On plots from which all seed-eating ants are removed, bait is both broadcast and applied to individual colonies.

Removal and continued exclusion of ants was accomplished by the use of poisoned bait (Myrex through 1980

Pogonomyrex desertorum
Desert Harvester Ant

and AMDRO thereafter), which was applied beginning in September 1977.

Four plots were assigned for removal of all granivorous ant species and four for selective removal of P. rugosus.


Our experiements at Portal are unlike the earlier study in the Sonoran Desert, these provided little evidence of strong competition between rodents and ants. There were some changes in the abundances of certain ant species associated with he removal of all of rodents or just kangaroo rats (Valone, et al.1994), but only one species, Pheidole rugulosa, showed a consistent, statistically significant increase inthe absence of rodents. Since this and is found predominantly in arid grasslands, it isuncertain whether its increase reflects release from competition with rodents or an indirect response to changes in vegetation caused by excluding kangaroo rats. There was no consistent evidence that any rodents increased significantly in response to removal of ants.

Early in the study, there was evidence of both direct and diffuse competition among the ant species (Davidson 1980, 1985). However the decline to extinction of the large Pogonomyremex species by the mid-1980s made inoperative the only manipulation that would have enabled us to address interspecific interactions among ants over the long term.

We have documented a number of indirect interactions as a consequence of our experimental manipulations at Portal. There appear to be hierarchies of competitive interactions among ant species, so that removal of the dominant species affects a second species, which in turn affects a third. The cascading effects of such hierarchical competition were seen in shifts of small-scale spatial distributions of the subordinate species.

Davidson (1980) found that the large, behaviorally dominant harvester ant (Po. rugosus) excludes its smaller congener (Po. desertorum) from the vicinity of its nest entrances, and the even smaller Pheidole ants tend to be clustered around Po. rugosus colonies, where they have a refuge from competition from Po. desertorum. When the dominant Po. rugosus was removed, this spatial arrangement broke down, with Po. desertorum moving its nest entrances toward and Pheidole moving its nests away from the abandoned Po. rugosus colonies (Davidson 1985).

Associated with significantly higher quantities of winter precipitation reported between 1978 and 1992 was a catastrophic decline in the 1980s of four species of harverster ants (Po. rugosis, P. barbatus, Po. desertorum, and A. cockerellii). In 1977 Po. rugosus was the dominant species with large, conspicuous burrow systems, but by the early 1990s was locally extinct on the study site. The number of colonies of Po. desertorum had decreased by more than 75% by 1992. A. cockerellii went extinct on our study site and also on Robert Chew's long-term site 7 km away in a somewhat different habitat (Chew 1995).

A common feature of these species is that, as granivores, they store seeds in large granaries within their burrow systems. a likely cause of their decline was wetting and spoilage of their seed stores (Chew 1995, Valone et al. 1995).

The small harverster ant, Pheidole xerophilia fluctuated but showed no clear long-term trend (Valone and Brown 1995, 1996; Brown et al. 1997).

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