Species interactions in streams - effects of predation, competition and ecosystem properties

Sammanfattning: Stream ecosystems are dynamic and they have an inherent environmental variability. Organisms that live in streams are adapted to the continuously changing environment and the presence or absence of species are determined by their environmental requirements, while abundance is most often affected by interactions, such as competition and predation, with other species that are found in the same habitat. My work mainly focused on interactions between three stream living species; stone loach, brown trout and signal crayfish that all co-occur in streams in southern Sweden where they dominate the predator guild. These species have overlapping food and habitat preferences and I have studied the effects of different environmental variables on species abundance, trophic niche width and niche overlap. I also examined the relative effects of predation and competition and how predator interactions affect prey survival and ecosystem attributes. I found predation to be important for foraging behaviour, prey survival and community composition. Although I have worked with effects of predation on several scales, from laboratory experiments to large scale data sets, the results still suggest that predation can be important in stream ecosystems. However, merging species into functional groups, like predators, may not always be appropriate since organisms can vary in the magnitude of effects on community processes due to certain species specific traits. For example, I have shown that different predator foraging modes can affect both prey survival and ecosystem attributes. Thus, predicting effects of predators in general can be difficult if not species specific traits are considered. Even though I found biotic factors, like abundance of juvenile and adult trout, to explain some variation in stone loach abundance, characteristics of the habitat explained even more. Stone loach abundance increased along a gradient from forested sites, high in canopy cover, to open sites with high periphyton biomass. Although no such relationship was found for the other species, crayfish seem to occur at higher densities at larger substratum grain sizes. Trophic feeding niche overlap between loach and juvenile trout was related to spatial habitat complexity, where increasing complexity decreased niche overlap. Hence, substrate variability is important for species interactions and maintaining or creating substrate with several size fractions could mediate coexistence, at least between loach and trout and would probably facilitate crayfish as well.