10 Brown and Davidson 1977 Competition ants rodents

1 5 the overlaps provide the potential for strong

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Unformatted text preview: ). The overlaps provide the potential for strong competition between rodents and ants, but other evidence is required to demonstrate whether they actually compete and to assess the magnitude of interaction and its impact on the ecosystem. Since 1973, we have performed a set of exclusion experiments to test directly for competition between rodents and ants. Replicated, circular plots, each 36 m in diameter, were established in relatively level, homogeneous desert scrub (Larrea-Franseria) habitat on the Silverbell Bajada, approximately 60 km northwest of Tucson, Arizona. Two plots were subjected to each of the following treatments. (i) Plots were fenced with 4-inch wire mesh to exclude seed-eating rodents, and those present were removed by trapping; (ii) granivorous ants were removed by repeated application of insecticide (mirex or chlordane) to individual colonies; (iii) both rodents and ants were excluded by fencing, trapping, and applying insecticide; and (iv) two plots were reserved as unmanipulated controls. We censused rodent and ant populations periodically. Rodent numbers were assessed by simultaneously livetrapping all of the plots with equal trapping effort and standardized grids. Ant colonies were counted directly once or twice each year during periods when most species were most active. In late summer of 1976, we used a photographic technique to sample populations of annual plants that had germinated in response to rain. An independent investigator (6) analyzed seed content of soils on our plots. Relative to populations on unmanipulated control plots, the number of ant colonies increased 71 percent on plots from which rodents had been excluded, and rodents increased 18 percent in number of 20 MAY 1977 o o individuals and 24 percent in biomass on plots where ants had been removed (Wilcoxon matched-pairs signed-ranks tests, P < .01) (Table 1). These results constitute direct evidence for strong competitive interactions. Our relatively short-term experiments measured competition only in contemporary ecological time. Since seed-eating rodents and ants probably have coexisted in arid habitats for at least 2 million years, evolutionary adaptations may reduce competition between the two groups; these would limit their potential numerical responses o to the sudden artificial removal of competitors. In the absence of either rodents or ants, the species of the remaining taxon that showed the greatest increase were those that tend to specialize on densely distributed seed resources. Thus, rodent biomass was enhanced more than the number of individuals because of increases in the relative abundance of the largest species, Dipodomys merriami, which specializes in harvesting dense clumps of seeds (7). Similarly, ant species of the genus Pheidole that forage in coordinated 0.3 RODENTS Fig. 1. Sizes of nativeseeds harvested by coexisting ants and rodents near Portal, Arizona. A total of 11,518seeds werecollected from the cheek pouches of 134 rodents representing five species, and 1052seeds were collected singly from ants representing seven species. These wer...
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This document was uploaded on 02/26/2014 for the course BIOLOGY 4368 at University of Houston.

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