Unrecognized Ant Megadiversity in the Australian Monsoonal Tropics: The Monomorium nigrius Forel Group Revisited

A previous analysis of diversity within the Monomorium nigrius Forel group, formally recognized as a single species, concluded that >200 species may occur in the Australian monsoonal tropics (AMT). Here, we test this conclusion by incorporating CO1 data from an additional 223 M. nigrius group specimens collected from the monsoonal tropics of the Northern Territory (NT). By integrating morphological variation, CO1 structure and divergence, and geographic distribution, we recognized 85 species from monsoonal NT. PTP and bPTP analyses, which consider CO1 structure and divergence alone, recognized 107 and 136 species, respectively. Using a distance-based clustering approach, the number of OTUs defined by a CO1 divergence threshold of 1–6% ranged from 163 to 37. Given that sampling remains highly patchy, many more than the 85 species recognized here are likely to occur in monsoonal NT. Only four of the 85 species are known from elsewhere in the AMT. Given such high rates of species turnover, our findings support the conclusion that the total number of AMT species in the M. nigrius group likely exceeds 200. Remarkable levels of unrecognized hyperdiversity occur in many ant species groups in the AMT, challenging the traditional understanding that peak ant diversity occurs in tropical rainforest biomes.