The spread of alien species is a major threat for biodiversity world-wide, it have caused many extinctions and can change properties of ecosystems. Invasive species also causes enormous economical costs. Ecological communities vary in their susceptibility to alien species and it is still not clear what determines this variation.
I use data on introductions of fish to Swedish lakes to study what properties of the ecological communities that determines whether or not the introductions have been successful, i.e. if the community resisted the invasion or not. In particular I am interested in the biotic resistance that the resident species can provide. Biotic resistance is commonly attributed to that more species rich assemblages can utilize resources more effectively and thus leaving little resource space for invading species. A community can however also be resistant to invasions due to one or a few strong competitors or predators.
The major hypothesis that I will test are:
• The resistance of a community is determined by its species richness
• The resistance of a community is determined by the species richness and the capacity of the habitat to support species, i.e. level of saturation
• Resident species vary in their contribution to resistance
• Invading species vary in their response to habitats and resident species