The American alligator (Alligator mississippiensis) has been frequently studied in large reservoirs and coastal marshes. Large ontogenetic shifts in their diet and morphology have been linked with changes in habitat use, with adult males using deep, open water and juveniles and nesting females relying on vegetated marsh. In certain regions of the inland portion of the alligator’s range, these different aquatic habitats are represented by seasonal wetlands and riverine systems that are separated by a terrestrial matrix. Ontogenetic habitat shifts, therefore, would require overland movements between systems, which has important implications for conservation of the species. I tested several commonly used methods of surveying alligator populations to determine the most effective method of studying alligators in seasonal wetlands. I then used systematic trapping, nest surveys and radio telemetry to determine habitat use and overland movement rates by different sex and size classes. I found that seasonal wetlands provided nesting and nursery sites for these inland alligator populations, but that both juveniles undergoing an ontogenetic shift and nesting females move between the wetlands and riverine systems. Overland movements by alligators between the wetland and riverine habitats establish a level of functional connectivity between these aquatic ecosystems. I constructed a habitat suitability index of both the wetlands and the surrounding landscape to determine which patch and landscape characteristics were important to wetland use by alligators. I found that both descriptive wetland characteristics and the spatial relationships between wetlands were important predictors of alligator use. Overland movement was related to upland landuse as well as distance between aquatic habitats. Conserving a variety of wetland sizes and types within an intact upland matrix is critical to maintaining connectivity across the landscape. Furthermore, understanding how species may act as mobile links between ecosystems, particularly those with ontogenetic niche shifts, illustrates the importance of approaching conservation from a landscape perspective.

The American alligator (Alligator mississippiensis) has been frequently studied in large reservoirs and coastal marshes. Large ontogenetic shifts in their diet and morphology have been linked with changes in habitat use, with adult males using deep, open water and juveniles and nesting females relying on vegetated marsh. In certain regions of the inland portion of the alligator’s range, these different aquatic habitats are represented by seasonal wetlands and riverine systems that are separated by a terrestrial matrix. Ontogenetic habitat shifts, therefore, would require overland movements between systems, which has important implications for conservation of the species. I tested several commonly used methods of surveying alligator populations to determine the most effective method of studying alligators in seasonal wetlands. I then used systematic trapping, nest surveys and radio telemetry to determine habitat use and overland movement rates by different sex and size classes. I found that seasonal wetlands provided nesting and nursery sites for these inland alligator populations, but that both juveniles undergoing an ontogenetic shift and nesting females move between the wetlands and riverine systems. Overland movements by alligators between the wetland and riverine habitats establish a level of functional connectivity between these aquatic ecosystems. I constructed a habitat suitability index of both the wetlands and the surrounding landscape to determine which patch and landscape characteristics were important to wetland use by alligators. I found that both descriptive wetland characteristics and the spatial relationships between wetlands were important predictors of alligator use. Overland movement was related to upland landuse as well as distance between aquatic habitats. Conserving a variety of wetland sizes and types within an intact upland matrix is critical to maintaining connectivity across the landscape. Furthermore, understanding how species may act as mobile links between ecosystems, particularly those with ontogenetic niche shifts, illustrates the importance of approaching conservation from a landscape perspective.

One common consequence of ontogenetic niche shifts is that juveniles and adults of the same species often use different resources and habitats. Theory suggests that variation in productivity and/or habitat size for the respective life stage regulates size distribution of individuals and population biomass. Low resources/habitat availability for juveniles relative to adults results in populations with many small juveniles with high mortality and few but large adults, while the opposite situation results in a population with small and slow growing adults with high densities. I tested this theory using lake-living brown trout (Salmo trutta), where adults inhabit lakes and use connected streams for spawning and nurseries for juveniles. My hypothesis was that the relative habitat availability for juveniles respectively adults determines the size structure of adult brown trout in lakes. This was done by quantifying available stream habitats in relation to lake area via GIS analysis of 101 allopatric brown trout lakes in Sweden and Norway. My results showed that the proportion of large trout, mean length and maximum length decreased with increasing juvenile habitat in relation to adult habitat availability. This suggests that relative variation in availability of juvenile and adult habitats can affect the size structure of lake-living brown trout, most likely due to size dependent niche shifts and competitive and cannibalistic interactions. As the lake brown trout is highly valued both for commercial and recreational purposes, these findings can be highly valuable for sustainable management of the ecosystems services that the brown trout provides.

