3 - Source–sink dynamics emerging from unstable ideal free habitat selection

Summary

Adaptive theories of source–sink regulation assume that dispersal maximizes individual fitness. Fitness in these models is improved through the long-term benefits of habitat-dependent dispersal rates, optimized habitat choices, pulsed dispersal, advantages to relatives, or natural selection of dispersal unrelated to habitat use. But source–sink dynamics may often simply represent ideal free habitat selection in unstable populations. The prevalence of source–sink systems suggests that there may be other evolutionary attractors. I explore two candidates: an inclusive fitness strategy that maximizes population growth, and a cooperative strategy whereby unrelated individuals form coalitions whose combined aggression forces the emigration of unaligned individuals. Although these forms of source–sink dynamics can displace ideal free habitat selection, they create high-fitness patches available for counter-invasion. Regardless of whether there are cycling evolutionary attractors for different forms of source–sink systems, computer simulations reveal crucial roles for sinks in damping otherwise unstable dynamics. Sinks, even when acting as ecological traps, increase the probabilities of persistence and, if the sink is not too severe, can create the illusion that they are unimportant in stabilizing source populations. Alteration or removal of these critical sinks can doom source populations to wildly fluctuating dynamics and extinction. The lesson for conservation in mosaic landscapes is clear: all habitat is critical.