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DDI_119.fm Page 2 Thursday, December 27, 2001 10:00 AM

2 M. Sarà and S. Morand

100km

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