Migration, Metapopulation Dynamics and Fugitive Co-existence

Many conclusions about migration in promoting persistence of metapopulations are based on very simple models, which ignore the effect of migration on local dynamics. These models are called patch models. Migration has typically several costs, including mortality of migrants en route, mortality owing to failure to establish a new population in an empty patch and, as costly migration decreases local population sizes, increased risk of extinction of local populations. We analyse metapopulation dynamics of single species and asymmetric competitors with models in which the equilibrium size of local populations is affected both by local processes and by migration. Our results clarify previous conclusions based on the simplified patch models which, when interpreted rigorously, deal only with colonization, not migration. Our results predict that species with an intermediate rate of migration are the ones which tolerate best habitat fragmentation. The general conclusion from these models is that "too much" migration can be as bad for metapopulation persistence as "too little" migration.