Extinctions and Ecosystem Stability

Licentiate thesis, 2019

Human society's expansion and demand for both biotic and abiotic natural resources exert a large pressure on ecosystems around the globe. Ecosystems are complex networks of species interacting with each other and their physical surroundings. Although they are in constant change due to incidental/fortuitous fluctuations as well as climate, migration and, evolution, in a human time-frame ecosystems are relatively stable, upholding certain qualities and functions. Stability of an ecosystem can refer to many different aspects but in general denotes an ability to keep the perceived qualities and functions in the face of external disturbances.

This thesis builds on the long heritage of trying to understand stability of ecosystems, and the more recent use of dynamical modelling for this purpose. A contested issue in ecosystem research is the role of complexity in facilitating stability. Complexity being an intuitive but not strictly defined concept including among others number of species, amount of interactions and structure of interactions. Irrespective of the role of complexity for ecosystem stability there is general agreement that there are limits to stability, in terms of some property, at which point an ecosystem if perturbed/pressured beyond it will transition to a qualitative different state.

This thesis shows that, contrary to previous conception, there are more limits of stability than one. The new limits revise the important transition points of an ecosystem and differentiate between different types of stability, which in turn have differing responses to disturbances of equal magnitude. Species extinctions are found as a mechanism to prevent collapse of an entire community and collapse is found to be divided into two types. The thesis also exposes certain types of constraints on the structures of interactions among species that have a large influence on the stability limits. Together these results give indication of important structures of ecosystems which determine response behaviour to a high degree, important when analysing systems and assessing their vulnerability in an uncertain environment.