Co-occurrence patterns in independently evolved groups of Mediterranean insectivorous vertebrates (lizards and shrews)

Abstract

Soricid mammals and lizards are small-sized, insectivorous vertebrates that are widespread and abundant in Mediterranean habitats. Because of their broad sympatry and their diet similarity, these taxa have been suspected to compete for food. Therefore, co-occurrence patterns between these taxa were studied at 72 sites in southern Italy by means of trapping methods. The assemblages were quite distinct depending on the site bioclimate: for the Lacertidae, Podarcis siculus dominated in the thermo-Mediterranean sites and P. muralis in the temperate sites, whereas, for the soricids, Suncus etruscus and two species of Crocidura were dominant in thermo-Mediterranean sites and three Sorex species in the temperate sites. The mean number of captured soricids was statistically higher in the temperate sites, and was positively related to the first component of a Principal Component Analysis summarizing three collinear study site variables (elevation, mean annual temperature, annual rainfall), the reverse being true for lizards. Co-occurrence analysis revealed that a non-segregated structure was present in the dataset, whereas a randomization algorithm showed that the assemblage of small mammals and lizards was non-randomly structured, with the frequency distribution of shrews being non-independent by site from that of lizards. However, when we divided the sites by their bioclimatic regime (thermo-Mediterranean versus temperate), the non-randomness of the community structure disappeared, thus demonstrating that interspecific competition was not the main force driving these assemblages of species. The number of shrews captured in each sampling site was however significantly negatively related to the number of lizards, this pattern being linked to the bioclimate of the various sampling sites. Overall, our data indicated that the assemblage of shrews and lizards was likely regulated essentially by local climate and not by synecological (interspecific competition) dynamics.