Competition, predation and interspecific synchrony in cyclic small mammal communities Otso Huitu, Kai Norrdahl and Erkki Korpima ¨ki Huitu, O., Norrdahl, K. and Korpima ¨ki, E. 2004. Competition, predation and interspecific synchrony in cyclic small mammal communities. / Ecography 27: 197  / 206. Although competition and predation are considered to be among the most important biotic processes influencing the distribution and abundance of species in space and time, the relative and interactive roles of these processes in communities comprised of cyclically fluctuating populations of small mammals are not well known. We examined these processes in and among populations of field voles, sibling voles, bank voles and common shrews in western Finland, using spatially replicated trapping data collected four times a year during two vole cycles (1987  /1990 and 1997  /1999). Populations of the four species exhibited relatively strong interspecific temporal synchrony in their multiannual fluctuations. During peak phases, we observed slight deviations from close temporal synchrony: field vole densities peaked at least two months earlier than those of either sibling voles or bank voles, while densities of common shrews peaked even earlier. The growth rates of all four coexisting small mammal species were best explained by theirown current densities. The growth rate of bank vole populations was negatively related to increasing densities of field voles in the increase phase of the vole cycle. Apart from this, no negative effects of interspecific density, direct or delayed, were observed among the vole species. The growth rates of common shrew populations were negatively related to increasing total rodent (including water voles and harvest mice) densities in the peak phase of the vole cycle. Sibling voles appeared not to be competitively superior to field voles on a population level, as neither of these Microtus voles increased disproportionately in abundance as total rodent density increased. We suggest that interspecific competition among the vole species may occur, but only briefly, during the autumn of peak years, when the total available amount of rodent habitat becomes markedly reduced following agricultural practices. Our results nonetheless indicate that interspecific competition is not a strong determinant of the structure of communities comprised of species exhibiting cyclic dynamics. We suggest that external factors, namely predation and shortage of food, limit densities of vole populations below levels where interspecific competition occurs. Common shrews, however, appear to suffer from asymmetric space competition with rodents at peak densities of voles; this may be viewed as a synchronizing effect. O. Huitu, K. Norrdahl and E. Korpima ¨ki, Sect. of Ecology, Dept of Biology, Univ. of Turku, FIN-20014 Turku, Finland. Competition and predation have repeatedly been con- sidered to be among the most important biotic processes influencing the distribution and abundance of species in space and time (Connell 1983, Sih et al. 1985, Morin 1999, Gurevitch et al. 2000, Chase et al. 2002). As competition between species and predation are by definition interactions between species, these processes to a great extent determine also the composition and structure of entire ecological communities (Morin 1999). Individuals of most species must, however, also engage Accepted 3 October 2003 Copyright # ECOGRAPHY 2004 ISSN 0906-7590 ECOGRAPHY 27: 197  /206, 2004 ECOGRAPHY 27:2 (2004) 197