Effects of Canopy Connectivity on Ant Community Assembly on a Shaded, Organic Coffee Farm

Abstract

Canopy connectivity influences foraging, dispersal, and competition in arboreal ant species, with implications for ant community assembly. Connectivity among the crowns of shade trees varies greatly with agricultural intensification in agroforestry systems, where some ant species have been shown to act as biological control agents against agricultural pests. Understanding how canopy connectivity affects arboreal ant communities could aid in the development of management practices that maximize biological control services from arboreal ant species. I used a manipulation of connectivity between the crowns of large shade trees to investigate the effects of canopy connectivity on arboreal ant species richness, composition, and co-occurrence rates in a coffee agroecosystem. Further, I examined the effects of the major dominant arboreal ant species, Azteca sericeasur, on ant species density and composition on trunks and crowns of upper shade trees. A linear mixed-effects analysis showed that the number of species observed at baits set in tree crowns increased significantly after the crowns had been connected with nylon ropes (p = 0.028). In trees occupied by A. sericeasur , lower numbers of species were observed at baits even in the crown (p = 0.067). Crowns that were connected increased in similarity of ant species composition, particularly between adjacent connected crowns. Composition also significantly differed between both trunks (P = 0.003) and crowns (P = 0.014) that contained A. sericeasur nests and those that did not. Overall C-scores combined with an analysis of co-occurrence rates of individual pairs of species indicate that this arboreal ant community is not characterized by high rates of segregation, and pairwise competitive interactions are not among the most important forces structuring community assembly here. In timed observations of connecting lines between tree crowns, only arboreal-nesting ant species were recorded, reinforcing the idea that canopy connections are most significant to strictly arboreal species. Connectivity may increase the number of species present in tree crowns by allowing ants to disperse and forage in the canopy while bypassing trunks with more aggressive, territorial species. While the keystone ant A. sericeasur makes heavy use of connections within lower vegetative strata, I found that other species, such as twig-nesting species, are more likely to make use of connections in the canopy above 11 m. Because some twig-nesting species in the upper crown have been shown to act as biological control agents, an increase in species density in tree crowns could have positive implications for agricultural pest control services.