One of the most important challenges facing ecologists and conservationists is to understand how species composition of communities changes across heterogeneous landscapes.  On the level of individual species, an important corollary is to understand ecological factors that enforce the limits of species’ ranges.  Tropical montane birds are characterized by their narrow, belt-like distributions along altitudinal gradients, resulting in high beta-diversity, or species turnover, in these landscapes at the community level. Along montane gradients, species often exhibit abrupt limits to their altitudinal distributions, which can coincide with range limits of closely related species such as congeners. Previous studies describing these “species replacements” suggest that interspecific competition is an underlying mechanism, but experimental evidence has been lacking.  In this study we used heterospecific song playback experiments to test whether interspecific competition, via interspecific territoriality, occurs at species contact zones in two genera of birds along an altitudinal gradient in the Tilarán Mountains of Costa Rica.  Playbacks were conducted within individual territories of birds found at contact zones and at increasing distances from contact zones. Territorial responses to congener, conspecific and control stimuli were measured.  

Results/Conclusions

We find that individuals at their altitudinal range boundaries responded aggressively to congener songs compared to control periods.  For most species tested, these responses did not differ from responses to conspecific playbacks.  Territorial responses to congener songs weaken as the distance of individuals from the contact zone increases.  Additionally, species in congener pairs did not always respond equally to playbacks of their congeners’ songs, revealing asymmetries in interspecific aggression at contact zones.  We conclude that interspecific competition can sharpen range limits of congeners at replacement zones along altitudinal gradients, but this likely requires sufficient contact between species to develop.  Asymmetric interspecific aggression, and the resulting dominance by aggressors, has important implications for species with behaviorally-mediated altitudinal range limits in light of future climate change, especially when high-elevation species are subordinate.  If warming temperatures in montane landscapes promote upslope range expansion by dominant competitors, then subordinate species could be forced into higher elevations, depending on smaller land areas to sustain viable populations.  If many species distributions are driven by biotic interactions, as can be expected for specialized tropical species, then models will have to incorporate these complex effects in order to produce accurate predictions of species’ range shifts in montane landscapes.