In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 66, no 2, p. 193-208Article in journal (Refereed)
    Abstract [en]

    Density dependent interactions between higher and lower trophic levels were studied in two consecutive whole lake experiments in a highly productive lake. In the first experiment, the zooplanktivorous fish species, roach (Rutilus rutilus) (primary carnivore), was reduced to 27% of its previous population size (53%) of biomass) by a selective rotenone treatment. In the second experiment, zander (Stizostedion lucioperca) (secondary carnivore), were stocked in the lake resulting in a reduction in the population size of roach to 51% of its previous population size (31% of biomass). In both experiments, seasonal average biomass of Daphnia cucullata) increased in the two years with the lowest roach biomasses. but no shift to larger zooplankton forms took place. In the years with increased Daphnia biomass, phytoplankton biomass and transparency showed an increased seasonal variation (measured as coefficient of variation). In contrast to manipulations in which planktivorous fish were totally removed, no effect on seasonal average phytoplankton biomass was observed. Due to the high reproductive capacity of roach, the system returned to previous conditions within a year or two after the perturbations. Under unperturbed conditions, the lake showed few indications of instability as suggested by the paradox of enrichment hypothesis. This lack of instability can. among other things, be related to high zooplanktivore predation pressure present in highly productive lakes preventing overexploitation of primary producers by grazers. In addition, the omnivorous feeding characteristics of roach (feeding on both zooplankton and algae/detritus) and interactions between the open water and the detritus nutrient pool are likely to increase the stability of the system. The two perturbation experiments plus a previous natural experiment (winter fish kill), which all were pulsed perturbations, provide no evidence for the presence of alternative stable states in highly productive lakes. Possibly, a sustained perturbation over several years may shift highly productive lakes from a dominance of phytoplankton production to a dominance of macrophyte production leading to an alternative state.

  • 39.
    Pratt, Jessica D.
    et al.
    Univ Calif Irvine, Dept Ecol & Evolutionary Biol, Irvine, CA 92697 USA.
    Keefover-Ring, Ken
    Umeå University, Faculty of Science and Technology, Umeå Plant Science Centre (UPSC). Umeå University, Faculty of Science and Technology, Department of Plant Physiology. Univ Wisconsin, Dept Entomol, Madison, WI 53706 USA.
    Liu, Lawrence Y.
    Univ Calif Irvine, Dept Ecol & Evolutionary Biol, Irvine, CA 92697 USA.
    Mooney, Kailen A.
    Univ Calif Irvine, Dept Ecol & Evolutionary Biol, Irvine, CA 92697 USA.
    Genetically based latitudinal variation in Artemisia californica secondary chemistry2014In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 123, no 8, p. 953-963Article in journal (Refereed)
    Abstract [en]

    Steep climatic gradients may select for clinal adaptation in plant functional traits with implications for interspecific interactions and response to future climate change. Terpenes are common in Mediterranean environments and mediate plant interactions with both the abiotic and biotic environment, including herbivores. Clines in traits such as terpenes have received much attention because they are linked to plant fitness and experience strong selection from the abiotic and biotic environment. In this study, we tested for intraspecific variation in Artemisia californica terpene chemistry in a common garden of plants sourced from populations spanning a large precipitation gradient (6 latitude) and grown in treatments of high and low precipitation. We found genetic variation in terpene richness, diversity, concentration and composition among A. californica populations spanning this species' range. Of these traits, terpene composition and monoterpene concentration varied clinally with respect to source site latitude. Regarding terpene composition, pairwise dissimilarity among populations increased in parallel with geographic distance between source sites. At the same time, monoterpene concentration decreased monotonically from plants of southern origin (source sites with high temperature, aridity, and precipitation variability) to plants of northern origin. Our precipitation manipulation suggests that phenotypic selection by precipitation may underlie this clinal variation in monoterpene concentration, and that monoterpene concentration and other aspects of terpene chemistry are not phenotypically plastic. In summary, this study provides novel evidence for a genetically based latitudinal cline in plant secondary chemistry and suggests that adaptation to a key aspect of the abiotic environment may contribute to this intraspecific variation. Accordingly, changes in terpene chemistry under projected future climates will likely occur solely through the relatively slow process of adaptation, with important consequences for plant interactions with the abiotic environment and a diverse community of associates.

