Ecological Entomology (2006), 31, 491–498 DOI:10.1111/j.1365-2311.2006.00805.x © 2006 The Authors Journal compilation © 2006 The Royal Entomological Society 491 Introduction New World army ants belong to the subfamily Ecitoninae, with behaviourally convergent species in the genus Simopelta (Gotwald & Brown, 1966; Longino, 2000; Brady, 2003). The dramatic foraging behaviour of the above-ground swarm raid- ing army ant species Eciton burchellii and Labidus predator have long attracted the interest of tropical biologists (Darwin, 1839; Wheeler, 1921). There is also a substantial, but relatively poorly studied, subterranean army ant fauna (Weissflog et al., 2000; O’Donnell et al., 2005). Among ants, army ants exhibit a unique type of mass foraging behaviour that involves the co- ordination of raid parties of thousands to hundreds of thousands of workers. The workers in a raid party cooperate to locate, sub- due, and harvest prey. Mass foraging makes the army ants highly successful top predators in tropical forests (Gotwald, 1995). As such, army ants function as ecological keystones (Franks & Bossert, 1983; Boswell et al., 1998, 2000). Army ants are of special interest from a conservation stand- point for two reasons. First, army ant colonies are nomadic (Gotwald, 1995; Brady, 2003). They do not construct or occupy permanent homeostatic nests, and thus they are more directly exposed to local climatic variation than other social insect taxa. For this reason, army ants may serve as particularly sensitive indicators of climatic variation and, possibly, of directional cli- mate change. Second, because army ants are top predators, they affect the evolution and community composition of their prey (Franks, 1982a; Rettenmeyer et al., 1983; Otis et al., 1986). Correspondence: Sean O’Donnell, Department of Psychology, Box 351525, University of Washington, Seattle, WA 98195, U.S.A. E-mail: sodonnel@u.washington.edu Microclimatic factors associated with elevational changes in army ant density in tropical montane forest SEAN O’DONNELL and ANJALI KUMAR Animal Behavior Program, Department of Psychology, University of Washington, Seattle, WA, U.S.A. Abstract. 1. The density (rate of encountering foraging raids) and species richness of army ants (Formicidae: Ecitoninae, and behaviourally convergent Ponerinae) was measured in montane tropical forest. Above-ground and subterranean army ant raids were sampled using standard protocols at four sites across an elevational gradient (1200–1650 m above mean sea level) in and near cloud forest in the area of Monteverde, Costa Rica. 2. Mean ambient temperature differed among sites, and decreased with elevation. For the above-ground foraging army ant species, raid rates also declined with elevation. Surface army ant raid rates, however, were not affected by day to day weather variation within sites (temperature, cloud cover, or precipitation). 3. For the underground foraging army ant species, raid rates did not vary directionally with elevation, and subterranean raid rates were not affected by day to day weather variation within sites. 4. Army ant species richness was not directionally related to elevation, and species sharing among sites was generally high. 5. Army ant community structure changes with elevation in Neotropical montane forest, and the results suggest that the strongest effects are of temperature regimes on the density of raids. These findings provide a baseline against which to detect changes in army ant communities that may accompany directional climate change in tropical cloud forests. Key words. Cloud forest, Costa Rica, Ecitoninae, Monteverde, Ponerinae, species richness.