A patch modeling approach to the community-level consequences of directional dispersal

Although plants and other sessile organisms often disperse in a prevailing direction, the ecological consequences of this are poorly understood. To explore patterns of plant diversity similar to those found in a California river system, I modeled perennial plant populations and communities occurring in a linear series of neighborhoods connected by dispersal, where dispersal occurs in a prevailing downstream direction. Simulations demonstrated that population size and species diversity correlated with dispersal and increased downstream, but only if fecundity and death rates were such that neighborhoods within the system depended on external propagule input for their persistence. Highly effective downstream dispersal and some upstream dispersal were also required. If these demographic and dispersal criteria were met, communities with intense competition and inhibition still developed downstream increases in diversity. Additionally, in such systems, directional dispersal could favor species coexistence since upstream neighborhoods provided a refuge for inferior competitors. The demographic and dispersal criteria were relaxed for systems recovering from scattered source populations following disturbance. I conclude that directional dispersal may be a key determinant of species coexistence and patterns of diversity in severely propagule-limited systems.