The impact of directed versus random movement on population dynamics and biodiversity patterns.

An improved understanding of dispersal behavior is needed to predict how populations and communities respond to habitat fragmentation. Most spatial dynamic theory concentrates on random dispersal, in which movement rates depend neither on the state of an individual nor its environment and movement directions are unbiased. We examine the neglected dispersal component of directed movement in which dispersal is a conditional and directional response of individuals to varying environmental conditions. Specifically, we assume that individuals bias their movements along local gradients in fitness. Random movers, unable to track heterogeneous environmental conditions, face source-sink dynamics, which can result in deterministic extinction or increase their vulnerability to stochastic extinction. Directed movers track environmental conditions closely. In fluctuating environments, random movers "spread their bets" across patches, while directed movers invest offspring in habitats currently enjoying propitious conditions. The autocorrelation in the environment determines each strategy's success. Random movers permeate entire landscapes, but directed movers are more geographically constrained. Local information constraints limit the ranges of directed movers and introduce a role for historical contingency in determining their ultimate distribution. These geographic differences have implications for biodiversity. Random movement maintains biodiversity through local coexistence, but directed movement favors a spatial partitioning of species.