Taxonomic level, trophic biology and the regulation of local abundance

1 Taxocenes - monophyletic ecological assemblages - are a key focus of macroecology. Abundance (individuals per area) is a basic property of taxocenes but has received less attention than diversity, although the two are probably related. Abundance reflects a taxocene's ability to harvest and sequester available energy and divide it among individuals. This paper explores how two properties of all taxocenes - trophic makeup and taxonomic level (e.g. genus, tribe, subfamily, family...)- may contribute to patterns of local abundance at geographical scales. 2 Forty-nine ground ant taxocenes, in habitats ranging from New World deserts to rain forests, were surveyed along a three-orders of magnitude productivity gradient using transects of 30 1-m(2) quadrats at each site. Abundance - the number of nests per transect - varied over two orders of magnitude. 3 Over 80% of the genera collected were omnivores. However, herbivore, omnivore, and predator taxa were added to ant taxocenes in roughly 1 order of magnitude steps up the productivity gradient. Specialist detritivores were added last. 4 Net primary productivity and mean monthly temperature both consistently entered regression models predicting abundance. However, while productivity was the dominant predictor of abundance for higher taxa (families, subfamilies), temperature was the dominant predictor of abundance for lower taxa (tribes, genera). The answer to the question 'What regulates the abundance of a taxocene?' is thus sensitive to the taxonomic level of analysis. 5 These data support the following scenario. Lower taxa are abiotic specialists given the insufficient number of genomes and generations required for the exploration of the entire abiotic envelope. Higher taxa, in contrast, consist of suites of abiotic specialists arrayed along the entire productivity gradient, with access to productivity everywhere the taxon occurs. If this scenario is true, individual species may respond to global changes in temperature; the higher taxa they belong to may most respond to global changes in productivity.