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Ontogenetic scaling and the development of within-cohort size structure
Umeå University, Faculty of Science and Technology, Ecology and Environmental Science.
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

It is increasingly recognized that individuals of the same species differ from each other and influence and respond to their environment in unique ways. This thesis deals with size variation among individuals that not only are of the same species but also of similar age. Such variation may develop even when individuals are born in the same environment, i.e. within a cohort. I have studied the sources and consequences of variation within and among cohorts from egg through early ontogeny using young-of-the-year (YOY) perch (Perca fluviatilis) as study organism.

In agreement with predictions based on model results only taking exploitative interactions among individuals into account, I found that the broader the initial size distributions were, the more did the degree of size variation among individuals decrease over time. Still, with initially small size variation among individuals, in several experiments also size divergence was observed. Furthermore, size variation among individuals increased more under high compared to at low densities. Increased size variation over time may be explained by size-dependent diet shifts allowing for initially larger individuals to make an early diet shift when the first resource becomes limiting. However, as size divergence also was observed in situations with only shared resources available, it can be concluded that diet shifts are not a prerequisite for size divergence in young animal cohorts. Hence, I also suggest that mechanisms not related to competition for limiting resources, such as genetic variation, stochasticity and behavioural traits must be taken into account, especially when initial size differences are small.

The importance of considering size variation among individuals within cohorts was demonstrated in a study of winter mortality in YOY perch cohorts. A large individual size in autumn was shown to increase overwinter survival within cohorts. However, late summer growth rather than average body size reached in autumn explained variation in overwinter survival between cohorts. Higher accumulation to lipid reserves and accordingly lower mortality over winter was observed in years with high growth rates late in the season. In another study I showed that apparent patterns of density-dependent growth can emerge among larval fish, but rather than a result of density-dependent resource limitation this was due to variation in size-selective predation pressure. Individuals in the right end of the size distributions grew in to a high predation pressure from cannibalistic perch when cannibal density was high, coinciding with high larval perch densities.

Finally, as substantial size variation among individuals can develop within cohorts, also intra-cohort cannibalism can occur. Using a physiologically structured population model it was shown that the development of size bimodality within cohorts as a result of intra-cohort cannibalism is critically dependent on long hatching periods, high victim densities and density-dependent feedbacks on shared resources.

Abstract [sv]

Det faktum att individer som tillhör samma art skiljer sig från varandra och påverkar och påverkas av sin omgivande miljö på ett unikt sätt tillskrivs allt större betydelse inom ekologin. Den här avhandlingen handlar framför allt om storleksvariation mellan individer som förutom att tillhöra samma art dessutom tillhör samma årsklass. Sådan storleksvariation kan till och med utvecklas mellan individer som föds och växer upp i samma miljö (inom en kohort). Jag har studerat orsaker bakom och konsekvenser av variation inom och mellan kohorter. Som studieorganism har jag använt mig av årsyngel av abborre (Perca fluviatilis).

I överensstämmelse med förutsägelser baserade på en modell som enbart tar hänsyn till konkurrens om en gemensam resurs visade det sig såväl i ett dammexperiment som i en naturlig sjö att ju bredare den initiala storleksfördelningen var desto mer minskade graden av variation i kroppsstorlek mellan individer över tid. Å andra sidan, när den initiala variationen var relativt liten observerades i flera oberoende experiment även storleksdivergens över tid mellan individer. Variationen i storlek ökade särskilt i miljöer med höga tätheter av konsumenter (abborrar). För att förstå de bakomliggande mekanismerna av sådana täthetseffekter måste man ta hänsyn till den återkoppling som sker mellan antalet konsumenter och mängden resurser. Ökad storleksvariation över tid skulle kunna förklaras med storleksberoende dietskiften som tillåter individer med en initial storleksfördel att genomgå ett tidigt dietskifte samtidigt som tillgången av den första resursen begränsar övriga individers tillväxt. Eftersom storleksdivergens även observerades i situationer där enbart en delad resurs var tillgänglig kan man dock dra slutsatsen att dietskiften inte är en förutsättning för storleksdivergens inom kohorter. Jag föreslår därför också att mekanismer som inte är relaterade till konkurrens om en begränsad resurs, såsom inneboende variation mellan individer och variation i beteendemönster bör beaktas för att förklara uppkomsten av storleksvariation, speciellt i de fall då den initiala variationen är liten.

