<p>Alternative stable states have been, based on theoretical findings, predicted to be common in ecological systems. Empirical data from a number of laboratory and natural studies strongly suggest that alternative stable states also occur in real populations, communities and ecosystems. Potential mechanisms involve population size-structure and food-dependent individual development. These features can lead to ontogenetic niche shifts, juvenile recruitment bottlenecks and emergent Allee effects; phenomena that establish destabilising positive feedbacks in a system and hence create alternative stable states.</p><p>I studied the consequences of population size-structure for community dynamics at different scales of system complexity. I performed laboratory and ecosystem experiments. Small poecilliid fishes and planktonic invertebrates with short generation times and life spans were used as model organisms. This allowed me to assess the long-term dynamics of the populations and communities investigated.</p><p>The main experimental results are: (a) An ontogenetic niche shift in individuals of the phantom midge <i>Chaoborus</i> made the population vulnerable to an indirect competitive recruitment bottleneck imposed by cladoceran mesozooplankton via rotifers. Consequentially the natural zooplankton food web exhibited two alternative attractors. (b) Body size determined the success of <i>Poecilia reticulata</i> invading resident population of <i>Heterandria formosa</i> and thus the type of alternative stable state that established. Small invaders were outcompeted by the residents, whereas large invaders excluded their competitor by predating on its recruits. (c) External juvenile and adult mortality altered the internal feedback structure that regulates a laboratory population of <i>H. formosa</i> in such a way that juvenile biomass increased with mortality. This biomass overcompensation in a prey population can establish alternative stable states with top-predators being either absent or present.</p><p>The major conclusion is that size-structure and individual growth can indeed lead to alternative stable states. The considerations of these ubiquitous features of populations offer hence new insights and deeper understanding of community dynamics. Alternative stable states can have tremendous consequences for human societies that utilise the ecological services provided by an ecological system. Understanding the effects of size-structure on alternative stability is thus crucial for sustainable exploitation or production of food resources.</p>

Alternative stable states have been, based on theoretical findings, predicted to be common in ecological systems. Empirical data from a number of laboratory and natural studies strongly suggest that alternative stable states also occur in real populations, communities and ecosystems. Potential mechanisms involve population size-structure and food-dependent individual development. These features can lead to ontogenetic niche shifts, juvenile recruitment bottlenecks and emergent Allee effects; phenomena that establish destabilising positive feedbacks in a system and hence create alternative stable states. I studied the consequences of population size-structure for community dynamics at different scales of system complexity. I performed laboratory and ecosystem experiments. Small poecilliid fishes and planktonic invertebrates with short generation times and life spans were used as model organisms. This allowed me to assess the long-term dynamics of the populations and communities investigated. The main experimental results are: (a) An ontogenetic niche shift in individuals of the phantom midge Chaoborus made the population vulnerable to an indirect competitive recruitment bottleneck imposed by cladoceran mesozooplankton via rotifers. Consequentially the natural zooplankton food web exhibited two alternative attractors. (b) Body size determined the success of Poecilia reticulata invading resident population of Heterandria formosa and thus the type of alternative stable state that established. Small invaders were outcompeted by the residents, whereas large invaders excluded their competitor by predating on its recruits. (c) External juvenile and adult mortality altered the internal feedback structure that regulates a laboratory population of H. formosa in such a way that juvenile biomass increased with mortality. This biomass overcompensation in a prey population can establish alternative stable states with top-predators being either absent or present. The major conclusion is that size-structure and individual growth can indeed lead to alternative stable states. The considerations of these ubiquitous features of populations offer hence new insights and deeper understanding of community dynamics. Alternative stable states can have tremendous consequences for human societies that utilise the ecological services provided by an ecological system. Understanding the effects of size-structure on alternative stability is thus crucial for sustainable exploitation or production of food resources.

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. / 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).

Invasive exotic species threaten biodiversity and ecosystem functions globally, creating need for and encumbering ecological restoration. When restoring exotic plant-dominated ecosystems, reinvasion pressure is the rate of new exotic recruitment following mature exotic removal. It can vary broadly among similarly invaded habitats and is crucial to restoration outcomes and costs, but is difficult to predict and poorly understood. Initial results from the experimental restoration of a wetland dominated by Triadica sebifera led us to develop the ‘outgrow the stress’ hypothesis. It holds: (1) Variation in reinvasion pressure is driven by differences in propagule abundance and spatiotemporal availability of realized recruitment windows, which are defined by abiotic conditions and biotic interactions. (2) Differences in reinvasion pressure become masked by exotic dominance when increases in niche breadth during development enable exotic persistence across sites where recruitment windows range from frequent to episodic. We validated this hypothesis. First, we used greenhouse and field experiments to quantify Triadica’s moisture niche early in development. By two months post-germination, seedling tolerances broadened to include conditions unsuitable for germination. This clearly demonstrated a rapid ontogenetic niche expansion, which could decouple mature Triadica density and average reinvasion pressure. Second, we used a greenhouse mesocosm experiment to quantify how recruitment window duration, competition and fertility impacted population-level Triadica establishment in stressful environments. As ‘outgrow the stress’ predicts, longer windows increased Triadica success and multi-factor interactions were common, with competition and fertility effects varying among environmental contexts. Third, we substantiated predictions of ‘outgrow the stress’ regarding propagule availability and soil moisture by manipulating these in a multi-site field experiment spanning eleven experimental restorations of Triadica-dominated habitats along a moisture gradient. Triadica reinvasion pressure varied broadly among sites but correlated with moisture and fertility. Propagule availability drove reinvasion in favorable environments, but availability of suitable conditions trumped propagules in extreme environments. Competition reduced Triadica performance and sometimes survival. Triadica prevalence reduced native plant prevalence. Six restorations require minimal Triadica management for success. This work advances our understanding and enables better predictions of reinvasion pressure and invasions in general. Accurate predictions enhance restoration efficiency by informing site selection and optimal management strategies.