Abstract
Climate change forces many species to move their ranges to higher latitudes or elevations. Resulting immigration or emigration of species might lead to functional changes, e.g., in the trait distribution and composition of ecological assemblages. Here, we combined approaches from biogeography (species distribution models; SDMs) and community ecology (functional diversity) to investigate potential effects of climate-driven range changes on frugivorous bird assemblages along a 3000 m elevational gradient in the tropical Andes. We used SDMs to model current and projected future occurrence probabilities of frugivorous bird species from the lowlands to the tree line. SDM-derived probabilities of occurrence were combined with traits relevant for seed dispersal of fleshy-fruited plants to calculate functional dispersion (FDis; a measure of functional diversity) for current and future bird assemblages. Comparisons of FDis between current and projected future assemblages showed consistent results across four dispersal scenarios, five climate models and two representative concentration pathways. Projections indicated a decrease of FDis in the lowlands, an increase of FDis at lower mid-elevations and little changes at high elevations. This suggests that functional dispersion responds differently to global warming at different elevational levels, likely modifying avian seed dispersal functions and plant regeneration in forest ecosystems along tropical mountains.
Highlights
Climate change forces many species to move their ranges to higher latitudes or elevations
A total of 52 species were restricted to the elevational level of the lowlands (i.e., 500 m), five species were restricted to the highlands (3000–3500 m, tree line), and no species occurred on all elevational levels
We combined Species distribution models (SDM) with an analysis of functional dispersion (FDis, a measure of functional diversity) to explore how climate change might affect the functional diversity of frugivorous bird assemblages along an elevational gradient in the Andes
Summary
Climate change forces many species to move their ranges to higher latitudes or elevations. Other physical or biotic barriers (e.g., the tree line) could limit upward movements of species on mountains and cause species extinctions because these barriers might not necessarily shift upslope at the same pace[18] Beyond these general theoretical expectations, little is known about the potential changes in the composition of ecological assemblages along mountains in response to climate change. FDis is defined as the mean distance of individual species to the centroid of all species in a multidimensional trait space[31] and measures the dispersion of species traits within the trait space of the assemblage To connect this measure of functional diversity with species distribution models, we applied a novel approach by weighting FDis with the SDM-derived probability of occurrence of each species in current and projected future assemblages. A reduced occurrence probability of species with traits located at the margin of the multi-dimensional trait space will lead to a reduction of FDis
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