Spatial niches and coexistence: testing theory with tarweeds

Competitive coexistence in a spatially heterogeneous environment is traditionally attributed to niche differences, but several recent theories have proposed important additional roles for propagule limitation and chance (e.g., neutral theory, stochastic niche theory, spatial storage effect). We tested whether propagule supply and timing of disturbance affected the coexistence of three ecologically similar plants that replace one another with partial overlap along a local soil gradient. We asked what prevents the species that dominates the most common habitat (Holocarpha virgata, open hillsides) from invading the habitats where the other two species are dominant (Calycadenia pauciflora, rocky hilltops; Hemizonia congesta, clay-rich bottomlands). We added abundant Holocarpha seeds into Calycadenia and Hemizonia habitats that were experimentally disturbed at different times of year. Initial Holocarpha seedling densities in Calycadenia and Hemizonia habitats equaled or exceeded those in unmanipulated Holocarpha habitat, but Holocarpha survival, adult size, and fecundity were much lower outside its own habitat. Holocarpha persisted in Calycadenia and Hemizonia habitats for three years, and springtime disturbance promoted this expansion. However, outside its own habitat Holocarpha showed below-replacement fitness and little competitive effect on the other two species. Our results were most consistent with a deterministic view of spatial niches. Nonetheless, chance events may often cause natural communities to include some transient populations at any given time, leading them to appear "unsaturated" with species.