The ant fauna of the remote Mitchell Falls area of tropical north-western Australia: biogeography, environmental relationships and conservation significance

This paper presents the first systematic description of the ant fauna of the dominant savanna landscapes of the remote northern Kimberley region of far northwestern Australia, an area of extremely high conservation value for other taxa. These conservation values are threatened by inappropriate fire regimes and invasion by the introduced cane toad Bufo marinus. Ants were sampled at 16 sites in the Mitchell Falls area, eight each on laterite and sandstone substrates, representing the two regionally dominant landforms. At each site, ants were sampled using 40 pitfall traps during the middle of the dry season (June/July) 2007, and a wide range of environmental variables was measured, covering vegetation and ground-layer structure, soil texture and chemistry, and time since fire. A total of 166 species from 33 genera were collected, with sample species richness ranging from 35 to 59 (mean 45.5) across the sites. The richest genera were Camponotus (21 species), Monomorium (20), Pheidole (14), Melophorus (14), Rhytidoponera (13), Meranoplus (12), Iridomyrmex (12), Polyrhachis (10) and Tetramorium (9). The dominant functional groups were Dominant Dolichoderinae (mostly species of Iridomyrmex) and Generalised Myrmicinae (mostly species of Monomorium and Pheidole), which together represented a relatively constant 80% of all ants at each site. The regional fauna is composed primarily of Torresian (tropical) and Eyrean (arid) taxa, which each contributed 42% of total species. The distributional range of individual species showed a strongly bimodal pattern, with about a third of total species known only from the Mitchell Falls area, and another third occurring throughout monsoonal northern Australia. Laterite and sandstone sites supported markedly different ant faunas. Ant distributions were only weakly related to time since fire. The overall functional group and biogeographical structure of the northern Kimberley ant fauna is similar to that of climatically comparable regions elsewhere in monsoonal northern Australia. However, the fauna is an especially significant one due to a high degree of regional endemism, which is attributed to the region's unusually high rainfall. Our findings indicate that the high conservation values attributed to the flora and vertebrate fauna of the northern Kimberley also apply to the region's insect fauna.