De resultat som presenteras visar på betydelsen av att ta hänsyn till storleksvariation mellan individer. Storlek efter den första tillväxtsäsongen var viktig för att förklara vilka individer inom en kohort av årsyngel som överlever sin första vinter. För att förklara variation i vinteröverlevnad mellan kohorter (mellan år och sjöar) var däremot hög tillväxt sent på säsongen (oberoende av medelstorlek på hösten) avgörande. Högre ackumulering av fettreserver och lägre mortalitet inom kohorter av årsyngel under vintern observerades under år med hög tillväxt under den senare delen av tillväxtsäsongen. I en annan studie visade jag att mönster som tyder på täthetsberoende tillväxt kan uppkomma hos fiskyngel men att detta fenomen snarare än täthetsberoende resursbegränsning förklaras av variation i storleksberoende predationstryck. De större individerna inom kohorterna växte in i ett högt predationstryck från kannibalistiska abborrar när tätheten av kannibaler var hög, något som samkorrelerade med höga tätheter av yngel. Slutligen, då en hög grad av storleksvariation mellan individer kan utvecklas inom kohorter är även kannibalism mellan individer inom en kohort möjligt. Genom att använda mig av en så kallad fysiologiskt strukturerad populationsmodell kunde jag visa att divergerande tillväxtkurvor mellan kannibaler och deras byten (vilket resulterar i storleksbimodalitet) som ett resultat av kannibalism inom kohorter är beroende av den tid det tar för ynglen att kläcka ut, antalet bytesfiskar per kannibal samt den återkoppling som finns mellan konsumenterna och deras gemensamma resurs (djurplankton).

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2009. , 34 p.
Keyword [en]
cannibalism, competition, density-dependence, resource feedbacks, hatching, growth, ontogenetic niche shifts, Perca fluviatilis, size variation, larval fish
National Category
Ecology
Research subject
Animal Ecology
Identifiers
URN: urn:nbn:se:umu:diva-25774ISBN: 978-91-7264-836-4 (print)OAI: oai:DiVA.org:umu-25774DiVA: diva2:233736
Distributor:
Ekologi, miljö och geovetenskap, 901 87, Umeå
Public defence
2009-10-02, Stora hörsalen (KB3B1), KBC, Umeå universitet, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2009-09-11 Created: 2009-09-02 Last updated: 2009-09-11Bibliographically approved
List of papers
1. The origin and development of individual size variation in early pelagic stages of fish
Open this publication in new window or tab >>The origin and development of individual size variation in early pelagic stages of fish
2007 (English)In: Oecologia, ISSN 0029-8549, Vol. 153, no 1, 57-67 p.Article in journal (Refereed) Published
Abstract [en]

Size variation among individuals born at the same time in a common environment (within cohorts) is a common phenomenon in natural populations. Still, the mechanisms behind the development of such variation and its consequences for population processes are far from clear. We experimentally investigated the development of early within-cohort size variation in larval perch (Perca fluviatilis). Specifically we tested the influence of initial variation, resulting from variation in egg strand size, and intraspecific density for the development of size variation. Variation in egg strand size translated into variation in initial larval size and time of hatching, which, in turn, had effects on growth and development. Perch from the smallest egg strands performed on average equally well independent of density, whereas larvae originating from larger egg strands performed less well under high densities. We related this difference in density dependence to size asymmetries in competitive abilities leading to higher growth rates of groups consisting of initially small individuals under high resource limitation. In contrast, within a single group of larvae, smaller individuals grew substantially slower under high densities whereas large individuals performed equally well independent of density. As a result, size variation among individuals within groups (i.e. originating from the same clutch) increased under high densities. This result may be explained by social interactions or differential timing of diet shifts and a depressed resource base for the initially smaller individuals. It is concluded that to fully appreciate the effects of density-dependent processes on individual size variation and size-dependent growth, consumer feedbacks on resources need to be considered.

Place, publisher, year, edition, pages
Berlin / Heidelberg: Springer, 2007
Identifiers
urn:nbn:se:umu:diva-16222 (URN)10.1007/s00442-007-0719-x (DOI)
Available from: 2007-08-31 Created: 2007-08-31 Last updated: 2009-09-10Bibliographically approved
2. Resource heterogeneity, diet shifts and intra-cohort competition: effects on size divergence in YOY fish
Open this publication in new window or tab >>Resource heterogeneity, diet shifts and intra-cohort competition: effects on size divergence in YOY fish
2008 (English)In: Oecologia, ISSN 0029-8549, Vol. 158, no 2, 249-257 p.Article in journal (Refereed) Published
Abstract [en]

