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Psyche 
[Vol. 88 
numbers of sympatric rainforest ant species are probably about one- 
quarter to one-half that experienced by purpurea in north Queens- 
land rainforest. 
It is unclear why there is a disproportionate decline in the 
utilization of stones as nest sites in the more tropical members of the 
impressa group (Table 3) and perhaps for tropical rainforest ants in 
general (cf. Wilson, 1959, p. 440). One possibility is that in 
subtropical and tropical rainforests on well-drained soils, stones 
frequently lie on subsoil below the thin organic horizon and offer an 
environment poorer in immediate food resources and more de- 
manding for nest excavation than rotting logs. In temperate and 
some subtropical rainforests of New South Wales, soil horizons 
tend to be less sharply stratified and/or litter decomposition is 
slower, so that humic material extends below the level of loose 
stones. 
Effects of Sympatry 
Nest site densities for sympatric and allopatric populations of 
confusa and chalybaea are given in Table 4. Both species occupy a 
significantly greater proportion of log nest sites in allopatric 
populations (contingency x 2 , p < .01 and p < .001, for confusa and 
chalybaea respectively) and confusa inhabits a greater proportion of 
stone nests sites allopatrically (x 2 = 5.4, p < .05). The lower 
sympatric densities of confusa and chalybaea could be a result of 
sympatric associations occurring in more marginal environments. 
However, the combined sympatric nest densities are very similar to 
the allopatric densities of both species. There are no significant 
differences between the total proportion of rotting logs occupied 
sympatrically and the proportion utilized allopatrically by either 
confusa (x 2 = 0.7) or chalybaea (x 2 = 1.8). The combined sympatric 
nest density under stones is the same as that for allopatric confusa 
populations. While these results could be coincindental, it seems 
more reasonable to conclude that sympatry has a depressant effect 
on relative abundance, and that competition for nest sites, food, or 
foraging space is important. 
Other Sympatric Congeners 
Other, more distantly related Rhytidoponera species also co- 
occur with members of the impressa group. R. victoriae (s.l.) is a 
common species (or complex of species) present in rainforest and 
other mesic habitats along the entire east coast of Australia. R. 
