species. In this sense, ‘island’ approaches have pro-
duced some dividends. It is now a question of
learning what they are, and differentiating between
those effects that are weak and those that are strong
(the world is neither wholly non-equilibrial nor
wholly equilibrial). It seems that we can work out
which sorts of species are most threatened and
some of the major reasons why they are threatened,
and it should therefore be possible to offer some
management solutions. Some of these insights have
indeed been derived from ‘island’ studies.
For many species, reserves are essential. Very
often this is because of specific threats posed by
people (or our commensals) and the protection that
can be afforded within the reserve. Simberloff
(1992) comes down in general on the side of large,
continuous blocks of habitat. He argues that no
existing theory adequately describes the joint
effects of loss of area and fragmentation, but that
there are empirical observations, such as those to
do with predation discussed earlier, to suggest that
for many species the combined impact is indeed
strongly deleterious: ‘Probably any species that has
evolved in large, relatively continuous habitat has
traits that are maladaptive in small, isolated frag-
ments’ (p. 85). Beyond this generalization, the
means, rate, and extent to which extinction is deliv-
ered, are likely to be idiosyncratic. The theoretical
basis for calculating likely regional and global
extinctions as a result of the rapid phase of habitat
loss currently under way remains highly uncertain.
For a careful review of the evidence and basis for
calculations see Whitmore and Sayer (1992).
Ecological hierarchies and fragmented landscapes
As with other branches of island theory, approaches
to the debates on reserve configuration that take
no account of hierarchical interactions between
organisms of differing trophic levels are incomplete
(Terborgh 1992). For many animal groups, plants
(collectively or individually) determine the ability
of particular animal species to occupy a habitat
island. Successional changes in plant communities
may often explain turnover in bird or butterfly
communities (Chapter 5; Thomas 1994). Equally
important may be the activities of animals as
dispersers and pollinators of plants (and as preda-
tors), both in contexts of forest maintenance and of
recolonization of disturbed areas. Different types of
animals disperse different, albeit overlapping,
suites of plant species. Studies of cleared areas in
the Neotropics have suggested that bats may be
more significant in seeding open habitat than are
day-flying birds. The efficacy of birds, in particular,
may also be positively influenced by the availabil-
ity of some woody cover in an otherwise open area
(Estradaet al. 1993; Gorchov et al. 1993; Guevara
and Laborde 1993). Thus, for instance, cheap plant-
ings of small clumps of plants may encourage
speedy return of forest around the inocula.
The Krakatau recolonization data demonstrate
the significance of birds and bats in transporting
seeds between sites, and thus also supports their
significance for population interchange between
forest patches in the tropics. Maintenance and
restoration of forest might thus necessitate protec-
tion for vertebrate species, if not in their own right,
for their interactions with plants. It is not ‘merely’
biodiversity which is at stake, as in cases there may
be huge economic costs to the loss of pollinator
species on which economically useful plants
depend (Pannell 1989; Fujita and Tuttle 1991; Cox
et al. 1992; Ricketts 2004). At the same time, the
Krakatau study demonstrated that particular
groups of plants, those dispersed by non-volant
frugivores and the large-seeded, wind-spread
species, are likely to be greatly hampered by patch
isolation (Whittaker and Jones 1994a,b; Whittaker
et al. 1997). These topics have recently seen a resur-
gence of research interest (Ewers and Didham
2005), with studies in both Africa and the
Neotropics demonstrating the negative impacts on
frugivore-mediated plant dispersal of forest frag-
mentation (e.g. Cordeiro and Howe 2001; Martínez-
Garza and Howe 2003).
It seems likely that some area effects involve
threshold responses and that these operate via hier-
archical links within ecosystems (Souléet al. 1988;
Terborgh 1992). For instance, below a certain size,
an isolated habitat island may be incapable of sup-
porting populations of large predatory vertebrates,
with the consequence that higher densities of theirDOES CONSERVATION BIOLOGY NEED ISLAND THEORY? 285