Stability and complexity of ecosystems - Global interaction constraints, landscape and extinctions

Abstract: 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 fortuitous fluctuations as well as migration, climate, and evolution, in a human time-frame ecosystems are relatively stable. Stability of an ecosystem can refer to many different aspects but in general denotes an ability to uphold 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 and specifically General Lotka-Volterra equations 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 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. Further, these stability aspects are found to hold when spatial extension is modelled explicitly. With spatial extension homogeneous landscapes are shown to enhance robustness by a larger spectrum of dynamics and in the limit of high dispersal heterogeneous landscapes can facilitate much higher complexity. 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 local and global features of ecosystems which determine response behaviour and stabilising dynamics to a high degree, important when analysing systems and assessing their vulnerability in face of environmental pressures.