The importance of biotic interactions and climate change on avifaunal range limits of the Albertine rift

Understanding species distribution ecology is essential given the growing need to understand how species will react to climate change and invasion. Understanding the processes by which distribution limits are set is necessary to build accurate distribution models, yet the role of biotic interactions (e.g. competition, predation, disease, parasitism) in determining range limits is uncertain, despite evidence that incorporating biotic information improves the accuracy of these models. By examining the elevational variability of Afromontane, avian range limits across several communities, we add to reports indicating that environmental conditions are generally more important in determining both high and low stress range limits across an environmental gradient. However, we note that biotic interactions may still be more important at low stress limits than they are at high stress limits and that the processes that limit ranges appear to be different at low and high stress limits. High stress range limits appeared to be much more variable, stimulating new and unexpected questions. We go on to document how distribution limits of avifauna have shifted in two Ugandan national parks. We tentatively conclude that range shifts may have occurred over the last decade, at a rate faster than one would have expected species to move merely based on the warming that has occurred. This indicated that warming may not be the only factor that is causing species’ upper limits to shift upwards in elevation.