Understanding the consequences of seed dispersal in a heterogeneous environment.

Plant distributions are in part determined by environmental heterogeneity on both large (landscape) and small (several meters) spatial scales. Plant populations can respond to environmental heterogeneity via genetic differentiation between large distinct patches, and via phenotypic plasticity in response to heterogeneity occurring at small scales relative to dispersal distance. As a result, the level of environmental heterogeneity experienced across generations, as determined by seed dispersal distance, may itself be under selection. Selection could act to increase or decrease seed dispersal distance, depending on patterns of heterogeneity in environmental quality with distance from a maternal home site. Serpentine soils, which impose harsh and variable abiotic stress on non-adapted plants, have been partially invaded by Erodium cicutarium in northern California, USA. Using nearby grassland sites characterized as either serpentine or non-serpentine, we collected seeds from dense patches of E. cicutarium on both soil types in spring 2004 and subsequently dispersed those seeds to one of four distances from their maternal home site (0, 0.5, 1, or 10 m). We examined distance-dependent patterns of variation in offspring lifetime fitness, conspecific density, soil availability, soil water content, and aboveground grass and forb biomass. ANOVA revealed a distinct fitness peak when seeds were dispersed 0.5 m from their maternal home site on serpentine patches. In non-serpentine patches, fitness was reduced only for seeds placed back into the maternal home site. Conspecific density was uniformly high within 1 m of a maternal home site on both soils, whereas soil water content and grass biomass were significantly heterogeneous among dispersal distances only on serpentine soils. Structural equation modeling and multigroup analysis revealed significantly stronger direct and indirect effects linking abiotic and biotic variation to offspring performance on serpentine soils than on non-serpentine soils, indicating the potential for soil-specific selection on seed dispersal distance in this invasive species.