The diversity and phylogenetic community structure of many organisms is negatively affected by factors that covary with elevation. On the Pacific slope of the Cordillera Guanacaste within Area de Conservación Guanacaste (ACG) in northwestern Costa Rica we found a negative relationship between elevation and ant diversity on each of three volcanos. This pattern was evident when diversity was measured through molecular operational taxonomic units (MOTU) or by phylogenetic diversity (PD) based on DNA barcodes or a multi-gene phylogeny. We observed an asymmetrical mid-elevation peak at approximately 600–800 m and we found high species turnover between sites on the same mountain and among the three mountains. At the highest elevation cloud forest sites we found evidence of significant phylogenetic clustering, the expected result of environmental filtering. The narrow elevational range of each species, coupled with the high diversity at each sampling point, emphasizes that climate change will bring strong changes in the location and composition of biodiversity on these mountains. The structure and composition of the hyperdiverse communities present at any one elevation is extremely vulnerable to a changing climate.