Species interactions and community structure

In this theses I have focused on how the mechanisms of species interactions affect community structure. It is well known that certain species may have a particularly strong influence on the structure of a community. What is not fully understood is how to characterize these species that have such a strong impact on community structure. In my work, I have work tried to find out what makes such species "keystones". This has been done both with models of small interacting units, but also with help of field data. I have attempted to estimate how direct as well as indirect effects among interacting species affect both the species abundance and the dynamics of the community. The analyse then implies that the effects of both direct as well as indirect interactions on species are of fundamental interest. Another issue I have focused on is how additional energy (enrichment) affects species interactions, stability, and particularly local stability properties of the ecological system. It is not fully understood how additional energy enters a system, neither how the additional resources are allocated among the producers. I have modeled a few scenarios with different allocation possibilities, and have shown that stability is crucially dependent on how the additional energy is spent by the producer level in the system. The local stability properties of a system is also very model dependent. The same model, but with different assumptions about how the enrichment is entering show very different dynamic outcomes. Finally, I have looked at how spatial structure increases stability in a community of interacting species. However, the opposite also seems to be true, i.e., that fragmented environments can have a negative effect on community persistence, when the number of patches is exceeding a certain threshold, a subsecuent loss of stability do occur.