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                                 Diversity and Distributions (2002)  8, 1–9

                                   BIODIVERSITY RESEARCH

                                 Island incidence and mainland population density: mammals
                                 Blackwell Science Ltd
                                 from Mediterranean islands

                                 MAURIZIO SARÀ  and SERGE MORAND       2  1 Dipartimento di Biologia Animale, Via Archirafi 18,
                                 90123 Palermo, Italy, E-mail:  Laboratoire de Biologie Animale  (Umr 5555 Cnrs),
                                 Centre de Biologie et d’Ecologie Tropicale et Méditerranéenne, Université de Perpignan, Av. de Villeneuve,
                                 66860 Perpignan Cedex, France, E-mail:

                                 Abstract. The reasons why some species are resist-  suggests that the distribution of mammals on these
                                 ant to extinction or are better invaders of islands  islands is not the result of a random process.
                                 than others remain unexplained. In this study, we  Using two comparative methods, we show that
                                 test the hypothesis that mammals living on the  mammal density on the mainland, corrected for
                                 mainland at higher density than predicted by the  body mass, is negatively correlated with island size.
                                 density/body mass relationship have a much greater  Mammals with a high density compared to the
                                 chance to colonize a small island successfully,  density/mass relationship are the best invaders
                                 and/or that they are less likely to become extinct  and/or probably have less chance of going extinct
                                 when living on small islands. For this, we used data  on small islands when population size is small.
                                 compiled on mammals from a number of Medi-
                                 terranean islands. We show a nested pattern for  Key words. Nested pattern, mammals, density,
                                 mammals on western Mediterranean islands, which  islands, extinction, biological invasions.

                                                                          characterized by several attributes, such as:
                                                                          abundance in its original range, a polyphagous
                                 Island biogeography has stimulated many theoret-  feeding habit, short generation times, high
                                 ical and empirical studies since the classical work  genetic variability, the ability of fertilized females
                                 of MacArthur & Wilson (1967). It appears that  able to colonize alone, a body size larger than
                                 the set of species inhabiting a particular island is  most relatives, an association with humans, and
                                 not a random sample drawn from the  main-  the ability to function in a wide range of physical
                                 land species pool. Since the work of Diamond  conditions. According to  Diamond (1984), the
                                 (1974) and  Lack (1976), fauna living in  insular  extinction risk of species is set by attributes such
                                 environments have been generally considered to be  as body size, trophic position, abundance and
                                 recruited from the available pool of  species on  habitat specialization. These potential attributes
                                 the basis of abundance on the mainland and  of invaders and those resistant to extinction have
                                 adaptability of their habitat requirements.  never been tested comparatively However,  some
                                   Two of the most important questions arising  features common to both invaders and those
                                 from these predictions are why are some species  resistant to extinction include our difficulty in
                                 good invaders of islands (Ehrlich, 1986) and why  distinguishing between colonization and extinc-
                                 are some resistant to extinction on islands  tion processes.
                                 (Diamond, 1984). Specifically, what are the attributes  Our aim is to test one of these determinants:
                                 of a successful invader and what are those of a  the relative population density on the mainland.
                                 species highly resistant to extinction? According  The data used consisted of the compiled data
                                 to Ehrlich (1986), a successful invader may be  on the occurrence of mammal species on west
                                 © 2002 Blackwell Science Ltd.           1
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