Coffee agroecosystems are constrained by a number of ecological factors, including pest damage. More than 200 different herbivores are cited as attacking coffee, several of which are considered economically important. Ants are ubiquitous in coffee agroecosystems and, while omnivorous, are also documented as important predators of multiple coffee pests. But species often shift foraging patterns depending on resource availability, and those resources will vary temporally based on precipitation patterns. Previous studies have demonstrated that insect predation by arthropods is higher during droughts, suggesting that water limitation results in dietary shifts towards greater prey consumption.
We performed a study of dietary niche breadth of ants in a coffee agroecosystem with various foraging strategies during the dry and rainy season. Using stable isotope analyses we compared seasonal and foraging differences in trophic signatures of ants. We hypothesized that ants that primarily forage in one vegetation stratum (i.e. trees or coffee bushes) should have a more narrow diet breath as compared with ants that forage across multiple strata. Similarly, ants should shift their strategy based on seasonal availability and should be more predaceous in the dry season as plant and nectar resources are not as widely available during the dry season.
Results/Conclusions
We collected 145 ant colonies representing eight different ant species across both the rainy and dry seasons. Four ant species were found nesting in both trees and in coffee bushes, 2 species were found only in trees and the remaining 2 were found only in coffee. For all colonies we collected leaf tissue from either the coffee or tree. The ants were collected from 20 different sites within a single 280-ha coffee farm. From each location we collected strict herbivore species and predator to provide relative comparisons of diet. Stable isotope analysis of d15N and d13C revealed overall, significantly higher d15N values in the dry relative to the rainy season. This pattern held in 6 of the 8 species when we examined the diet differences by species. Species that foraged both on trees and coffee had a significantly different diet composition than species that found exclusively on one stratum. Ant species found only in coffee had the highest overall d15N and ants found only in the trees had the lowest d15N. As future climatic shifts are expected to impact seasonality in these systems, this study contributes to understanding how such changes may alter ant diet preferences and predatory ant functions.