  • 40. Sammul, Marek
    et al.
    Oksanen, Lauri
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Mägi, Merike
    Regional effects on competition-productivity relationship: a set of field experiments in two distant regions2006In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 112, no 1, p. 138-148Article in journal (Refereed)
    Abstract [en]

    We studied the effect of productivity on competition intensity and the relationship between competition intensity and community species richness, using a removal experiment with the perennial plant Solidago virgaurea. The experiment was conducted in 16 different communities from two geographically distant areas (western Estonia and northern Norway). The results were compared with the results of previous experiments with Anthoxanthum odoratum from the same areas. Removal of neighbors had a positive effect on the biomass of both Solidago and Anthoxanthum, and this response was stronger in communities with higher productivity. Thus, the corrected index of relative competition intensity, CRCI, increased with increasing community productivity. Species richness was negatively correlated with CRCI in Estonia but not in Norway and not in the case of the pooled material. The results suggest that competitive exclusion operates at least in these communities which species pool is large.

    Our results indicate that the relationship between competition intensity and productivity is non-linear. In our data, competition prevails in communities where living plant biomass exceeds 200 g m−2, whereas in less productive communities, competition remains undetected and direct plant–plant relationships might at times be even mutualistic. Moreover, we found that the relationship between competition intensity and productivity is strongly dependent on regional differences and is intimately connected to a concordant variation in the intensity of grazing. The least productive communities both in Estonia and in Norway are characterized by intensive grazing, which reduces importance of competition. Hence, the contrasting results corroborates the predictions of the hypothesis of exploitation ecosystems, predicting that trophic dynamics account for the relationship between competition intensity and primary productivity.

  • 41.
    Schröder, Arne
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Persson, Lennart
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    de Roos, Andre M
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Complex shifts between food web states in response to whole-ecosystem manipulations2012In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 121, no 3, p. 417-427Article in journal (Refereed)
    Abstract [en]

    Food webs can respond in surprising and complex ways to temporary alterations in their species composition. When such a perturbation is reversed, food webs have been shown to either return to the pre-perturbation community state or remain in the food web configuration that established during the perturbation. Here we report findings from a replicated whole-lake experiment investigating food web responses to a perturbation and its consecutive reversal. We could identify three distinct community states in the food web that corresponded to the periods before, during and after the perturbation. Most importantly, we demonstrate the establishment of a distinct post-perturbation food web configuration that differed from both the pre- and during-perturbation communities in phytoplankton biomass and micro- and mesozooplankton species composition. We suggest that the pre- and post-perturbation food web configurations may represent two alternative stable community states. We provide explanations for how each of the contrasting communities may be maintained through altered species interactions. These findings add to the discussion of how natural food webs react to environmental change and imply that the range of potential ecosystem dynamics in response to perturbations can be wider and more complex than is often recognized.

  • 42.
    Schröder, Arne
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Persson, Lennart
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    de Roos, André M.
    Inst. of Biodiversity and Ecosystem Dynamics, Univ. of Amsterdam, Netherlands.
    Direct experimental evidence for alternative stable states: a review2005In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 110, no 1, p. 3-19Article in journal (Refereed)
    Abstract [en]

    A large number of studies have presented empirical arguments for the existence of alternative stable states (ASS) in a wide range of ecological systems. However, most of these studies have used non-manipulative, indirect methods, which findings remain open for alternative explanations. Here, we review the direct evidence for ASS resulting from manipulation experiments. We distinguish four conclusive experimental approaches which test for predictions made by the hysteresis effect: (1) discontinuity in the response to an environmental driving parameter, (2) lack of recovery potential after a perturbation, (3) divergence due to different initial conditions and (4) random divergence. Based on an extensive literature search we found 35 corresponding experiments. We assessed the ecological stability of the reported contrasting states using the minimum turnover of individuals in terms of life span and classified the studies according to 4 categories: (1) experimental system, (2) habitat type, (3) involved organisms and (4) theoretical framework. 13 experiments have directly demonstrated the existence of alternative stable states while 8 showed the absence of ASS in other cases. 14 experiments did not fulfil the requirements of a conclusive test, mostly because they applied a too short time scale. We found a bias towards laboratory experiments compared to field experiments in demonstrating bistability. There was no clear pattern of the distribution of ASS over categories. The absence of ASS in 38% of the tested systems indicates that ASS are just one possibility of how ecological systems can behave. The relevance of the concept of ASS for natural systems is discussed, in particular under consideration of the observed laboratory bias, perturbation frequency and variable environments. It is argued, that even for a permanently transient system, alternative attractors may still be of relevance.

