prey species may be maintained, with potentially
significant consequences further down the food
chain. Thus, the densities of medium-sized terres-
trial mammals are between 8 and 20 times greater
on Barro Colorado Island, which lacks top preda-
tors, than in a comparable ‘mainland’ site in which
they occur. Terborgh (1992) suggests that this may
be having important selective impacts on plant
regeneration. On some smaller islets nearby in Lake
Gatun, not only are the (true) predators absent, but
so too are those smaller mammals that act as seed-
predators, the squirrels, peccaries, agoutis, and
pacas which have such high densities on Barro
Colorado island. These small islets became domi-
nated in the 70 or so years since their isolation by
large-seeded tree species. This is plausibly because
these plants no longer suffer high rates of attrition at
the point of germination and establishment, and
have therefore been able to out-compete the smaller-
seeded species to a greater extent than usual.
Predators undoubtedly have a crucial role to play
in fragmented landscapes, not just in terms of
predation within the habitat island, but also within
the matrix. Simberloff (1992) notes that in the
Manaus fragmentation study the bat falcon (Falco
rufigularis) has repeatedly been seen to chase birds
flying over cleared areas from fragment to fragment.
It may therefore be unsurprising that most under-
storey birds will not willingly cross gaps of even
80 m. Similarly, in the USA, the northern spotted
owl (Strix occidentalis caurina), of metapopulation
theory fame, previously was a species which rarely
left closed forest. In the newly fragmented land-
scapes, as many as 80% of yearling males die,
apparently from predation by great horned owls
(Bubo virginianus) and goshawks (Accipiter gentilis)
as they disperse over cleared areas. This was one
consideration that persuaded the interagency
federal committee responsible for the management
plan to shift their strategy towards larger, more con-
tinuous blocks. This brief consideration of predation
helps us recall the point that the particular mecha-
nisms responsible for failures to sustain populations
or to recolonize habitat islands may thus be poorly
predicted by ‘species–area’ type-approaches, or for
that matter by metapopulation models.
There may also be important dynamic features,
such as the spatial and temporal oscillations that
characterize elephant populations in the Tsavo
National Park in Kenya, on a cycle of roughly
50 years. Even with very large reserves, migratory
animals may ignore park boundaries and, espe-
cially in lean years, may move into areas outside,
where they are most unwelcome and perhaps also
unsafe. These patterns can provide big manage-
ment problems, even when a very large area is
enclosed as a reserve (Woinarski et al. 1992). The
elephants illustrate another idea: that of the
keystone species, meaning a species that is so
critical to the functional character of an ecosystem
that its removal would cause a chain of alterations
(e.g. Soulé 1986; Pimm 1991). Elephants have a key
role as grazers. Without them successional changes
occur in the vegetation, altering the carrying
capacity of the system for other species. High
densities of elephant, on the other hand, can lead
to overgrazing and widespread destruction of
trees. In such cases, conservation management
naturally focuses around managing the keystone
species.Climate change and reserve systems
Range shifts driven by significant climatic change
have been a feature of the Quaternary period. There
is no reason to consider these climatic fluctuations
to be at an end, and there is also mounting concern
that humans have initiated a phase of rapid climatic
warming. Researchers are now focusing on the
abilities of plant and animal species to track
their required climatic envelope across the now
fragmented landscapes (Huntley et al. 1995). Key
factors are the speed of climatic change, the
dispersal abilities of the target species, and
the characteristics of the landscapes across which
species may have to move, which may range from
the optimal to the benign to the hostile. Grappling
with such scenarios requires a variety of research
tools (Bush 1996, 2002; Pearson and Dawson 2003;
Araújoet al. 2005b), amongst which are the tools of
island biogeography reviewed in these pages
(Boggs and Murphy 1997).286 ISLAND THEORY AND CONSERVATION