Adjusting Foraging Effort to Resources in Adjacent Colonies of the Leaf- Cutter Ant, Atta colombica

Abstract

Two adjacent colonies of leaf-cutter ants Atta colombica were studied to examine the influence of resource availability on foraging. A colony located in older, secondary forest used 130 different resource species and had a dietary diversity equal to 30.7 equallyused species. This colony showed a cyclical annual pattern of gradually changing diet on a Bray-Curtis ordination. An adjacent colony in younger, secondary forest used 103 plant species and had a dietary diversity of only 11.8 equally-utilized species. Foraging by this colony was governed by the availability of a small number of high quality resource species. Tree species eliciting a strong recruitment of foragers from the colony were denser in the younger forest suggesting that the colony in the younger forest was specializing in a richer resource environment. Regression analyses indicate that the location of small patches of high quality resources determines the spatial distribution of foraging in the younger forest but much less so in the older forest. A weak correlation between maximum harvest rates and the synchrony of leaf production may indicate that the phenology of leaf production serves as a predator-avoidance mechanism. ONE OF THE CORNERSTONES OF FORAGING THEORY has been the premise that species adjust their foraging behavior to local conditions under the pressure of natural selection. Animals that identify productive patches of their ranges and concentrate their foraging effort on these small areas will forage most efficiently and be favored on an evolutionary time scale. For individual foragers, this is done with such behavioral mechanisms as changing turning rates and move distances (Pyke et ail. 1977, Krebs and Davies 1978); for social insects, recruitment behavior and trail systems play an important role (H6lldobler 1976, Holldobler and Lumsden 1980, Shepherd 1982). Animals appear to rank resources and concentrate their foraging effort on the best items, generally supporting the prediction (MacArthur and Pianka 1966; Schoener 1969, 1971) that foragers will specialize in times or areas of plenty and generalize in times or areas of scarcity (see Krebs 1979 for a recent review). A changing resource environment may also modify forager behavior. For example, Bernstein (1975) found that Veromessor pergandei switches from an individual to a group searching method during times of resource scarcity. Acromyrmex versicolor uses individual foraging on small dispersed items, but group foraging on large clumped items (Gamboa 1975). The interaction between a forager and the resource pool is thus very dynamic and most species will probably defy any simple categorization of their foraging behavior. The complexity of resource environments, as well as formidable practical difficulties, often prevents the kind of close examination of resources required to demonstrate fine-scale adjustment of a forager's behavior to changes in its resources. Leaf-cutter ants, however, seem ideal subjects for studying the interaction of foragers and resources. A colony forages as a single unit whose foraging effort is easily quantified by the numbers and distribution of workers out of the nest. Workers gather forage from large, stationary, identifiable resources (trees and lianas) and are easy to observe while they forage. For these reasons, leaf-cutter ants were used to answer the following questions: 1) How do differences in available resources affect long-term foraging behavior? 2) How does plant phenology affect exploitation by these generalist herbi-