Diane W. Davidson

Competition between seed-eating rodents and ants in desert ecosystems.

Three kinds of evidence indicate that desert rodents and ants compete for seeds: (i) extensive overlaps in diet, (ii) reciprocal increases when one taxon is experimentally excluded, and (iii) complementay patterns of diversity and biomass in georadients of productivity. The effect on seed resources and annual plan geoseems to be similar whether rodents, ants, or both are foraging.

Species Diversity and Community Organization in Desert Seed-Eating Ants

Patterns of species diversity and community organization in desert seed-eating ants were studied in 10 habitats on a longitudinal gradient of increasing rainfall extending from southeastern California, through southern Arizona, and into southwestern New Mexico. Local communities of harvester ants include 2-7 common species, and at least 15 species from five genera of Myrmecines compose the total species pool in these deserts. Ant species diversity is highly correlated with mean annual precipitation, an index of productivity in arid regions. Communities are structured on the basis of competition for food, and interspecific differences in worker body sizes and colony foraging strate- gies represent important mechanisms of resource allocation. Seed size preferences, measured for native seeds and in food choice experiments with seeds of different size but uniform nutritional quality, are highly correlated with worker body sizes. Species of similar body size can coexist within local habitats if they differ in foraging strategy. Interspecific aggression and territorial defense and mi- crohabitat partitioning all appear to be relatively unimportant in these ant communities. Patterns of species diversity and community organization in harvester ants are strikingly similar to those reported for communities of seed-eating rodents that occupy many of the same desert habitats. Separate regressions of within-habitat species diversity against the precipitation index of productivity for the two groups correspond closely in slope, intercept, and proportion of explained variation. Resource allocation on the basis of seed size characterizes local communities of both ants and rodents. Parallels between these two groups suggest that limits to specialization and overlap may be specified by parameters such as resource abundance and predictability that affect unrelated taxa similarly.

FORAGING ECOLOGY AND COMMUNITY ORGANIZATION IN DESERT SEED-EATING ANTS'

Granivorous ants in the southwestern deserts of the United States are characterized by species-specific colony foraging behaviors that determine their efficiencies at utilizing seeds from different density distributions. Workers search for food either in groups or as individuals, and these feeding strategies represent specializations for high and low density resources respectively. While gathering experimental seeds, simultaneously supplied in clumped and dispersed distributions, feeding by group foragers focused on the high density resource patches, while individual foragers harvested seeds predominantly from the dispersed distribution, which required that prey be independently dis- covered. Collecting native seeds from unmanipulated environments, individual foragers tended to spend proportionately more time searching and experience lower foraging success than did group foragers in the same local habitat. During a period of high seed abundance, group foragers collected a much narrower range of prey types than did individual foragers. Colonies of Pogonomyrmex rugosus exhibited a mixed foraging strategy, with the most distinct feeding columns occurring during a period of peak seed abundance. Among group foragers, greatest activity coincided with periods of relatively high seed densities and low climatic stress, and these species utilized tactics such as seed storage, hibernation, and estivation to weather less favorable periods in a resting state. Individual foragers were active at intermediate levels during less favorable periods. Although high and low density seed resources are not renewed independently of one another, density specialization appears to promote coexistence between group and individual foragers. Their capacity to stably partition resources in this way should depend on (1) the degree of difference in their efficiencies at exploiting different density distributions of seeds and (2) the availability of seeds in the habitat as a function of seed density. In more mesic habitats, where seeds are renewed more frequently and probably also in larger pulses, group foraging species are proportionately more abundant.

