Page 6 - sarà_2002
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                                    6  M. Sarà and S. Morand

                                        A                                    1998), as has its consequences in term of energy
                                                                             used (Damuth, 1981).
                                       5                       n = 31        invader’s success, but he did not use either exten-
                                                                               Ehrlich (1986) failed to find any rules for an
                                      Maximum density  3       P < 0.0001    sive data or any appropriate comparative
                                                               r = 0.69
                                                                             method. Our results clearly show that mammals
                                                                             with high density compared to their body mass
                                                                             are the best invaders, or that they have probably
                                                                             less chance of going extinct when living in small
                                                                             populations on small islands. Hence, body size by
                                                                             itself cannot explain the potential ability to
                                      –1                                     invade an island. Body size is rather a confound-
                                         0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5   ing variable as it covaries with many life traits
                                                     Body mass               (e.g. fecundity and survival).
                                                                               Both extinction and invasion processes may
                                                                             thus have played a role through time. The area–
                                         B                     n = 29        species relationship can be helpful for describing
                                      Contrasts in island area size  –1 2 1 0  r = 0.46  Cheylan (1984) investigated the area-species rela-
                                                                             the degree of insularity of a fauna and the equi-
                                                                             librium of the extinction and invasion processes.
                                                               P = 0.012
                                                                             tionship for insular mammals in the Provence islets
                                                                             and in some western Mediterranean islands; and
                                                                             later Sarà (1998) repeated it for a larger sample
                                                                             including eastern Mediterranean islands. Both
                                                                             studies proved that mammal species richness is
                                                                             sion slope (the  z-value of Preston, 1960) indi-
                                       –3                                    well correlated to island area size, but the regres-
                                                                             cated a low degree of insularity (z = 0.20 in both
                                        –1.5  –1.0  –0.5   0   0.5   1.0
                                                                             analyses). Large islands have a ‘continental’ slope
                                          Contrasts in residuals of mammal density
                                                                             (z = 0.13) and their mammal fauna can thus be
                                                                             considered a subsample of that living on the
                                    Fig. 3 (A) Relationship between maximum mammal
                                    density and body mass. (B) Relationship between  adjacent mainland; whereas, mammals living in
                                    island area size and density of mammals on  small islands show a relatively high degree of
                                    mainland  (corrected  for  body  mass)  using  insularity (z = 0.24) (Sarà, 1998).
                                    independent contrasts.                     Man, driving extinction and favouring colon-
                                                                             ization (see  Table 3), is considered as one of the
                                                                             major agents and determinants of mammalian
                                                                             diversity in such islands (Blondel & Vigne, 1993;
                                    This study provides evidence that a nested pat-  Masseti, 1998). The reconstruction of this process
                                    tern exists for mammals on western Mediterra-  of faunal change in several large islands throughout
                                    nean islands, which suggests that the distribution  time is well documented (see reviews in Cheylan,
                                    of mammals on these islands is not the result of  1991; Sarà, 1998; Blondel & Aronson, 1999).
                                    a random process. Patterson & Atmar (1986) gave  Whatever the major determinant might be, the
                                    the following explanations for nested patterns:  outcome is that the pattern of mammal distribu-
                                    (a) an ability for dispersion and colonization,  tion is nested and ordered according to Atmar
                                    (b) a relative proneness to extinction, and (c) the  & Patterson’s (1995) model. The mammal fauna
                                    adverse effects of parasites.            living today on islands is thus a specific assemblage
                                      The negative relationship between body size  of species and their characteristics are worthy of
                                    and density has been observed and discussed  further investigation.
                                    extensively in the ecological literature (Damuth,  Regarding the lack of any distance effect, we
                                    1981; Silva & Downing, 1995; Morand & Poulin,  must remember that all the previously cited
                                                                        © 2002 Blackwell Science Ltd, Diversity and Distributions, 8, 1–9
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