... the scarcity of kinds—the richness in endemic forms
in particular classes or sections of classes,—the absence of
whole groups, as of batrachians, and of terrestrial mam-
mals notwithstanding the presence of aerial bats,—the
singular proportions of certain orders of plants,—herba-
ceous forms having developed into trees, &c.,—seem to
me to accord better with the view of occasional means of
transport having been largely efficient in the long course
of time, than with the view of all our oceanic islands hav-
ing been formerly connected by contiguous land with the
nearest continent...
(Darwin 1859, p. 384)3.1 Introduction: the global significance of island biodiversity
The term biodiversityis a contraction of biological
diversity, and broadly defined refers to the variabil-
ity of life from all sources, including within species,
between species, and of ecosystems (Matthews et al.
2001). We may at times be concerned with higher or
lower points in the taxonomic hierarchy, such that
familial, generic, subspecific, or even gene fre-
quency data may be analysed under the header
‘biodiversity’, but the most commonly used diver-
sity unit is undoubtedly the species. As a first-order
generalization, islands are species poor for their
size but rich in forms found nowhere else, i.e.
endemicto that island or archipelago. They are
thus ‘hotspots’ of biodiversity (see Box 3.1). Given
that the total number of living species on the planet
is not yet known to within an order of magnitude
(Groombridge 1992), it is difficult to provide a firm
measure of the relative contribution of islands to
global biodiversity. Yet, for particular taxa there are
sufficient data to demonstrate that, taken collectively,
islands contribute disproportionately for their area
to global species totals.
In order to appreciate the special significance of
island biotas it is important to consider some of
their general properties, and in what ways they are
peculiar. This chapter sets out to do that, distin-
guishing the notion of their compositional distinc-
tiveness or ‘disharmony’ from simple species
poverty. We also explore aspects of the historical
biogeographical context and the importance of dis-
persal limitations in determining the balance of
island assemblages.
The affinities of island biotas to continental source
areas have fascinated biogeographers for more than
a century, providing some key insights for evolu-
tionary theory (e.g. Darwin 1859; Wallace 1902).
First steps are to establish how island biotas are
related to continental biotas, where they ultimately
derive from, and how far the unique (i.e. endemic)
forms found on islands have developed from the
colonizing forms. Once this context has been devel-
oped, we can move on to look at the mechanisms
and processes involved in the evolution of island
biotas, which we do in later chapters. In addition, as
discussed later in this volume, islands are reposito-
ries for many of the world’s threatened species and
are worthy of special attention in conservation pri-
oritization decisions. Scientific guidance as to where
particular islands fit in global and regional planning
frameworks is to be derived from the application of
biogeographical techniques and theories, under the
recently recognized (but fairly long-developing)46
CHAPTER 3