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In a world where even documenting species declines in tropical systems is challenging enough, Klimes et al. raise the bar by addressing the deceptively simple, yet inherently complex, question of why species richness is lower in secondary forests. Using the first plot-scale inventory of arboreal ant nests, combined with an innovative rarefaction technique, they quantify the relative importance of a range of successional factors and highlight the contribution of beta diversity to the higher richness in primary forest., (© 2012 The Author. Journal of Animal Ecology © 2012 British Ecological Society.)

1. Competition is considered a key factor structuring many communities, and has been described as the 'hallmark' of ant ecology. Dominant species are thought to play a key role structuring local ant assemblages through competitive exclusion. 2. However, while there have been many studies demonstrating competitive exclusion and consequently reduced richness at baits, it is not clear whether such regulation of 'momentary' diversity at clumped food resources can scale up to the regulation of richness at the site or assemblage level. 3. In this study, ant assemblages were sampled in three different savanna habitats in South Africa using both baiting and pitfall trapping. 4. As has been found in previous studies, there was a unimodal relationship between dominant ants and species richness at baits, with high abundances of dominant ants regulating species richness through competitive exclusion. Analysis of pitfall samples revealed strong convergence in pattern, and results from null model co-occurrence analyses supported the findings. 5. The importance of competition in structuring local ant assemblages was, however, only apparent at one of the three savanna habitats suggesting that a full range of extreme environments is needed to produce the full unimodal relationship at the assemblage level. 6. Although the relative importance of competition varied with habitat type, the study demonstrated that in some habitats, dominant ants can control species richness at the assemblage level.