Most organisms exhibit a substantial size variation among individuals due to individual differences in experienced biotic and abiotic environmental conditions and because individuals undergo growth and development during most of their life time. One important issue in this context is how size variation within cohorts may develop over time. Here we tested the hypothesis, in gape-limited animals such as fish, that size divergence among individuals within a cohort depends on the opportunity to undergo size-dependent diet shifts, by allowing initially larger individuals to make an early diet shift when the first resource becomes limiting. We used young-of-the-year perch (Perca fluviatilis) as our study organism. Competitive intensity and the opportunity to undergo a diet shift from zooplankton to macroinvertebrates affected both mean growth rates and the extent to which inter-individual variation in growth was manifested. As predicted, increased competition combined with the presence of both zooplankton and benthic macroinvertebrates increased the degree of size variation. However, size divergence was also observed among individuals when only the initial resource, zooplankton, was available. We argue that only non-exploitative interactions, such as dominance structures and social interactions could have caused this latter pattern, as exploitative competition is expected to lead to size convergence due to the superior competitive ability of smaller individuals. Our results suggest that diet shifts are not a prerequisite for size divergence in animal cohorts and that dominance and social interactions may have similar effects on size variation within cohorts. Finally, development of size variation is suggested to have strong implications for overall cohort performance.

Place, publisher, year, edition, pages
Berlin / Heidelberg: Springer, 2008
Identifiers
urn:nbn:se:umu:diva-11362 (URN)10.1007/s00442-008-1140-9 (DOI)
Available from: 2008-12-16 Created: 2008-12-16 Last updated: 2009-09-10Bibliographically approved
3. Effects of ontogenetic scaling on resource exploitation and cohort size distributions
Open this publication in new window or tab >>Effects of ontogenetic scaling on resource exploitation and cohort size distributions
2010 (English)In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 2, 384-392 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
John Wiley & Sons, Inc., 2010
National Category
Ecology
Research subject
Animal Ecology; Limnology
Identifiers
urn:nbn:se:umu:diva-25765 (URN)10.1111/j.1600-0706.2009.17897.x (DOI)000275557500021 ()
Note
Felstavat ord i titeln: effetcsAvailable from: 2009-09-02 Created: 2009-09-02 Last updated: 2011-02-24Bibliographically approved
4. Growing through predation windows:: effects on body size development in young fish
Open this publication in new window or tab >>Growing through predation windows:: effects on body size development in young fish
2010 (English)In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, 1796-1804 p.Article in journal (Refereed) Published
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.

National Category
Ecology
Research subject
Animal Ecology; Limnology
Identifiers
urn:nbn:se:umu:diva-25767 (URN)10.1111/j.1600-0706.2010.18475.x (DOI)000283165700008 ()
Available from: 2009-09-02 Created: 2009-09-02 Last updated: 2011-03-15Bibliographically approved
5. Intra-cohort cannibalism and size bimodality: a balance between hatching synchrony and resource feedbacks
Open this publication in new window or tab >>Intra-cohort cannibalism and size bimodality: a balance between hatching synchrony and resource feedbacks
2010 (English)In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 119, no 12, 2000-2011 p.Article in journal (Refereed) Published
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.

National Category
Ecology
Research subject
Animal Ecology
Identifiers
urn:nbn:se:umu:diva-25769 (URN)10.1111/j.1600-0706.2010.18454.x (DOI)000284371000018 ()
Available from: 2009-09-02 Created: 2009-09-02 Last updated: 2012-02-21Bibliographically approved
6. Influence of growth history on the accumulation of energy reserves and winter mortality on young fish
Open this publication in new window or tab >>Influence of growth history on the accumulation of energy reserves and winter mortality on young fish
Show others...
2008 (English)In: Canadian Journal of Fisheries and Aquatic Sciences, ISSN 0706-652X, E-ISSN 1205-7533, Vol. 65, no 10, 2149-2156 p.Article in journal (Refereed) Published
Abstract [en]

In seasonal environments accumulated energy reserves are important to avoid starvation mortality during periods of low resource levels. Here we investigated patterns of energy accumulation and the importance of growth history for winter survival in young-of-the-year Eurasian perch (Perca fluviatilis). Under simulated winter conditions in aquaria’s we showed that high winter mortality most likely relate to the depletion of energy reserves in small perch. Correspondingly in a field study, using 4 lakes covering 3-6 lake years each, overwinter survival within cohorts was positively related to individual size. However, average size in autumn did not explain the variation in overwinter survival between cohorts. Instead we showed that seasonal growth history is an important factor. High growth rates late in season may increase cohort survival over winter irrespective of average size, related to a positive growth dependent increase in allocation to energy reserves when approaching winter. Mechanisms regulating within-season temporal dynamics of growth rates are therefore suggested to be important for overall cohort performance.

Place, publisher, year, edition, pages
EBSCO Host, 2008
Identifiers
urn:nbn:se:umu:diva-11369 (URN)10.1139/F08-115 (DOI)
Available from: 2008-12-16 Created: 2008-12-16 Last updated: 2010-11-26Bibliographically approved

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