Assembly of a twig-nesting ant community: Disturbance as a window to non-neutral processes

Background/Question/Methods The enigmas associated with structure and diversity of ecological communities have long fascinated ecologists. One factor receiving much attention as a potential driving force is disturbance. In shaded coffee agroecosystems, several disturbances may influence assembly of ants living in hollow coffee twigs, and allow for examining multiple hypotheses of community assembly. Twig-nesting ants are nest-site limited and do not sort along a nest-size niche axis, thus competition for and colonization of nest sites are likely important. I examined the impact of large-scale disturbance (shade tree pruning) conducted over two years with the objective of examining for non-random changes in the ant community with time since disturbance. During June 2007 and June 2008, farmers pruned trees in two distinct areas within a permanent 45-ha plot, and left another area uncut. I sampled ants during July 2008, forming a treatment gradient of time since disturbance (uncut = control; cut one year ago = c1; cut one month ago = c2). In each treatment area, I randomly chose 2 or 3 hectares, and sampled ants on the coffee bush nearest to each tagged tree. Then, I compared the occupation rate, species richness, and relative abundance of ants within the three areas. Results/Conclusions I encountered 19 species of twig-nesting ants from 534 nests. Availability of nest sites did not differ with treatment, but a significantly higher proportion of available nest sites were occupied in c2 (82.9%) than in c1 (70.7%) and even fewer were occupied in uncut (57.7%). Ant richness was highest in c2 (16 species), lower in c1 (13) and lowest in uncut (11). Species relative abundance drastically differed in the three areas. Two species (Pseudomyrmex simplex and Procryptocerus hyleaus) accounted for nearly half of nests encountered, and the number of nests occupied by each did not greatly differ with treatment. Four species only occurred in the most recently cut site, and an additional 4 species occurred only in either or both of the cut sites. Relative abundance of two species (Myrmelachista mexicana and Nesomyrmex echinatinodis) drastically declined with time since disturbance, while abundance of two other species (Pseudomyrmex ejectus and Pseudomyrmex PSW-53) greatly increased with time since disturbance. Differences in occupation, richness, and relative abundance indicate that disturbance may have a large impact on community assembly of ants in human-managed habitats. Further, the directional changes of species composition signal that that non-random processes may be driving assembly in undisturbed sites.