Three colonies each of two species of leaf-cutting ants (A tta colombica Guer. and A Ita cephafotes L.) were studied for 1 yr in the Guanacaste Province of Costa Rica to determine the following: (I) Do colonies of Atta cut leaves from a limited number of species (i.e., are they selective)? (2) What determines which plant species are attacked? (3) Do leaf-cutters optimize foraging in terms of energy expended by attacking palatable plants closest to the nest or do they distribute their efforts more or less evenly throughout their foraging territory? Several lines of evidence support the hypothesis that A. ccphalotes and A. colombica are selective in terms of plant material attacked. The ants sampled a large majority of the plant species present, but concentrated their foraging on a restricted subset of species. The mature leaves of only 31.4% and 22.0% of the species present were readily acceptable to A. colombica and A. ci'phalotes, respectively. The new leaves of another 12-16 plant species were acceptable. Different colonies of the same A tta species consistently attacked the same plant species if they were available, cut leaves from the same species at similar rates, and even attacked the same species of plants at the same time of year. The amount of material cut from host plant species was not correlated with host plant abundance in four of the six colonies studied, and was even slightly negatively correlated in two of these four. An analysis of plant distance from the nest versus amount harvested shows that palatable plant species close to the nest have a greater probability of being visited by the ants, but do not necessarily suffer more defoliation than other such plants within 50-60 m of the nest. Amount harvested decreases greatly for plants farther than 60-80 m from the nest. Thus colonies of A tta do not normally concentrate their efforts on plants closest to the nest, but foraging cannot be described as evenly distributed either.The factor responsible for selectivity in colonies of A tta is probably the internal chemistry of the material selected. An A tta colony must provide the fungus garden with a proper balance of nutrients and moisture without overloading it with secondary compounds from the plants selected. The ants' attempt to solve this problem may explain some of the complexities of leaf-cutter foraging behavior.

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A month-long study was conducted on the comparative foraging behavior of 20 colonies of the leafcutting ant, Atta cephalotes L. in Santa Rosa National Park, Guanacaste Province, Costa Rica. The study was conducted during the middle of the wet season, when trees had mature foliage and the ants were maximally selective among species of potential host plants. The colonies always gathered leaves from more than a single tree species but on average one species constituted almost half the diet with the remaining species being of geometrically decreasing importance. Colonies exhibited greater diversity in their choice of leaves and lower constancy of foraging when the average quality of resource trees was lower, as predicted by elementary optimal foraging theory. Furthermore, the ants were more selective of the species they attacked at greater distances from the nest. However, the ants sometimes did not attack apparently palatable species, and often did not attack nearby individuals of species they were exploiting at greater distances.A classical explanation for why leafcutting ants exploit distant host trees when apparently equally good trees are nearer, is that the ants are pursuing a strategy of conserving resources to avoid long-term overgrazing pressure on nearby trees. We prefer a simpler hypothesis: (1) Trees of exploited species exhibit individual variation in the acceptability of their leaves to the ants. (2) The abundance of a species will generally increase with area and radial distance from the nest, so the probability that at least one tree of the species will be acceptable to the ants also increases with distance. (3) The ants forage using a system of trunk-trails cleared of leaf litter, which significantly reduces their travel time to previously discovered, high-quality resource trees (by a factor of 4- to 10-fold). (4) Foragers are unware of the total pool of resources available to the colony. Therefore once scouts have chanced upon a tree which is acceptable, the colony will concentrate on harvesting from that tree rather than searching for additional sources of leaves distant from the established trail.

The landscape within the Chesapeake Bay watershed has been and continues to be impacted by human modifications. Understanding if such anthropogenic disturbances influence organisms that are dependent upon estuarine wetlands remains unclear. We developed an index of marsh bird community integrity (IMBCI) to evaluate marsh bird communities and wetland condition. During the 2002 and 2003 summers, we detected 30 bird species at 219 point count locations distributed among 96 wetlands. IMBCI scores for each wetland were used to determine whether wetland habitat characteristics and urban/suburban development, agriculture, and forest at three different spatial scales (watershed, 1000-m buffer, and 500-m buffer) influenced marsh bird community integrity. We found no relationship between IMBCI scores and wetland habitat characteristics, implying that marsh bird community integrity is not related to any single plant community. Nonparametric changepoint analysis indicated that marsh bird community integrity was significantly reduced when the amount of urban/suburban development within 500 m and 1000 m of the marsh exceeded 14% and 25%, respectively. There was no effect of urban/suburban development on IMBCI scores at the watershed scale. The results of our study demonstrate that marsh bird community integrity shows a threshold response to urban/suburban development at local scales. IMBCI scores, combined with the identification of a land-use threshold, can be easy to interpret and may help communicate complex ecological data to natural resource managers and conservation planners.

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