  • 43. Sniegula, Szymon
    et al.
    Johansson, Frank
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson-Örtman, Viktor
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Differentiation in developmental rate across geographic regions: a photoperiod driven latitude compensating mechanism?2012In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 121, no 7, p. 1073-1082Article in journal (Refereed)
    Abstract [en]

    Genetic differentiation and phenotypic plasticity in growth rates along latitudinal gradients may benefit our understanding of latitudinal compensating mechanisms in life history patterns. Here we explore latitudinal compensatory growth mechanisms with respect to photoperiod in northern and southern populations of two damselfly species, Coenagrion puella and C. pulchellum. In addition we compared size of field-collected adults from southern and northern populations. Eggs from females in copulating tandems were collected at two or three localities for each species in each geographic region. Eggs were transported to the laboratory and the experiment started when the eggs hatched. The role of photoperiod on the expression of larval growth rate was evaluated under controlled laboratory conditions. Both species had lower growth rate when reared in the northern photoperiod, which is counter to expectations if species use photoperiodic cues to trigger compensatory growth. Instead, both species displayed countergradient variation in growth rates, which probably enable northern populations to compensate for the shorter growth season in the north. The smaller size of field-collected adults from northern populations also supports the view that these species compensate for the shorter growth season by investing in growth and development but accomplish this at the expense of decreased final size.

  • 44.
    Stenberg, Johan A.
    et al.
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Heijari, Juha
    Dept of Ecology and Environmental Science, Univ. of Kuopio, Finland.
    Holopainen, Jarmo K.
    Dept of Ecology and Environmental Science, Univ. of Kuopio, Finland.
    Ericson, Lars
    Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
    Presence of Lythrum salicaria enhances the bodyguard effects of the parasitoid Asecodes mento for Filipendula ulmaria2007In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Oikos, Vol. 116, no 3, p. 482-490Article in journal (Refereed)
    Abstract [en]

    This paper reports significant effects of a co-occurring plant species (Lythrum salicaria, Lythraceae) on the reproductive success of the perennial herb Filipendula ulmaria (Rosaceae). We studied 15 Filipendula populations in the Skeppsvik Archipelago; seven of which were monospecific and eight mixed with Lythrum. All the Filipendula populations studied harbored the chrysomelid beetle Galerucella tenella, and in 2005 seed set was strongly negatively correlated with the percentage leaf area consumed. Moreover, data from 2004 showed that 25–100% of the G. tenella larvae were parasitized by the hymenopteran parasitoid Asecodes mento, and we found a strong cascading top-down effect of parasitism in 2004 on Filipendula seed set in 2005. In 2004, parasitism (at the population level) was negatively correlated with percentage leaf area consumed and positively correlated with seed set in 2005. The parasitoid Asecodes also parasitized G. calmariensis, which is monophagous on Lythrum. Mixed populations of Filipendula and Lythrum supported higher densities of their shared ‘bodyguard’Asecodes. Further, Y-tube bioassays showed that floriferous Filipendula attracted more than twice as many gravid Asecodes females as floriferous Lythrum. Taken together, these findings suggest that coexistence of the two plants results in ‘associational resistance’ for Filipendula and ‘associational susceptibility’ for Lythrum. This scenario was supported for Filipendula since, for this species, we found lower leaf consumption followed by higher seed production in mixed than in monospecific populations. Considered together, our results show that bodyguards may increase the reproductive fitness of a perennial herb, and that the strength of the cascading ‘bodyguard’ effect can be strongly influenced by co-occurring plants through ‘apparent competition’. This is the first paper to demonstrate that, in the wild, plant species may use odors to compete for ‘bodyguards’, thereby causing asymmetrical ‘apparent competition’ between the herbivores involved. Our data emphasize the need to consider community factors in studies of trophic interactions.

  • 45. Sundqvist, Maja K.
    et al.
    Giesler, Reiner
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Graae, Bente J.
    Wallander, Håkan
    Fogelberg, Elisabeth
    Wardle, David A.
    Interactive effects of vegetation type and elevation on aboveground and belowground properties in a subarctic tundra2011In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 120, no 1, p. 128-142Article in journal (Refereed)
    Abstract [en]

