Structuring factors for macroinvertebrate communities and the importance for ecosystem processing

Sammanfattning: Aquatic macroinvertebrates play important roles in our ecosystems and are essential for the ecosystem processing. An improved knowledge of structuring factors, such as competition and predation, and the way in which they operates in the macroinvertebrate community is crucial for our understanding of not only the macroinvertebrates but also the ecosystem in general. Furthermore, today our ecosystems are also faced with a new range of threats including brownification and invasive species. The objective of my thesis is therefore to enhance the knowledge of the structuring factors and how it affects the ecosystem processes. In this theses I have studied (both in field and in the laboratory), both the general and interactive effects of predation risk, competition, brownification and invasive species, on macroinvertebrate ( Radix balthica, Cloeon dipterum, Gammarus pulex, Asellus aquaticus and Potamopyrgus antipodarum ) fitness factors (growth and mortality) as well as ecosystem processing (grazing and decomposition). I found that indirect effects of predation affected ecosystem processing; decomposition increased whereas grazing decreased in treatments with predator cues. Furthermore, predator cues had a positive effect on R. balthica growth, most likely due to high quality leaf biofilm. Competition, both intraspecific and interspecific, had a negative effect on both ecosystem processing as well as fitness factors. However, I also found facilitation between species, cascading further to affect ecosystem processing. Furthermore, water colour had a significant effect on predation. In clear water fish predated selectively on R. balthica , whereas there was no difference in prey preference in brown water. An increase in water colour had a negative effect on both decomposition and grazing, most likely due to changes in species composition of periphyton and leaf biofilm. Increased water colour also had a negative effect on R. balthica growth. In contrast, the invasive species P. antipodarum was more tolerant, both to predation as well as the effect of water colour, explaining their success as an invasive species. P. antipodarum also had higher grazing rates than the native species, which may have implications for the invaded ecosystems. My results illustrate the importance of competition, indirect effects of predation, water colour as well as the introduction of invasive species, not only for the individual fitness of macroinvertebrates but also for ecosystem possessing. Furthermore, it also shows the complexity of macroinvertebrate population dynamics and how the outcome of interactions is greatly dependent on which species are involved and in what combinations.