Amphibia-Reptilia 27 (2006): 325-337 Null models, co-occurrence patterns, and ecological modelling of a Mediterranean community of snakes Luca Luiselli 1 , Ernesto Filippi 2 Abstract. A Mediterranean community of snakes, inhabiting a hilly area in central Italy (Monterano, Tolfa Mountains) was statistically modelled to investigate whether: (i) it shows non-random structure as predicted by competition theories, (ii) there are proximate elements of the landscape that may predict the presence/absence of the various snake species at the study area, and (iii) the snakes select ‘resting sites’ which are relatively simple or relatively complicated in landscape structure. To investigate issue (i) we used co-occurrence pattern analyses and Monte Carlo simulations. To investigate issue (ii) we used logistic regression analysis on presence/absence data of eleven ‘proximate landscape’ variables around sites of presence for the snakes and around random (= absence) sites at the same study area. To investigate issue (iii), we used expected-versus- observed χ 2 to test whether snake presences were associated more often than random with a combination of variables, i.e. with at least four different variables for each point of presence. The snake community was constituted by eight species, but only the five commonest species (i.e. Coluber viridiflavus, Elaphe longissima, Elaphe quatuorlineata, Natrix natrix, and Vipera aspis) were included in our models. Our modeling and simulation analyses suggested that: (i) the community structure was not competitively structured both in terms of habitat resource and in terms of ‘proximate landscape’ variables influencing snake presences; (ii) all five snake species were positively influenced by at least two of the eleven variables considered in the logistic regression model; (iii) four out of five species clearly selected sites with complicated structure, the only exception being Coluber viridiflavus. Introduction Studies on snake community ecology have been in general descriptive and not predictive (e.g., Toft, 1985; Luiselli, 2006a). General evidence suggests that many communities are organized via resource partitioning patterns, where food is the main resource to be partitioned, contrary to what happens in other reptiles where habitat is the main resource to be partitioned (Toft, 1985). Although also re-analysis of available data on snake communities worldwide by means of sim- ulation analyses confirmed that (i) most com- munities effectively partitioned the food re- source, and (ii) many of these communities are competitively structured along the trophic niche dimension (Luiselli, 2006a, b), nonetheless it remains that the use of the habitat structure is 1 - F.I.Z.V.(Ecology) and Centre of Environmental Studies ‘Demetra’ s.r.l., via Olona 7, I-00198 Roma, Italy e-mail: lucamlu@tin.it 2 - F.I.Z.V. (Herpetology) and ‘Altair’ Environmental Stud- ies Centre, piazza Capri 20, I-00141 Roma, Italy e-mail: ernesto.filippi@tin.it still a fundamental aspect to be studied for prop- erly understanding snake community ecology. It is now clear that most, perhaps all, snake species use their available habitats in a non- random way, and this non-random distribution of habitats does not seem to be the immediate result of differential survival in adjacent habi- tats (Reinert, 1997; Pringle et al., 2003). It has been demonstrated that habitat preferences in snakes may vary intraspecifically (e.g., Rein- ert, 1984; Shine, 1986; Burger and Zappalorti, 1989; Luiselli et al., 1994; Shine et al., 2003), geographically (e.g., Sweet, 1985), seasonally (e.g., Shine and Lambeck, 1985; Seigel, 1986; Reinert, 1997; Luiselli et al., 2005), and even ontogenetically (e.g., Beatson, 1976; Reinert, 1997). In addition, it has been also demon- strated that prey resource distribution may sig- nificantly influence the habitat use by snakes, and that snake species may enterprise signifi- cant migrations to follow their favoured preys (e.g., Madsen and Shine, 1996; Shine and Mad- sen, 1997). One problem of the great majority of stud- ies in snake habitat selection (for reviews see © Koninklijke Brill NV, Leiden, 2006. Also available online - www.brill.nl