    An improved knowledge of how contrasting types of plant communities and their associated soil biota differ in their responses to climatic variables is important for better understanding the future impacts of climate change on terrestrial ecosystems. Elevational gradients serve as powerful study systems for answering questions on how ecological processes can be affected by changes in temperature and associated climatic variables. In this study, we evaluated how plant and soil microbial communities, and abiotic soil properties, change with increasing elevation in subarctic tundra in northern Sweden, for each of two dominant but highly contrasting vegetation types, namely heath (dominated by woody dwarf shrubs) and meadow (dominated by herbaceous species). To achieve this, we measured plant community characteristics, microbial community properties and several soil abiotic properties for both vegetation types across an elevation gradient of 500 to 1000 m. We found that the two vegetation types differed not only in several above- and belowground properties, but also in how these properties responded to elevation, pointing to important interactive effects between vegetation type and elevation. Specifically, for the heath, available soil nitrogen and phosphorus decreased with elevation whereas fungal dominance increased, while for the meadow, idiosyncratic responses to elevation for these variables were found. These differences in belowground responses to elevation among vegetation types were linked to shifts in the species and functional group composition of the vegetation. Our results highlight that these two dominant vegetation types in subarctic tundra differ greatly not only in fundamental aboveground and belowground properties, but also in how these properties respond to elevation and are therefore likely to be influenced by temperature. As such they highlight that vegetation type, and the soil abiotic properties that determine this, may serve as powerful determinants of how both aboveground and belowground properties respond to strong environmental gradients.

  • 46. Svanbäck, Richard
    et al.
    Rydberg, Cecilia
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Leonardsson, Kjell
    Englund, Göran
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Diet specialization in a fluctuating population of Saduria entomon: a consequence of resource or forager densities?2011In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 120, no 6, p. 848-854Article in journal (Refereed)
    Abstract [en]

    Intraspecific competition has been shown to favor diet specialization among individuals. However, the question whether the competition takes the form of interference or exploitative in driving diet specialization has never been investigated. We investigated individual diet specialization in the isopod Saduria entomon, in relation to forager and resource biomasses in a system that exhibits predator–prey fluctuations in density. We found that individual diet specialization was only affected by the biomass of their preferred prey (Monoporeia affinis) and not by Saduria biomass; diet specialization was higher when Monoporeia biomass was low compared to when there were high Monoporeia biomass. Population diet breadth increased at low Monoporeia biomass whereas individual diet breadths were marginally affected by Monoporeia biomass. Overall, this led to the increase in diet specialization at low Monoporeia biomass. This study shows that predator–prey dynamics might influence diet specialization in the predator and that resource biomass, not forager biomass might be important for individual diet specialization.

  • 47. Ujvari, Beata
    et al.
    Andersson, Stefan
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Brown, Gregory
    Shine, Richard
    Madsen, Thomas
    Climate-driven impacts of prey abundance on the population structure of a tropical aquatic predator2010In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 1, p. 188-196Article in journal (Refereed)
    Abstract [en]

    In the present study we explore how annual variation in climate (late wet-season rainfall) affects population demography in a gape-limited obligate piscivorous predator, the Arafura filesnake Acrochordus arafurae in the Australian tropics. These aquatic snakes display extreme sexual dimorphism, with body sizes and relative head sizes of females much larger than those of males. Two consecutive years with low rainfall during the late wet season reduced the abundance of small but not large sized fish. Although snake residual body mass (RBM, calculated from a general linear regression of ln-transformed mass to ln-SVL) decreased after the first year with low prey availability, it was not until the second year that reduced prey abundance caused a dramatic decline in filesnake survival, and hence in population numbers. Thus, our results suggest that most snakes survived the first year of reduced prey abundance, but a successive year with low prey availability proved fatal for many animals. However, the effects of prey scarcity on RBM and survival fell disproportionately on some size classes of snakes. Medium-sized animals (large males and intermediate-sized females) were affected more dramatically than were small or large snakes. We attribute the higher survival of small snakes to their lower energy needs compared to medium-sized individuals, and the higher survival of large snakes to the continued abundance of large prey (mainly large catfish). Two successive years with low abundance of smaller sized prey thus massively modified the size-structure of the filesnake population, virtually eliminating large males and intermediate-sized females. Our field data provide a clear demonstration of the ways in which stochastic variation in climatic conditions can have dramatic effects on predator population demography, mediated via effects on prey availability.

  • 48. Veen, G. F. (Ciska)
    et al.
    De Long, Jonathan R.
    Kardol, Paul
    Sundqvist, Maja K.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. Center for Macroecology, Evolution and Climate, Natural History Museum of Denmark, Univ. of Copenhagen, Copenhagen, Denmark.
    Snoek, L. Basten
    Wardle, David A.
    Coordinated responses of soil communities to elevation in three subarctic vegetation types2017In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 126, no 11, p. 1586-1599Article in journal (Refereed)
    Abstract [en]

