Ecological and fitness consequences of species coexistence: A removal experiment with wood warblers

Local guilds define groups of species that share common resources and coexi st in space and time. Local guilds have historically been a major focus of community ecology; however, studies of local guilds rarely measure conseque nces of coexistence for fitness-related traits or test predictions of alter native hypotheses for how species may interact. We studied consequences of coexistence for Orange-crowned Warblers (Vermivora celata) and Virginia's W arblers (V. virginiae), which have overlapping breeding territories in cent ral Arizona. We used reciprocal removal experiments to examine (1) whether coexistence results in ecological consequences with respect to access to ne st sites, access to food resources, nest predation, and adult female predat ion, 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 re moved 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 War bler nest sites, (2) increased feeding rates during both the incubation and nest:ling periods, and (3) suffered reduced nest predation rates, compared with control plots where Orange-crowned Warblers were present. When the tw o species coexist, increased nest predation rates for both species appear t o 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 selec tion and building periods, and from Orange-crowned Warbler preemption of ne st sites preferred by both species. The mechanisms whereby Orange-crowned W arblers may reduce access to food resources for coexisting Virginia's Warbl ers, however, are not yet fully understood. Both. Orange-crowned and Virgin ia's Warblers fledged between 78% and 129% more young per nest on plots whe re the opposite species had been removed, indicating that both species suff er substantial fitness costs of coexistence. Overall, results illustrate th at (I) Orange-crowned and Virginia's Warblers do not coexist independently of each other, (2) interactions between the two species are complex and asy mmetrical, (3) interactions between the two species result in substantial f itness costs of coexistence for both species, and (4) ecological interactio ns between the two species extend far beyond competition for food resources which has dominated studies of terrestrial vertebrate communities.