Tropical mountains are the most species rich regions on Earth, but are especially vulnerable to biodiversity loss because of the high proportion of range-restricted species with narrow elevational distributions. Shade coffee plantations, in contrast to open monocultures, may mitigate biodiversity loss on tropical mountainsides by providing structure and food to native bird species. The following chapters explore the effects of shade coffee cultivation on bird diversity along the northwest slope of the Sierra Nevada de Santa Marta of northern Colombia.;The first chapter examines how coffee plantation and natural forest bird communities differ in their compositional turnover along elevational and horizontal spatial gradients. I surveyed bird communities at 23 shade coffee plantation sites and 20 natural forest sites along a 1000-meter elevational gradient. Compared to natural forest, I found shade coffee plantations to be diminished in beta diversity despite a greater average number of bird species per site. Coffee plantations also exhibited less variance in all vegetation measures reflecting their homogenized structure. This homogeneous feature of shade coffee plantations is a potential cause of their reduced avian beta diversity across elevational and horizontal distances.;The second chapter assesses the role of environmental filtering in reshaping bird communities of shade coffee plantations relative to nearby natural forest. I analyzed the phylogenies and traits of bird communities in these two habitats of the Sierra Nevada de Santa Marta in order to address the hypothesis that shade coffee plantations filter avian communities by limiting their phylogenetic and functional composition. I found that coffee plantation bird communities demonstrated greater phylogenetic clustering and lower expected phylogenetic diversity than natural forest communities. Coffee plantation bird communities also displayed less functional diversity than natural forest communities in terms of the birds' foraging strata, dietary guild, and body mass. Reductions in understory foragers, frugivores, and body mass variance coupled with increases in canopy foragers and omnivores contributed to the functional and corresponding phylogenetic filtering of coffee plantations.;The third chapter explores the role of land use in determining how climate change will impact bird communities in the Sierra Nevada de Santa Marta. I used species distribution models to predict changes in occurrence rates by 2050 for 160 bird species I had detected in shade coffee plantations and natural forest sites. Most natural forest bird communities showed greater average projected declines than adjacent shade coffee bird communities under future warming and drying. The interaction between land use and elevation was a significant predictor of changes in occurrence rates. Consequently, the projected decline of shade coffee bird communities was relatively muted with increasing elevation. A likely contributing cause of this projected pattern is the more homogeneous community composition (diminished beta diversity) of shade coffee plantations across the elevational gradient.
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