Adaptive evolution in source-sink environments: Direct and indirect effects of density-dependence on niche evolution

The most basic adaptation of a species is its ecological niche - that set of environmental conditions, resources, etc. which permit its populations to persist without immigration. Immigration from habitats with conditions within the niche (''source'' populations) can sustain populations in habitats outside the niche (''sink'' populations). Recent theoretical work on adaptive evolution in source-sink environments suggests that niche evolution should often be conservative, favoring the improvement or maintenance of adaptation to sources, at the expense of improved adaptation in sinks. Niche conservatism requires sinks to remain sinks over evolutionary time-scales. This paper uses models of coupled population and evolutionary dynamics in a spatially heterogeneous environment to examine the effect of density-dependence on niche conservatism and evolution. Density-dependence in birth, death, or dispersal rates can influence the relative evolutionary importance of source and sink habitats. Three distinct dispersal modes are compared. Species with density-dependent, ideal-free dispersal should exhibit niche conservatism. Species with passive or density-independent dispersal are more likely to show niche evolution, but only if rates of dispersal are sufficiently high and fitness in the sink is not too low. Species with active interference leading to a non-ideal free habitat distribution should be expected to be evolutionarily labile in their ecological niches. I conclude that the nature of density-dependence in dispersal may crucially govern the direction of niche evolution.