Regulation of "Momentary" Diversity by Dominant Species in Exceptionally Rich Ant Communities of the Australian Seasonal Tropics

Abstract

I report observations on exceptionally diverse ant communities in the seasonally arid tropics of northern Australia that indicate that interference competition from dominant species regulates the number of ant species foraging at rich food sources in a manner analogous to humped diversity models applicable to communities of plants and sessile marine invertebrates. I adopt two novel approaches to the study of humped diversity patterns. First, I look at the effect of temporal changes in the foraging abundance of dominant species, presumably linked to temporal changes in climatic favorability, on "momentary" diversity within a community. Second, I relate diversity directly to the abundance of dominant species rather than to some environmental variable that is assumed to control their abundance. Iridomyrmex spp. and Oecophylla smaragdina were dominant ants at the forest and savanna sites studied: they occurred at a large proportion (35%-85%) of tuna baits, they dominated and monopolized most (>80%) of the baits at which they occurred, they had the highest mean abundance scores at baits, and they were far more abundant at baits than expected from their representation in pitfall catches. The total abundance of nondominant ants, species richness, and species diversity at baits all exhibited humped relationships with the combined abundance of Iridomyrmex and Oecophylla, which indicates that dominant species suppressed the foraging abundances and diversity of other species under climatically favorable conditions. There were important site differences in the expression of this humped pattern: the abundance of dominant species was never low in the savanna, and here only the descending part of the hump was expressed. This study appears to be the first time that any humped diversity pattern caused by competitive exclusion has been documented in terrestrial animal communities.