Predator driven niches vary spatially among co-occurring damselfly species

Determining how niche differences contribute to local species coexistence is a vexing problem. Previous work has shown that the ecological and evolutionary processes shaping niche differentiation can vary among populations, suggesting that the strength of niche differences among species should likewise vary geographically. Most tests of this idea compare different species in different locations, not the same species in different locations. Thus, it is unclear whether niche differences vary spatially because of variation in community composition or because populations of the same species experience differences in the strength of niche effects. To test this latter hypothesis, we used field experiments to manipulate the relative abundances of the same pair of Enallagma damselfly species at two lakes. Manipulating relative abundances allowed us to quantify the demographic signature of niche differences that could stabilize coexistence, because if species are niche differentiated, they should experience lower mortality in response to their shared fish predator, and higher growth in the face of resource competition, when rare. We found that both species experienced lower mortality when rare in one location but not the other. No differences in growth were detected, indicating that competition for prey resources may not be a key factor affecting coexistence. These results suggest the species are ecologically differentiated among populations in ways shaping survivorship in response to a shared predator, which should promote their coexistence. We discuss several factors that could contribute to the differences we observed, focusing on the ideas that either (1) niche differentiation between species evolves locally, or that (2) spatial variation in environmental factors affects the manifestation of species niche differences. We therefore argue that the problem of ‘species coexistence’ is not a problem of species, but rather is one of understanding if species’ populations coexist. Such results imply a role for microevolutionary processes in structuring communities.