Dam construction in lakes may have significant negative effects on aquatic ecosystems, and it is therefore considered as a major threat to aquatic biodiversity. Many studies have been conducted to investigate how fish are affected by dam removal. Dams can impact the flora and fauna of lakes by influencing the structure of food webs and nutrient dynamics. In this study, I investigated whether or not dam removal in lake outlets affect the proportion of piscivorous perch, including a separate analysis of perch with lengths between 120-180 mm (the length interval at which the diet changes from insects to fish). I used data from 27 lakes located in Värmland, Dalarna and Västmanland. The proportion of piscivorous perch was 43% higher where the dam had been removed. The results showed a (non-statistically significant) trend on the effect of dam removal on the mean proportion of piscivorous perch. Dam removal had no effect on the mean proportion in the length interval 120>180 mm. Dams alter the composition, structure, and function of the surrounding environment; therefore, the outcomes of dam removals can be unpredictable. There are few published studies on how lentic fish communities respond to dam removals in lake outlets; therefore, such knowledge would be valuable and can provide relevant information to water managers, hydropower companies and decision-makers. / Dammkonstruktion i sjöar har betydande negativa effekter på akvatiska ekosystem, och anses därför som ett stort hot mot akvatisk biodiversitet. Det har gjorts många studier på hur fisk påverkas av dammar, och utrivning av dessa, eftersom de kan ha effekter på sjöars flora och fauna, genom att påverka näringsvävens struktur och näringsämnesdynamik. I denna studie undersökte jag om dammutrivningar i sjöutlopp påverkar proportionen av fiskätande abborrar, samt om medel-proportionen av fiskätande abborrar mellan 120–180 mm påverkas av utrivning (det längdintervall när dieten ändras från insekter till fisk). Jag använde data från 27 sjöar som var lokaliserade i Värmland, Dalarna och Västmanland. Resultaten visade att proportionen av fiskätande abborrar var 43% större där dammen hade rivits ut. Dammutrivning visade en (icke statistisk signifikant) trend på att det fanns en effekt av dammutrivning på medelproportionen av fiskätande abborrar. Dammutrivning hade ingen signifikant effekt alls på medelproportionen av fiskätande abborre i längdintervallet 120>180 mm. Dammar förändrar sammansättningen, strukturen och funktionen i den omgivande miljön, därför kan dammutrivningars resultat vara oförutsägbart. Det finns få publicerade studier av hur fiskesamhällen reagerar på dammutrivningar i sjöutlopp, därför skulle sådan kunskap vara värdefullt och kan ge relevant information till förvaltare av vatten, såsom myndigheter, kraftbolag och beslutsfattare.

Size-dependent differences between individuals in size-structured organisms have fundamental effect on population and community dynamics. Intraguild predation (IGP) is one specifically interesting constellation that often arises when two size-structured populations interact. Ontogenetic bottlenecks that determine population size-structure are affected by both population intrinsic as well as population extrinsic factors, and are therefore context-dependent. Surprisingly, size-structured IGP systems have mainly been investigated theoretically and especially long-term empirical studies are widely lacking. In this thesis I investigate empirically how habitat complexity, interaction strength, and stage-specific resource availabilities affect population processes and their effects on the dynamics of a size-structured IGP system. I conducted multi-generation experiments in a size-structured IGP system, with the Least Killifish (Heterandria formosa) as IG prey and the Common Guppy (Poecilia reticulata) as IG predator. With no alternative resource next to the shared resource, IG predator and IG prey could not coexist. Weak interactions only increased IG prey and IG predator persistence times and observed exclusion patterns depended on habitat complexity. An alternative resource for either the juvenile IG predator or the juvenile IG prey on the other hand promoted coexistence. However, this coexistence was context-dependent. Ontogenetic bottlenecks played a central role in the dynamics of the size-structured IGP system in general. In the final study I show that an ontogenetic bottleneck can, through changes in stage-specific resource availabilities, be affected in a way that leads to increased trophic transfer efficiency with potential effects on higher trophic levels. Overall, the results emphasize importance of the broader context in which size-structured communities are embedded. Especially, when managing natural communities it is important to account for the combined effects of size-structure, stage-specific resource availabilities, and habitat structure. Specifically, when managing species that connect habitats or ecosystems all life-stages’ environmental conditions must be consider in order to ensure strong predictive power of tools used for ecosystem management planning.