Functional groups and patterns of organization in North American ant communities: a comparison with Australia

Ant distribution and behavioural dominance is examined at nine sites along an elevational gradient (1400–2600 m) in south eastern Arizona, in order to classify North American species according to a functional group scheme used extensively in Australia. The functional groups are then used as a basis for determining patterns of community structure along the environmental gradient, and for comparing community structure between Australia and North America. Quantitative information on species com‐ position was obtained from pitfall traps, and patterns of ant abundance at tuna baits were used to determine relative behavioural dominance among taxa. A total of eighty‐three species from twenty‐eight genera was recorded along the elevational gradient, with site species richness ranging from four (high elevation Douglas fir forest) to thirty‐three (mid elevation oak–juniper woodland). There was a strong correlation between ant abundance and richness, which was not an artefact of sampling intensity. The most common ants were species of Forelius, Monomorium, Crematogaster and Pheidole at the three desert sites, species of Formica, Pheidole and Crematogaster at the three woodland sites, and species of Prenolepis and Formica at one forest site. No species were abundant at two other forest sites. The most common species in traps also tended to be the most common species at baits. In terms of behavioural dominance, highly competitive ants included species of Solenopsis, Forelius, Monomorium and Liometopum. Species of Pheidole and Crematogaster tended to be moderately competitive, whereas species of Dory‐ myrmex, Myrmica, Camponotus and Formica (fusca gp) had low competitive ability. On the basis of these results and on published records of other taxa, North American ants were assigned to functional groups as follows (major taxa only given here): Dominant Dolichoderinae—Forelius, Liome‐ topum; Subordinate Camponotini—Camponotus; Hot Climate Specialists—Pogonomyrmex, Myrmecocystus; Cold Climate Specialists—Formica (rufa, exsecta and microgyna groups), Leptothorax, Stenamma, Lasius, Prenolepis; Cryptic Species—Smithistruma, Solenopsis (subgenus Diplorhop‐ trum), Acanthomyops; Opportunists—Formica (fusca group), Myrmica, Paratrechina, Dorymyrmex; Generalized Myr‐ micinae—Pheidole, Crematogaster, Monomorium; Specialist Predators—no major taxa. Functional group composition varied systematically along the elevation gradient: Dominant Dolichoderinae, Generalized MyrÃmicinae and Hot Climate Specialists were predominant at desert sites; Generalized Myrmicinae and Opportunists were predominant at woodland sites; and Opportunists and Cold Climate Specialists were predominant at forest sites. These patterns are consistent with published studies from elsewhere in North America. Almost all North American taxa can be matched with what appear to be ecologically equivalent taxa in Australia, and biogeographic patterns of functional group composition are broadly similar across the two continents. The major differences are that Australian ant communities are far richer in species, and are almost always dominated by dolichoderines, particularly species of Iridomyrmex. Generalized myrmicines are subdominant to dolichoderines in Australia, but are the behaviourally dominant ants throughout the warmer parts of North America. In cool‐temperate North America, species of Formica (especially rufa and exsecta groups) are behaviourally dominant, as they are throughout the Palearctic. Some major features of the North American fauna can be linked to its poor representation of Dominant Dolichoderinae, including (1) the relatively low degree of physiological, morphological and behavioural specialization of Hot Climate Specialists; (2) behavioural dominance by formicines in cool‐temperate habitats; and (3) the susceptibility to invasion by behaviourally dominant species such as the imported fire ant Solenopsis invicta and the Argentine ant Linepithema humile.