Invasion resistance, species build-up and community collapse in metapopulation models with interspecies competition

Islands or habitat patches in a metapopulation exist as multi-species communities. Community interactions link each species' dynamics so that the colonization of one species may cause the extinction of another. In this way, community interactions may set limits to the invadability of an island and to the likelihood of resident species extinctions upon invasion. To examine the nature of these limits, I assemble stable multi-species Lotka-Vollerra competition communities that differ in resident species number and the average strength (and variance) of species interactions. These are then invaded with species whose properties are drawn from the same distribution as the residents. The invader success rate and the extinction rate of resident species is determined as a function of community-and species-level properties. I show that the probability of colonization success for an invader decreases with species number and the strength and variance of interspecific interactions. Communities comprised of many strongly interacting species limit the invasion possibilities of competing species. Community interactions, even for a superior invading competitor, set up a sort of ‘activation barrier’ that repels the invader. This ‘priority effect’ for residents is not assumed a priori in my description for the individual population dynamics of these species, rather it arises because species-rich and strongly-interacting species sets have alternative stable states that tend to disfavour species at low densities. These models point to community-level rather than invader-level properties as the strongest determinant of differences in invasion success. If an invading species is successful it competitively displaces a greater number of resident species, on average, as community size increases. These results provide a logical framework for an island-biogeographic theory based on species interactions and invasions and for the protection of fragile native species from invading exotics.