NEST‐SITE LIMITATION IN COFFEE AGROECOSYSTEMS: ARTIFICIAL NESTS MAINTAIN DIVERSITY OF ARBOREAL ANTS

Abstract

Nest sites are a limiting resource for arboreal twig‐nesting ants, and nest sites may be increasingly limited with habitat modification. One such habitat modification is the conversion of traditional coffee farms, where coffee is cultivated under a dense, diverse shade canopy, to more intensive production systems with reduced canopy cover and lower diversity, height, and density of shade trees. As a result of such management intensification, ant diversity declines. We ask here if: (1) nest sites are a limiting resource for arboreal twig‐nesting ants in coffee farms, especially in intensively managed systems and (2) nest‐site limitation is a mechanism causing loss of ant diversity with coffee management intensification. During 2000–2003, we investigated occupancy, species richness, and species composition of arboreal twig‐nesting ants using natural (hollow coffee twig) and artificial (bamboo stem) nests in farms either with high or low diversity and density of shade trees. In both high‐ and low‐shade sites ants occupied a majority (>55%) of natural nests and occupied some (>15%) artificial nests, and significantly more artificial nests were occupied in low‐shade sites. In both high‐ and low‐shade sites, ant richness was higher in artificial than in natural nests. More species occupied natural nests in low‐shade sites, and more species occupied artificial nests in high‐shade sites. Furthermore, species composition differed between nest types, with more ant species found more often or only in artificial nests. These results indicate that, although ants are not strongly nest‐site limited in coffee agroecosystems, nest limitation increases somewhat with increasing management intensification. Reductions in numbers of nest sites may be a mechanism causing ant diversity loss with coffee management intensification. Interestingly, because relatively fewer species colonized artificial nests in the low‐shade site, ants may be recruitment limited in the low‐shade sites, possibly maintaining low ant richness in these sites.