GRANIVORY IN A DESERT ECOSYSTEM: EXPERIMENTAL EVIDENCE FOR INDIRECT FACILITATION OF ANTS BY RODENTS'

Two major groups of desert granivores, ants and rodents, coexist as permanent residents of local desert habitats in southwestern North America. At our Sonoran Desert study site, both of the major taxa exhibited short-term increase in density when the other taxon was experimentally removed. Over the longer term, density compensation continued at a relatively constant level for rodents in the absence of ants. In contrast, beginning : 2 yr after initiation of experiments, ant populations on rodent removal plots showed a gradual but significant decline relative to densities on control plots. Indirect interactions, mediated through ant and rodent resources, may account for these differences. Removal of harvester ants leads to higher annual plant densities only in small-seeded species. These plants are relatively poor competitors and do not displace the large-seeded annuals, on whose seeds rodents specialize. In contrast, rodent removal leads to a differential increase in large-seeded annuals, which competitively displace the small-seeded resource species of ants. The decline of ant populations on rodent removal plots preceded by several years the first detectable evidence for competitive suppression of small-seeded annuals. Because ants do not excavate buried seed, they probably experienced resource depression before buried seed reserves were exhausted through germination and subsequent competitive inhibition.

A PRELIMINARY STUDY OF SEED PREDATION IN DESERT AND MONTANE HABITATS

Multifactorial experiments in which domestic seeds in shallow glass containers were distributed in desert and montane habitats provided data on identity of seed predators (whether rodents or ants), spatial and temporal pattern of their foraging activities, and their preferences for sizes and species of seeds. The results indicate that in some desert ecosystems both rodents and ants are important and efficient collectors of seeds. These two taxa overlap greatly in several parameters of seed utilization, suggesting that they are potentially close competitors. Rodents removed much more seed than ants, perhaps because they are more efficient at locating and harvesting large clumps. This technique has considerable promise for assessing the significance of competitive interactions between distantly related taxa in natural ecosystems.

Experimental tests of the optimal diet in two social insects

Summary1.Experimental studies of dietary specialization in two species of harvester ants (genus Pogonomyrmex) reveal correspondence to several qualitative predictions of optimal diet theory.2.If the proportions of available seed types are held constant while overall abundance is varied, dietary specialization in colonies of P. barbatus is enhanced in more productive environments.3.Colonies of P. rugosus take a narrower range of prey types as distance to the foraging area increases.4.Ants appear to rank prey (barley particles) on the basis of size (caloric value), and the order of incorporation of prey types in the diet of P. rugosus corresponds to the sequence of profitabilities of these food types. Contrary to the predictions of theory, partial preferences are expressed by ant colonies of both species and may be explained in part by within-colony worker variation in the choices of individual workers, as well as by a possible requirement for workers to continuously sample the resource environment. Trends in dietary specialization at the colony level with changes in resource density and distance probably reflect the summed responses of individual workers that evaluate the resource environment and alter their diets in the predicted way.

Pruning of host plant neighbors by ants: an experimental approach.

Some ants of myrmecophytic plants either obligately or facultatively prune vegetation surrounding their host trees. Pruning behavior occurs at higher frequency in ants with sting defenses than in those with chemical defenses, which may generally convey an advantage in aggressive encounters between ants. Experiments in tropical moist forest of Peru tested the hypothesis that pruning may reduce the threat of invasions by potentially dangerous alien ants. When unseverable contacts were made between Triplaris americana 1-3 m in height and neighboring plants, Crematogaster ants invaded more frequently and in greater num- bers than on control plants and inhibited activities of behaviorally subordinate resident Pseudomyrmex dendroicus. Invaders occasionally carried away brood and usurped hollow stem nests of resident ants. Pseudomyrmex cut petioles of leaves on plants contacting their hosts as well as leaves of their own host plants, when these leaves formed bridges for major invasions. Basal clearings maintained by Pseudomyrmex around their hosts appear to reduce the likelihood that Crematogaster workers will occur in the vicinity of the tree trunk. On Cordia nodosa inhabited by Allomerus demararae, numbers of invading ants did not differ on experimental and control trees, perhaps because larger ants cannot invade through dense trichomes. Allomerus workers attacked encroaching vines only when these vines bore large numbers of Crematogaster.