Variation in biodiversity and abundance of functional groups of arthropods along a tropical elevational gradient in Puerto Rico

Elevational gradients represent platforms for exploring the effects of environmental variation on biodiversity. The environmental correlates of these spatial gradients are likely to be modified during the Anthropocene, as species respond to global change drivers including warming and increased frequency of extreme events. We quantified variation in the abundance of four functional groups of canopy arthropods (i.e., folivores, sap-suckers, detritivores, and predators), as well as in aspects of biodiversity on each of six host-plant species along two elevational transects in the Luquillo Mountains of Puerto Rico: a mixed forest transect, traversing tabonuco, palo colorado, and elfin forests, and a palm forest transect, comprising only patches dominated by sierra palm (Prestoea acuminata). We expected gradients in arthropod abundance and biodiversity to be host-tree specific, and for gradients on palm to differ between transects due to a combination of mechanisms associated with host selection, rescue effects, habitat structure, and source pool dynamics. In general, abundance and biodiversity declined with elevation. The ways in which abundance declined with increasing elevation was contingent on host tree identity and on arthropod functional group, whereas all aspects of biodiversity declined with elevation in consistent manners regardless of host tree identity or transect. Similarly, turnover (beta components of biodiversity between sequential elevational strata) did not differ between transects. Decreases in productivity with increasing elevation may be responsible for gradients in abundance or biodiversity. However, host-specific and functional group-specific gradients suggest that elevational effects manifest differently depending on tree species identity and resource bases that are consumer specific.