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DDI_119.fm Page 1 Thursday, December 27, 2001 10:00 AM

Diversity and Distributions (2002) 8, 1–9

BIODIVERSITY RESEARCH

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

1
MAURIZIO SARÀ and SERGE MORAND 2 1 Dipartimento di Biologia Animale, Via Archirafi 18,
2
90123 Palermo, Italy, E-mail: mausar@unipa.it 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: morand@university-perp.fr

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:
INTRODUCTION
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. http://www.blackwell-science.com/ddi 1