Shade tree isolation in pastures modulates diversity of epiphyte-dwelling spiders: The role of epiphyte biomass and species dispersal capacity

In Mexico, most forest habitats have been lost, leaving mostly agroecosystems such as coffee plantations. In some areas, coffee plants are commonly planted under a closed canopy of shade trees and sometimes these coffee plantations are transformed into cattle pastures, in which scattered and isolated trees remain. Isolated trees (i.e. trees without contact between their crowns) left in disturbed environments such as pastures represent an extreme of habitat reduction for tree-dwelling organisms. However, these isolated trees can also be biodiversity reservoirs, serving as regeneration nuclei and connection points between terrestrial habitats. In this study, we evaluated whether the alpha and beta diversities of epiphyte-dwelling spiders differ between isolated trees in a pasture and shade trees (i.e. which sustain a continuous canopy above the crop) in a coffee plantation, and whether epiphyte biomass and differences in the dispersal capacity of the spider species could explain these changes. We found that spider alpha diversity was almost 50% higher in isolated trees than in shade trees. Isolated trees had the highest epiphyte biomass and there was a positive relationship between spider diversity and epiphyte biomass in these trees but not in shade trees. Spider community composition differed between tree conditions. We found that compositional similarity decreased and turnover increased with distance between trees only for spiders with low dispersal capacity in isolated trees. The high subset of unique species and high species richness in isolated trees highlight their importance in preserving spider communities in agrolandscapes.