REVIEW Trade-offs in community ecology: linking spatial scales and species coexistence Jamie M. Kneitel* and Jonathan M. Chase Department of Biology, Washington University, Campus Box 1137, St Louis, MO 63130, USA *Correspondence: E-mail: kneitel@biology2.wustl.edu Abstract Trade-offs in species performances of different ecological functions is one of the most common explanations for coexistence in communities. Despite the potential for species coexistence occurring at local or regional spatial scales, trade-offs are typically approached at a single scale. In recent years, ecologists have increasingly provided evidence for the importance of community processes at both local and regional spatial scales. This review summarizes the theoretical predictions for the traits associated with trade-offs under different conditions and at different spatial scales. We provide a spatial framework for understanding trade-offs, coexistence and the supportive empirical evidence. Predictions are presented that link the patterns of diversity observed to the patterns of trade-offs that lead to coexistence at different spatial scales. Recent evidence for the evolution of trade-offs under different conditions is provided which explores both laboratory microcosm studies and phylogenetic tests. Examining trade-offs within a spatial framework can provide a strong approach to understanding community structure and dynamics, while explaining patterns of species diversity. Keywords Coexistence, community ecology, diversity, local and regional scales, metacommunity, spatial scales, species traits, trade-offs. Ecology Letters (2004) 7: 69–80 INTRODUCTION In evolutionary biology, the ‘Darwinian demon’ reigns supreme in the world of life histories, by asexually reproducing with immeasurable frequency and number while living forever. Trade-offs between survival and reproduction, however, constrain any organism from realistically resembling that creature. Similarly, community ecology could have its analogous, ‘Hutchinsonian demon’, whereby one species in a community dominates because it is the best at colonizing new patches, utilizing all the resources, avoiding predators and resisting stresses (Tilman 1982 termed these ‘superspecies’). Here, interspecific trade- offs are invoked to tame this demon, in that the benefits of performing one ecological function well (e.g. consuming one type of resource) comes at a cost of performing another function (e.g. consuming a different type of resource). Trade-offs within this community context represent niche differentiation among species, which emerge from individ- ual-level constraints within an environmental context (e.g. resources, abiotic factors, presence of competitors or predators; Chase & Leibold 2003). Trade-offs are ultimately exhibited as a negative functional interaction between traits (e.g. growth and reproduction; Stearns 1989; Zera & Harshman 2001). These differences in life-history traits can have consequences for population parameters (growth rate and carrying capacity), body size differences and ecological traits (MacArthur & Wilson 1967; Pianka 1970; Boyce 1984; Gleeson & Tilman 1990). For example, the trade-off between seed size and seed number has been used as a proxy for the competition– colonization trade-off for species coexistence (Turnbull et al. 1999; Levine & Rees 2002); competitive ability is enhanced by production of fewer, larger seeds, whereas colonization ability is improved by production of many small seeds. Other examples include allocation to aboveground–below- ground growth (e.g. Gleeson & Tilman 1990) and power- efficiency growth (growth at low and high resource levels; e.g. Sommer 1985). However, it is not always necessary to invoke character divergences because habitat characteristics (e.g. refugia; Mouquet et al. in press) and neutral models (e.g. Hubbell 2001) can also explain coexistence. Nevertheless, we argue that niche differences exhibited as trade-offs among species provide a more convincing explanation for Ecology Letters, (2004) 7: 69–80 doi: 10.1046/j.1461-0248.2003.00551.x Ó2003 Blackwell Publishing Ltd/CNRS