Ecological communities all over the world are loosing biodiversity due to different kinds of human activities and there is an urgent need of understanding how those losses affect the function of the ecosystems on which we all depend. The community's response to species losses is likely to depend on both the structure of the local community as well as its interactions with surrounding communities. Also the characteristics of the species going extinct do affect how the community structure changes. The main purpose of this thesis is to study how local population dynamics and regional processes in food webs affect ecological communities' response to species loss, especially the risk of cascading extinctions.

In Paper I we use a set of model food webs with different shapes and connectance to look at how the structure of the community affects its resistance to species loss. We also investigate how the resistance is affected by which species, according to trophic level and connectivity, that is lost initially. What we find is that food webs with lower connectance seem to be more vulnerable than more connected communities. The loss of a species at low trophic level and / or with high connectivity triggers the on average highest number of secondary extinctions. We also discuss about the structure of the post­ extinction community and compare our analysis with topological studies.

In paper Il we use as set of metacommunities with different connectances and different number of patches and we vary the dispersal distances and migration rates. The aim of this paper is to investigate how web connectance of local communities, number of habitat patches and dispersal patterns affects a metacommunity's response to the global loss of a species. We find that asynchrony among patch dynamics may arise from relatively low rates of migration, and that the inclusion of space significantly reduces the risk of global cascading extinctions. It is shown that communities with sparsely connected food webs are the most sensitive to species loss, but also that they are particularly well stabilised by the introduction of space. In agreement with theoretical studies of non-spatial habitats, species at the highest trophic level are the most vulnerable to secondary extinction.

Paper III is a book chapter that emerged from a working group during a food web symposium in Giessen, Germany 2003. It is dealing with ideas of how to look at spatial aspects of food webs.