Ecological and Fitness Consequences of Species Coexistence: A Removal Experiment with Wood Warblers

Local guilds define groups of species that share common resources and coexist in space and time. Local guilds have historically been a major focus of community ecology; however, studies of local guilds rarely measure consequences of coexistence for fitness-related traits or test predictions of alternative hypotheses for how species may interact. We studied consequences of coexistence for Orange-crowned Warblers (Vermivora celata) and Virginia’s Warblers (V. virginiae), which have overlapping breeding territories in central Arizona. We used reciprocal removal experiments to examine (1) whether coexistence results in ecological consequences with respect to access to nest sites, access to food resources, nest predation, and adult female predation, and (2) whether ecological consequences result in fitness consequences with respect to reproductive success (clutch size, number of young fledged per nest), or adult female survival (within a breeding season). When we removed Virginia’s Warblers, Orange-crowned Warblers experienced reduced nest predation rates compared with control plots where Virginia’s Warblers were present. When we removed Orange-crowned Warblers, Virginia’s Warblers (1) shifted their nest sites to sites indistinguishable from Orange-crowned Warbler nest sites, (2) increased feeding rates during both the incubation and nestling periods, and (3) suffered reduced nest predation rates, compared with control plots where Orange-crowned Warblers were present. When the two species coexist, increased nest predation rates for both species appear to result from density-dependent functional shifts in nest predator behavior (short-term apparent competition). Reduced access to preferred nest sites for Virginia’s Warblers coexisting with Orange-crowned Warblers appears to result from both Orange-crowned Warbler interference during nest site selection and building periods, and from Orange-crowned Warbler preemption of nest sites preferred by both species. The mechanisms whereby Orange-crowned Warblers may reduce access to food resources for coexisting Virginia’s Warblers, however, are not yet fully understood. Both Orange-crowned and Virginia’s Warblers fledged between 78% and 129% more young per nest on plots where the opposite species had been removed, indicating that both species suffer substantial fitness costs of coexistence. Overall, results illustrate that (1) Orange-crowned and Virginia’s Warblers do not coexist independently of each other, (2) interactions between the two species are complex and asymmetrical, (3) interactions between the two species result in substantial fitness costs of coexistence for both species, and (4) ecological interactions between the two species extend far beyond competition for food resources which has dominated studies of terrestrial vertebrate communities.