In mixed tropical landscapes, savanna and rain-forest vegetation often support contrasting biotas, and this is the case for ant communities in tropical Australia. Such a contrast is especially pronounced in monsoonal north-western Australia, where boundaries between rain forest and savanna are often extremely abrupt. However, in the humid tropics of north-eastern Queensland there is often an extended gradient between rain forest and savanna through eucalypt-dominated tall open forest. It is not known if ant community structure varies continuously along this gradient, or, if there is a major disjunction, where it occurs. We address this issue by sampling ants at ten sites distributed along a 6-km environmental gradient from rain forest to savanna, encompassing the crest and slopes of Mt. Lewis in North Queensland. Sampling was conducted using ground and baited arboreal pitfall traps, and yielded a total of 95 ant species. Mean trap species richness was identical in rain forest and rain-forest regrowth, somewhat higher in tall open forest, and twice as high again in savanna woodland. The great majority (78%) of the 58 species from savanna woodland were recorded only in this habitat type. MDS ordination of sites based on ant species composition showed a continuum from rain forest through rain-forest regrowth to tall open forest, and then a discontinuity between these habitat types and savanna woodland. These findings indicate that the contrast between rain forest and savanna ant communities in tropical Australia is an extreme manifestation of a broader forest-savanna disjunction.

Rainforest in the tropical savanna landscapes of monsoonal Australia is of high biogeographic interest because it occurs as numerous small and isolated patches, and supports a fauna that contrasts markedly with that of the surrounding savanna. The overall ant fauna of monsoonal rainforest is known reasonably well, but with just one exception (Holmes Jungle in Darwin) the faunas of individual rainforest patches have been only superficially studied. This paper comprehensively documents an ant fauna of a spring-fed rainforest patch at the Territory Wildlife Park (TWP) near Darwin, contrasts it with that of surrounding savanna, and compares it with the Holmes Jungle fauna. Ants were sampled at 21 locations within the TWP rainforest, using Winkler sacs for litter-dwelling species (160 samples), standard pitfall traps for ground-active species (75 traps), and baited pitfall traps taped to trees for arboreal species (75 traps). In total, 43 species were collected, with 35 recorded in leaf litter, 24 in ground pitfall traps and 12 in arboreal pitfall traps. Species of Paratrechina and Pheidole collectively comprised 75% of all individuals recorded. Species richness was far higher in the surrounding savanna, and the rainforest and savanna faunas had markedly different species and functional group composition. The rainforest fauna at TWP was remarkably similar to that at Holmes Jungle. The rainforest/savanna contrast in ant diversity across northern Australia is the reverse of that occurring in Neotropical savanna landscapes.