The geometry of coexistence: Resource configuration alters richness and limiting similarity in an ant assemblage

Background/Question/Methods

An important way in which community ecologists can assist conservation biologists is by elucidating the habitat parameters that are most conducive to the maintenance of species richness. Traditional theories of competition and coexistence predict that organisms using the same resource will be behaviorally and/or morphologically differentiated to mitigate competition. However, there are many examples of seemingly undifferentiated organisms that appear to use the same resources, especially when the resources are patchily distributed in space and time. The importance of resource patchiness for the co-occurrence of morphologically similar ants was tested in a set of field experiments in a tropical forest in Barro Colorado Island, Panama, and in various habitats across Oklahoma, USA. Six grams of food were placed upon wax paper in four different dispersion levels: a single piece, or four, sixteen, or sixty-four pieces. Empty pieces of wax paper were used as controls. Ants on the wax paper were collected after approximately 90 minutes. In Panama, eleven replicates of each treatment level were placed in the same locations every other day for a month, and the leaf litter communities under the bait stations were destructively sampled at the end of the experiment. Species richness and the proportion of morphologically similar species at each bait station were expected to increase as resources became more finely dispersed. 

Results/Conclusions

The short-term collections in Panama were consistent with both predictions: species richness and the proportion of morphologically similar species increased as the resources became more finely divided, although mean richness peaked at the sixteen-piece treatment instead of at sixty-four. However, the data from Oklahoma did not show any significant trends, which is consistent with observations that species packing is "tighter" in the tropics. The data from the destructive samples at the end of the Panama experiment showed a monotonically positive relationship between the proportion of morphologically similar species and resource dispersion, but a nearly opposite species richness-dispersion relationship: the sixteen-piece dispersion treatments had the fewest species, and the lowest overall levels of ant abundance. Taken together, these data suggest that more finely dispersed resources might be more conducive to niche-overlap at broader spatial scales, but at the scale of 0.25 m², the most friendly places for an ant to eat might not be the most friendly places for an ant to live.