Page 2 - sarà_2002
P. 2  Page 2  Thursday, December 27, 2001  10:00 AM

                                    2  M. Sarà and S. Morand


                                    Fig. 1 Map of the western Mediterranean Sea showing the coast line (dotted line) at the last maximum ice
                                    period (18 000 ± 20 000 years BP; modified after Agnesi  et al., 1997).

                                    Mediterranean islands (Sarà, 1998). These islands  (Damuth, 1981, 1987), have much more chance
                                    have a low degree of geographical isolation and  to colonize a small island successfully, and/or
                                    can be considered as a group of continental  have less chance to go extinct when living on
                                    islands, as the African and Euro-Asiatic  land-  small islands.
                                    masses surround them. The most isolated
                                    islands are Sardinia (225 km from mainland) and
                                    Minorca (220 km); however,  they are part of  MATERIALS AND METHODS
                                    wider insular systems (the Sardinian-Corsican
                                                                             Source data
                                    and the Balearics) that decrease their real dis-
                                    tance from mainland.                     Data on presence and absence of mammal species
                                      First, we followed Atmar & Patterson’s (1993)  on 45 islands of the west Mediterranean Sea were
                                    approach of using a direct measure of order  compiled by Sarà (1998) (see  Appendix 1). For
                                    based on entropy to investigate patterns in spe-  each host species, it was then possible to estimate
                                    cies  distribution between islands (or isolated  the minimum area size and the related distance
                                    habitats). In what they called a ‘cold system’, the  to the mainland for each island where the mam-
                                    extinction order of species would be perfectly  mal species occurs  (Fig. 1). Data on maximum
                                    determined independently by random processes  recorded mainland density were obtained from
                                    that may occur on each island (or isolated hab-  published studies, and were selected where pos-
                                    itat). If the temperature of the system rises, then  sible to include studies of mainland Mediterra-
                                    the extinction order will be less predictable.  nean ecosystems (Table 1).
                                    Hence, the temperature of the system becomes a
                                    measure of the order from 0 (complete order and
                                                                             Measure of order
                                    predictable extinction) to 100 (unpredictable
                                    extinction and disorder).                Rather than measuring the index of nestedness,
                                      Second, we hypothesized that mammals living  Atmar & Patterson (1993) and  Patterson &
                                    on mainland at higher density than that pre-  Atmar (2000) suggested using a measure of
                                    dicted by the density/body mass relationship  order, which is similar to a measure of the
                                                                        © 2002 Blackwell Science Ltd, Diversity and Distributions, 8, 1–9
   1   2   3   4   5   6   7