Seed selection by an exceptionally rich community of harvester ants in the Australian seasonal tropics

1. We used the composition of nest middens to describe the extent of dietary partitioning in an exceptionally diverse assemblage of harvester ants in the Kakadu region of northern Australia. Eight of the >15 harvester species known from adjacent 30 x 30-m plots, comprising four species of Meranoplus and two species each of Monomorium and Pheidole, maintained conspicuous nest middens. 2. All these eight harvester species are monomorphic (or functionally so in the context of seed collection), with total body length ranging from 2.0 to 5.0 mm. The two species of Monomorium are column foragers, whereas the others apparently forage solitarily. Nests averaged 0.1 m(2), but were highly patchy both within and between plots. 3. Seed fragments from a total of 33 plant species were identified from 50 middens sampled during the end of the wet season, with up to 19 seed species recorded from a single midden. Seed species diversity was greatest in middens of Meranoplus minimus, Monomorium sp. 1 (rothsteini group) and Pheidole sp. 1 (mjobergi group), and least in middens of the diversus group of Meranoplus. 4. Despite a high overall diversity of seed species, each harvester species collected primarily two or three seed species (with one exception, all grasses), and there was virtually no overlap in these species between harvesters. The pronounced dietary separation of harvester species was demonstrated by multivariate analysis of midden composition. 5. The extent to which differential seed selection is determined by intrinsic preferences, vs. interactions between, harvester species is unknown, nor is it clear why the diversity of harvester ants is so high. However, it is likely that differential seed selection plays a significant role in species coexistence.