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The effects of stage-specific differences in energetics on community structure
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Aquatic Ecology)
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When intraspecific individuals differ in resource intake, scramble competition occurs among inferior individuals growing food-dependently. Scramble can be released through predation mortality. As a consequence of this release, production rates in inferior individuals increase and biomass overcompensation in the subsequent life-stages may occur. When intraspecific individuals do not differ in their resource intakes biomass overcompensation does not occur. If an individual changes its resource intake over ontogeny, the balance of intake and losses, its energetics, will change over ontogeny. Furthermore, differences will arise between the energetics of different life-stages. The predominant volume of interspecific competition theory is based on studies assuming no stage-specific differences in energetics, neglecting the influence of ontogeny on community dynamics altogether. We study how an stage-specific differences in energetics affect expectations from conventional competition theory.

We use a stage-structured biomass model consistently translating individual life history processes, in particular food-dependent growth in body size, to the population level. The stage-structured population can be reduced to an unstructured population, if the energetics of all individuals are assumed to be equal.  The stage-structured model, however can also describe population dynamics when this equality is broken. We use the stage-structured biomass model to contrast the stage-specific differences resulting in a stage-structured population model, with an unstructured population model assuming no differences between stages.

We show that stage-specific differences in energetics can affect competition on various trophic levels. I: In stead of outcompeting each other, a predator can be facilitated by another preying a scrambling prey life-stage of the same prey population. II: In coexistence with their prey, omnivores with an ontogenetic diet shift, where juvenile omnivores feed on resource and adults on prey, affect community structure only as predators, not as competitors to their prey. We show coexistence of omnivore and prey is not possible if the dominating interaction is competition. Feeding on prey, however, alleviates competition with prey and facilitates the introduction of omnivores. III: An ontogenetic diet shift creates niche partitioning, where without it this would result in neutral coexistence of two consumers competing for two resources. IV: Furthermore, predators can change resource requirements of diet shifters such that diet shifters can reduce resources to lower equilibria and sustain higher predator biomass than consumers without stage-specific differences in energetics.

Place, publisher, year, edition, pages
Umeå: Department of Ecology and Environmental Science, Umeå University , 2010. , 47 p.
Keyword [en]
ontogenetic diet shift, stage-structured biomass model, food-dependence, growth, development
National Category
Earth and Related Environmental Sciences Natural Sciences
Research subject
Earth Sciences with Specialization Environmental Analysis
URN: urn:nbn:se:umu:diva-33444ISBN: 978-91-7264-996-5OAI: diva2:312457
Public defence
2010-05-24, KBC-huset , Lilla Hörsalen, KB3A9, Umeå universitet, Linneus väg 6, Umeå, 10:00 (English)
Available from: 2010-05-03 Created: 2010-04-24 Last updated: 2010-05-07Bibliographically approved
List of papers
1. Stage-specific predator species help each other to persist while competing for a single prey
Open this publication in new window or tab >>Stage-specific predator species help each other to persist while competing for a single prey
2008 (English)In: Proceedings from the National Academy of Science of the United States of America, ISSN 0027-8424, EISSN 1091-6490, Vol. 105, no 37, 13930-13935 p.Article in journal (Refereed) Published
Abstract [en]

Prey in natural communities are usually shared by many predator species. How predators coexist while competing for the same prey is one of the fundamental questions in ecology. Here we show that competing predator species may not only coexist on a single prey but even help each other to persist, if they specialize on different life history stages of the prey. By changing the prey size distribution a predator species may in fact increase the amount of prey available for its competitor. Surprisingly, a predator may even not be able to persist at all unless its competitor is also present. The competitor thus increases significantly the range of conditions for which a particular predator can persist. This “emergent facilitation” is a long-term, population-level effect that results from asymmetric increases in the rate of prey maturation and reproduction when predation relaxes competition among prey. Emergent facilitation explains observations of correlated increases of predators on small and large conspecific prey as well as concordance in their distribution patterns. Our results suggest that emergent facilitation may promote the occurrence of complex, stable community food webs and that persistence of these communities could critically depend on diversity within predator guilds.

