The filtering metaphor revisited: competition and environment jointly structure invasibility and coexistence
Filtering, or the reduction in species diversity that occurs because not all species can persist in all locations, is thought to unfold hierarchically, at large scales due to the environment and at small scales due to competition. However, the ecological effects of competition and the environment are not independent, and observational approaches preclude investigation into their interplay. We use a demographic approach with 30 plant species to experimentally test (i) the effect of competition on species persistence in two soil moisture environments, and (ii) the effect of environmental conditions on the mechanisms underlying competitive coexistence. We find that competitors cause differential persistence of species across environments even when these effects are lacking in the absence of competition, and that the traits that determine persistence depend on the competitive environment. Changing environmental conditions generated idiosyncratic effects on coexistence outcomes, increasing competitive exclusion of some species while promoting coexistence of others. Our results highlight the importance of considering environmental filtering in light of, rather than in isolation from, competition, and challenge community assembly models and approaches to projecting future species distributions.