Relating niche and spatial overlap at the community level Ulrich Hofer, Louis-Fe ´lix Bersier and Daniel Borcard If interspecific competition is a strong structuring force of communities, ecologically similar species should tend to have spatial ranges at local scale that do not overlap. Experimental testing of this hypothesis becomes impracticable with large communities. One possibility to tackle this issue is a correlational approach, by comparing the matrix of niche overlap with that of spatial overlap. The use of the standard Mantel test is however impaired by the non-linearity in the relationship of the two descriptors: in a competitively structured assemblage, specieswith high niche overlap are expected to be segregated spatially, but species with small niche overlap may or may not exhibit high spatial overlap. To overcome this problem, we devised an original randomization test, which was run for three data-sets comprising frogs, lizards, and birds along altitudinal gradients. The test yielded intriguing results: reptiles and birds revealed an adjustment that would reduce the potential for interspecific competition, while amphibians showed the opposite trend, that is, ecologically similar species co-occurred more than expected by chance. Frogs may be more constrained by resource requirements, possibly breeding sites, than by competition. Our test will help to assess the generality of this pattern with other data-sets. U. Hofer, Dept of Vertebrates, Natural History Museum, Bernastrasse 15, CH-3005 Bern, Switzerland. L.-F. Bersier, Zoological Inst., Univ. of Neucha ˆtel, rue Emile-Argand 11, CH-2007 Neucha ˆtel, Switzerland (louis.felix.bersier@unine.ch) andChairof Statistics, Dept of Mathematics, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland. / D. Borcard, De ´pt de Sciences Biologiques, Univ. de Montre ´al, C.P. 6128, Succursale ‘‘Centre-Ville’’, Montre ´al, Que ´bec H3C 3J7, Canada. The effect of competitive interactions on community structure has been confirmed by several well-known reviews of field experiments on interspecific competition (Connell 1983, Schoener 1983, Goldberg and Barton 1992, Gurevitch et al. 1992). Evidence usually came from manipulative experiments that involved two or three potential competitor species (Schoener 1983). One common result of competition was differences in spatial distribution of species, e.g. vertical zonation in barnacles (Connell 1961) or altitudinal zonation in terrestrial salamanders (Hairston 1980a, 1980b). At the same time, prominent observational studies of bird assem- blages also revealed differences in spatial distribution, which their authors claimed to result from competition. Diamond (1975) interpreted complementary distribu- tions of birds among islands of the Bismarck Archipe- lago to result from interspecific competition. Terborgh (1985) concluded that direct and diffuse competition accounted for two-thirds of the limits of avian eleva- tional ranges along an elevational gradient in the Peruvian Andes. In contrast to those from manipulative experiments, the conclusions of such ‘‘natural experi- ments’’ (Cody 1974) have been at the heart of heated debates (Connor and Simberloff 1979, Diamond and Gilpin 1982), inviting reanalyses and stimulating the search for adequate analytical procedures, which are usually null model tests (Sanderson et al. 1998, Gotelli 2000, Gotelli and Entsminger 2001). Within the non-experimental domain of community ecology, null model analyses of whole assemblages have Published in Oikos 106, issue 2, 366 - 376, 2004 which should be used for any reference to this work 1