Abstract
AimAlthough patterns of biodiversity across the globe are well studied, there is still a controversial debate about the underlying mechanisms and their generality across biogeographic scales. In particular, it is unclear to what extent diversity patterns along environmental gradients are directly driven by abiotic factors, such as climate, or indirectly mediated through biotic factors, such as resource effects on consumers.LocationAndes, Southern Ecuador; Mt. Kilimanjaro, Tanzania.MethodsWe studied the diversity of fleshy‐fruited plants and avian frugivores at the taxonomic level, that is, species richness and abundance, as well as at the level of functional traits, that is, functional richness and functional dispersion. We compared two important biodiversity hotspots in mountain systems of the Neotropics and Afrotropics. We used field data of plant and bird communities, including trait measurements of 367 plant and bird species. Using structural equation modeling, we disentangled direct and indirect effects of climate and the diversity of plant communities on the diversity of bird communities.ResultsWe found significant bottom‐up effects of fruit diversity on frugivore diversity at the taxonomic level. In contrast, climate was more important for patterns of functional diversity, with plant communities being mostly related to precipitation, and bird communities being most strongly related to temperature.Main conclusionsOur results illustrate the general importance of bottom‐up mechanisms for the taxonomic diversity of consumers, suggesting the importance of active resource tracking. Our results also suggest that it might be difficult to identify signals of ecological fitting between functional plant and animal traits across biogeographic regions, since different species groups may respond to different climatic drivers. This decoupling between resource and consumer communities could increase under future climate change if plant and animal communities are consistently related to distinct climatic drivers.
Highlights
The distribution of biodiversity on earth has fascinated scientists for over two centuries (e.g., Darwin, 1859; von Humboldt, 1808)
We evaluated the resulting SEM through a χ2 test, the root mean square error of approximation (RMSEA) and the comparative fit index (CFI) (Grace et al, 2012; Hooper et al, 2008)
We did not find a link between the functional diversity of birds and plants, even though functional diversity was quantified based on matching traits that mediate bird-plant interactions. These findings suggest that bottom-up mechanisms can drive taxonomic diversity of avian frugivores across large environmental gradients, underlining the importance of resource tracking by frugivores
Summary
The distribution of biodiversity on earth has fascinated scientists for over two centuries (e.g., Darwin, 1859; von Humboldt, 1808). The patterns of biodiversity along spatial and environmental gradients are very well documented (Gaston, 2000). Biodiversity generally peaks in the tropics and decreases toward higher latitudes (Allen et al, 2002; Hillebrand, 2004) and decreases along elevational gradients in mountain systems (e.g., Rahbek, 1995). Despite the extensive knowledge on biodiversity and its distribution, understanding the mechanisms behind these patterns still constitutes a major challenge (Lewinsohn & Roslin, 2008; Mittelbach et al, 2007). A large body of evidence links species coexistence and the resulting patterns of species diversity to present-day abiotic and biotic drivers, such as energy availability (Guégan et al, 1998; Rosenzweig, 1992; Roy et al, 1998) or biotic interactions (Bascompte, 2009)
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