Place, publisher, year, edition, pages
Washington, USA: The National Academy of Sciences of the United States of America, 2008
emergent facilitation, food-dependent prey developmentpredator coexistence, prey stage, stage-specific predation
urn:nbn:se:umu:diva-11414 (URN)10.1073/pnas.0803834105 (DOI)
Published online before print September 8, 2008, doi: 10.1073/pnas.0803834105 Free via Open Access: OA This research was supported by grants from the Swedish Research Council and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning to L.P. T.S. was supported by the Lake Ecosystem Response to Environmental Change (LEREC) project. Available from: 2009-01-08 Created: 2009-01-08 Last updated: 2010-05-03Bibliographically approved
2. The role of predation and competition in a stage-structured intraguild predation system
Open this publication in new window or tab >>The role of predation and competition in a stage-structured intraguild predation system
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Omnivorous species can simultaneously prey on and compete with other species, a type of interaction referred to as intraguild predation (IGP). Theory predicts that coexistence in IGP systems requires a balance between predation and competition interactions, which occurs when the consumer is a superior resource competitor and resource productivity is restricted to intermediate levels. Mixed competition/predation interactions between a predator and a consumer can, however, also result from ontogenetic niche shifts (life history omnivory). When young, a life history omnivore competes with the species that becomes its prey later in life.  Resource competition with superior consumers can hence limit the development of young predators, while adult predators can cultivate a favourable environment for their young by suppressing these consumers. We formulate and analyze a model in which predators interact with consumers and resources through a mixture of basic intraguild predation and life history omnivory. The model predicts increasing coexistence when predators change to life history omnivores. Furthermore, we show that the crucial assumption enabling coexistence in case of basic intraguild predation, that consumers are superior resource competitors, demotes coexistence when predators are life history omnivores. In coexistence community dynamics are shaped primarily by predation with competitive interactions playing a marginal role. As a result, community dynamics in stage-structured IGP systems, in which predators are life history omnivores, largely resemble those of a three-species linear food chain.

urn:nbn:se:umu:diva-33443 (URN)
Available from: 2010-04-24 Created: 2010-04-24 Last updated: 2010-05-03Bibliographically approved
3. Ontogenetic Diet Shifts Result in Niche Partitioning between Two Consumer Species Irrespective of Competitive Abilities
Open this publication in new window or tab >>Ontogenetic Diet Shifts Result in Niche Partitioning between Two Consumer Species Irrespective of Competitive Abilities
2010 (English)In: American Naturalist, ISSN 0003-0147, E-ISSN 1537-5323, Vol. 176, no 5, 625-637 p.Article in journal (Refereed) Published
Abstract [en]

Tilman's theory predicts the outcome of competition between two consumers sharing two resources on the basis of the shape of zero net-growth isoclines (ZNGIs). In his theory, intra-specific differences in resource use are not accounted for. Here we extend this theory to include situations where organisms undergo ontogenetic diet shifts, as these characterize the life histories of many species. In a situation that without diet shifts would lead to neutral coexistence of consumer species, we investigate whether ontogenetic diet shifts lead to niche partitioning. We analyze a model describing competition for two resources between two competitors with distinctive diets over ontogeny, using copepods (showing ontogenetic diet shifts) and daphnids (not showing ontogenetic diet shifts) as appropriate representatives. We show that an ontogenetic diet shift affects the shape of the ZNGI, changing it from reflecting perfectly substitutable resources to reflecting essential resources. Furthermore, we show that resource supply determines population stage structure and stage-dependent resource consumption in copepods and influences the competitive outcome with daphnids. In particular, we show that in itself, an ontogenetic diet shift can provide a competitive advantage if the supply of the adult resource is lower than the supply of the juvenile resource but that it always causes a disadvantage if the supply of the adult resource exceeds that of the juvenile resource.

urn:nbn:se:umu:diva-33441 (URN)10.1086/656488 (DOI)000282369900011 ()
Available from: 2010-04-24 Created: 2010-04-24 Last updated: 2011-02-23Bibliographically approved
4. Ontogenetic diet shifters profit competitively from predation
Open this publication in new window or tab >>Ontogenetic diet shifters profit competitively from predation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Integrating the simultaneous impacts of predation and resource competition on ecological communities is a major topic in community ecology. A number of studies have provided mechanisms that cause predation to affect competition between consumers for common resources especially in relation to resource productivity. We investigated how competition for two resources between two consumers of which one underwent an ontogenetic diet shift whereas the other was a generalist over its entire life time was affected by the presence of a predator feeding on both consumer populations. By assuming that consumers in the absence of  ontogenetic niche shifts showed neutral coexistence, we were able to study the effects of  ontogenetic niche shifts per se on competitive interactions. Predators differentially affected the resource requirements of diet shifters and generalist consumers and increased the region of resource supply where diet shifters competitively excluded generalists. Even in the case when generalists were superior competitors independent of resource supply, diet shifters could profit competitively from predators and even exclude the superior competitor when predators were present. Overall, diet shifters thus gained competitively from predation.

urn:nbn:se:umu:diva-33416 (URN)
Available from: 2010-05-02 Created: 2010-04-23 Last updated: 2010-05-07Bibliographically approved

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