Ant Fauna of a Mangrove Community in the Australian Seasonal Tropics, With Particular Reference to Zonation

Abstract

The ant fauna of Australia's tropical rainforests is composed primarily of Indo-Malayan taxa, and is remarkably disjunct from the surrounding savanna ant fauna, which is dominated by autochthonous, arid-adapted species. Mangrove communities, which like the rainforests have closed canopies, are major components of the Australian tropics; however, their ant faunas remain poorly known, despite ants being the dominant insect group in them. This paper describes the ant fauna of a mangrove community in Darwin Harbour, and compares it with the regional savanna and rainforest fauna. Ants were sampled at baits located on the ground, foliage, and tree trunks, along three transects following the elevation/tidal gradient. Sixteen ant species were recorded in mangroves, including three savanna species, six rainforest species (one, Monomorium ?turneri, not previously recorded in Northern Territory), five habitat generalists, and two species restricted to mangroves, Polyrhachis constricta and P. sokolova, which remarkably nest in intertidal sediments and are subject to inundation. Species of Crematogaster and Polyrhachis were most abundant, and Polyrhachis was the richest genus with six species. The biogeographical affinities of the ant fauna were very similar to those of local rainforest faunas, but with a higher incidence (50%) of obligately arboreal species. Five of the Polyrhachis species occurred in distinct zones along the elevational/tidal gradient, thus exhibiting the zonation so characteristic of mangrove plant species, although the ant zonation did not appear to be mediated by floristic changes. Potential causes of this zonation are both abiotic and biotic, but their relative importances remain a matter of conjecture.