Competition, predation, and elevational limits of tropical birds
Tropical mountains harbor some of the world’s most diverse ecological communities due to high species turnover generated by narrow elevational ranges of species. Our research investigates the importance of biotic factors in reinforcing elevational ranges of tropical birds, with the goal to understand how such high species turnover is maintained in these landscapes. Specifically, we focus on interspecific competition and nest predation, two biotic interactions that may have widespread effects on avian species range limits. It has long been thought that parapatric distributions between closely related species along elevational gradients are maintained by competitive interactions. Recent empirical work examining territorial aggression in contact zones between species pairs in Costa Rica supports this hypothesis. We explore whether such competitive interactions underlie species elevational replacements more generally by comparing results from heterospecific song playback experiments conducted in two regions, the Tilarán Mountains of Costa Rica and the Southern Peruvian Andes. In the tropics, nest predation plays an important selective role in shaping avian life history traits and ecology, although the intensity of predation varies with elevation. We examine whether variation in nest predation rates and predator frequency could influence community-level patterns of species turnover along elevational gradients.
Our results showed that with interspecific competition, species with parapatric distributions exhibit high variation in interspecific aggression. Importantly, local densities of congeners within contact zones may determine the presence and strength of interspecific competition. With nest predation, we found a striking threshold shift in predation rates with elevation, where high nest predation at lower elevations drops abruptly above the foothills, the zone in which predation by snakes and primates also declines steeply. Remarkably, community turnover in birds was highest between zones in which nest predation rates and composition of the predator community changed the most. We suggest that dramatic shifts in nest predator communities and nest predation rates can explain variation in nesting success of species and could thereby limit elevational distributions of tropical montane birds, with community-wide effects. Taken together, we show that biotic interactions can have important species-specific and community-level consequences, and the strength of interactions may vary in predictable ways across species and landscapes. Tropical mountains are expected to undergo extensive transformations and community reorganization as species respond to warming temperatures. Empirical data on the strength of biotic interactions will be crucial to make informed predictions for how species and communities will respond to shifting climate regimes.