The vertical stratification patterns of tropical forest vertebrates

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Edmund Basham, J. Alex Baecher, David H. Klinges and Brett R. Scheffers, Wildlife Ecology and Conservation, University of Florida, Gainesville, FL

Background/Question/Methods
Tropical forests harbor the highest levels of terrestrial biodiversity and represent some of the most complex ecosystems on the Earth, with a significant portion of this diversity above ground. Although the vertical dimension of forests is a central aspect of the ecology of forest communities there is little consensus as to prominence, evenness, and consistency of community-level stratification from ground to canopy. We ask, where are tropical forest vertebrates found between the ground and canopy? Here, we collate the results of 62 studies across the tropics to synthesize and assess broad patterns of vertical stratification in vertebrate abundance and richness. Our review of the literature yielded sufficient data for bats, small non-volant mammals, birds and amphibians. We use generalized linear mixed models to show that variation in the stratification of abundance and richness exists within and among all taxa considered, and find that some macro-ecological variables can partially explain this variation.
Results/Conclusions
Bat richness stratification was variable among studies but trended towards greater richness in the canopy, and bat abundance was strongly weighted towards the canopy. Bird richness and abundance stratification was variable, though slightly weighted towards the canopy. On the contrary, both amphibians and small mammals showed consistent patterns of decline in abundance and richness towards the canopy. We distil and discuss research trends in localised stratification drivers, but analyse the influence of macroecological variables on stratification patterns, finding elevation to be a key predictor of bird stratification in particular. Prominent differences among taxa are likely due to taxa-specific interactions with local drivers such as vertical habitat structure, food distribution, and vertical climate gradients, which may vary considerably across macroecological gradients such as elevation and biogeographic realm. Our study showcases the complexity of tropical forest ecosystems in the way communities organize in space, but we also see that significant effort is required to fill research gaps in terms of the adequate sampling across various under-sampled regions, taxa, and environments.