Oecologia DOI 10.1007/s00442-006-0453-9 123 COMMUNITY ECOLOGY Habitat complexity facilitates coexistence in a tropical ant community M. Sarty · K. L. Abbott · P. J. Lester Received: 13 January 2006 / Accepted: 26 April 2006  Springer-Verlag 2006 Abstract The role of habitat complexity in the coexis- tence of ant species is poorly understood. Here, we examine the inXuence of habitat complexity on coexis- tence patterns in ant communities of the remote PaciWc atoll of Tokelau. The invasive yellow crazy ant, Anop- lolepis gracilipes (Smith), exists in high densities on Tokelau, but still coexists with up to seven other epigeic ant species. The size-grain hypothesis (SGH) proposes that as the size of terrestrial walking organisms decreases, the perceived complexity of the environment increases and predicts that: (1) leg length increases allo- metrically with body size in ants, and (2) coexistence between ant species is facilitated by diVerential habitat use according to body size. Analysis of morphological variables revealed variation inconsistent with the mor- phological prediction of the SGH, as leg length increased allometrically with head length only. We also experimentally tested the ability of epigeic ants in the Weld to discover and dominate food resources in treat- ments of diVering rugosity. A. gracilipes was consis- tently the Wrst to discover food baits in low rugosity treatments, while smaller ant species were consistently the Wrst to discover food baits in high rugosity treat- ments. In addition, A. gracilipes dominated food baits in planar treatments, while smaller ant species dominated baits in rugose treatments. We found that the normally predictable outcomes of exploitative competition between A. gracilipes and other ant species were reversed in the high rugosity treatments. Our results support the hypothesis that diVerential habitat use according to body size provides a mechanism for coexis- tence with the yellow crazy ant in Tokelau. The SGH may provide a mechanism for coexistence in other ant communities but also in communities of other terres- trial, walking insects that inhabit a complex landscape. Keywords Anoplolepis gracilipes · Discovery-dominance trade-oV · Invasive ants · Morphology · Size-grain hypothesis Introduction InterspeciWc competition plays a key role in structuring ant communities (Blüthgen and Fiedler 2004; Blüthgen et al. 2004; Davidson 1998; Lach 2005; Savolainen and Vepsäläinen 1988). Coexistence in ant communities can be mediated by trade-oVs in competitive abilities, for example the ability to discover versus the ability to dominate and consistently recruit to a food resource (the discovery–dominance trade-oV; Fellers 1987). Most Weld studies of recruitment dynamics have been conducted in two dimensional environments, with food baits placed on the ground or on tree trunks, but at the scale of an individual ant landscapes are much more Communicated by Konrad Fiedler Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at http://dx.doi.org/10.1007/s00442-006-0453-9. M. Sarty (&) · K. L. Abbott · P. J. Lester School of Biological Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand e-mail: megan.sarty@maf.govt.nz Present Address: M. Sarty Biosecurity New Zealand, Ministry of Agriculture and Forestry, P.O. Box 2526, Wellington, New Zealand