Seeds of War: Ants and Rodents

The great Swiss myrmecologist Auguste Forel (1848 – 1931) once observed that “the greatest enemies of ants are other ants, just as the greatest enemies of men are other men.” In general, this maxim appears true – with exceptions. Once such exception was reported in 1977 in the journal Nature, by myrmecologists James H. Brown and Diane W. Davidson. These two researchers found that seed-harvesting ants compete with seed-eating rodents (!) in the Silverbell Bajada near Tucson, Arizona.

Unlikely foes, perhaps. Images: Wikimedia Commons (left) and Alex Wild (right)

Through a series of seed baits and exclusion procedures (to exclude either ants using insecticides, or rodents using barriers), Brown and Davidson found a strong inverse correlation between seed-eating rodent abundances and seed-harvesting ant abundances. In other words, the more rodents, the fewer ants, and the more ants, the fewer rodents. They also note a broad overlap in the sizes of seeds that each community of seed-eaters gather (although the ants, naturally, more frequently gather some smaller seeds compared to the rodents).

Perhaps the most interesting finding is that the species of rodents and ants that were most affected by the exclusion of the other group were those species that “specialize in harvesting dense clumps of seeds”. On the rat side of the equation, this primarily meant Dipodomys merriami, and on the ant side this meant Pheidole species that utilize coordinated foraging trails to locate and harvest clumped aggregates of seeds (both species groups pictured above). So, not only do two completely unrelated groups of animals directly compete for food in ecologically meaningful ways, but the specific dynamics of that interaction appear to also have similarities across both groups.

Brown and Davidson note that their findings show how important it is for researchers to do what they can to expand their view beyond their own taxon of interest, such as ants, to more properly understand the broad range of interactions that might be significantly affecting species’ ecology and evolution. Yet, at least in myrmecology, the paradigm remains rather “Forelian”, focusing nearly exclusively on the competitive interactions between ant species with little attention to similar and important interactions that might be occurring with distantly related animals. There are some signs of change, however: For example, the work of K. Supriya, at the University of Chicago and the Field Museum of Natural History, who is investigating competitive interactions between ants and birds in the Eastern Himalayas.