    Global warming has begun to have a major impact on the species composition and functioning of plant and soil communities. However, long-term community and ecosystem responses to increased temperature are still poorly understood. In this study, we used a well-established elevational gradient in northern Sweden to elucidate how plant, microbial and nematode communities shift with elevation and associated changes in temperature in three highly contrasting vegetation types (i.e. heath, meadow and Salix vegetation). We found that responses of both the abundance and composition of microbial and nematode communities to elevation differed greatly among the vegetation types. Within vegetation types, changes with elevation of plant, microbial and nematode communities were mostly linked at fine levels of taxonomic resolution, but this pattern disappeared when coarser functional group levels were considered. Further, nematode communities shifted towards more conservative nutrient cycling strategies with increasing elevation in heath and meadow vegetation. Conversely, in Salix vegetation microbial communities with conservative strategies were most pronounced at the mid-elevation. These results provide limited support for increasing conservative nutrient cycling strategies at higher elevation (i.e. with a harsher climate). Our findings indicate that climate-induced changes in plant community composition may greatly modify or counteract the impact of climate change on soil communities. Therefore, to better understand and predict ecosystem responses to climate change, it will be crucial to consider vegetation type and its specific interactions with soil communities.

  • 49. Vrede, Tobias
    et al.
    Drakare, Stina
    Eklöv, Peter
    Hein, Arne
    Liess, Antonia
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Olsson, Jens
    Persson, Jonas
    Quevedo, Mario
    Ragnarsson Stabo, Henrik
    Svanbäck, Richard
    Ecological stoichiometry of Eurasian perch: intraspecific variation due to size, habitat and diet2011In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 120, no 6, p. 886-896Article in journal (Refereed)
    Abstract [en]

    The turnover and distribution of energy and nutrients in food webs is influenced by consumer stoichiometry. Although the stoichiometry of heterotrophs is generally considered to vary only little, there may be intraspecific variation due to factors such as habitat, resources, ontogeny and size. We examined intraspecific variation in Eurasian perch Perca fluviatilis stoichiometry, a common species that exhibits habitat and resource specialization, ontogenetic niche shifts and a large size range. This study investigated the elemental stoichiometry of a wide size range of perch from littoral and pelagic habitats. The mean C:N:P stoichiometry of whole perch was 37:9:1 (molar ratios). However, %C, %P, C:N, C:P and N:P varied with size, morphology, habitat and diet category. These factors together explained 24–40% of the variation in C:N:P stoichiometry. In contrast, perch stoichiometry was not related to diet stoichiometry, suggesting that the former is homeostatically regulated. The results suggest that the high P content of perch may result in stoichiometric constraints on the growth of non-piscivorous perch, and that piscivory is an efficient strategy for acquiring P. Resource polymorphism, individual diet specialization and intraspecific size variation are widespread among animals. Thus changes in stoichiometry with size, habitat, morphology and resource use, and therefore also stoichiometric demands, are probably common.

  • 50. Wardle, David A
    et al.
    Bellingham, Peter J
    Kardol, Paul
    Giesler, Reiner
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Tanner, Edmund V J
    Coordination of aboveground and belowground responses to local-scale soil fertility differences between two contrasting Jamaican rain forest types2015In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 124, no 3, p. 285-297Article in journal (Refereed)
    Abstract [en]

    There is growing interest in understanding how declining soil fertility in the prolonged absence of major disturbance drives ecological processes, or ecosystem retrogression'. However, there are few well characterized study systems for exploring this phenomenon in the tropics, despite tropics occupying over 40% of the Earth's terrestrial surface. We studied two types of montane rain forest in the Blue Mountains of Jamaica that represent distinct stages in ecosystem development, i.e. an earlier stage with shallow organic matter and a late stage with deep organic matter (hereafter mull' and mor' stages). We characterized responses of soil fertility and plant, soil microbial and nematode communities to the transition from mull to mor and whether these responses were coupled. For soil abiotic properties, we found this transition led to lower amounts of both nitrogen (N) and phosphorus (P) and an enhanced N to P ratio. This led to shorter-statured and less diverse forest, and convergence of tree species composition among plots. At the whole community (but not individual species) level foliar and litter N and P diminished from mull to mor, while foliar N to P and resorption efficiency of P relative to N increased, indicating increasing P relative to N limitation. We also found impairment of soil microbes (but not nematodes) and an increasing role of fungi relative to bacteria during the transition. Our results show that retrogression phenomena involving increasing nutrient (notably P) limitation can be important drivers in tropical systems, and are likely to involve aboveground-belowground feedbacks whereby plants produce litter of diminishing quality, impairing soil microbial processes and thus reducing the supply of nutrients from the soil for plant growth. Such feedbacks between plants and the soil, mediated by plant litter and organic matter quality, may serve as major though often overlooked drivers of long term environmental change.

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