and for abundances of wildlife (e.g. see Gascon et al.
1999; Gates and Donald 2000).
Conservation requires pragmatic decision-
making. As we continue to fragment landscapes,
island effects may inform such decision-making,
but should not be oversimplified. There is no single
message, and no single island effect; indeed,
insularity may, in at least a minority of cases, bring
positive as well as negative effects (Lockwood and
Moulton 1994). Island effects may be weak or strong.
The implications of insularity vary, depending on
such factors as the type(s) of organism involved, the
type of landscapes involved, the nature of the
environmental dynamics, the biogeographical
setting, and the nature of human use and involve-
ment in the system being fragmented. It is unfortu-
nate that the term ‘island biogeography theory’ has
become largely synonymous in a conservation
setting with a limited conception of island ecology,
stressing the inevitability of stochastically driven
trends to equilibrium. Both pure and applied island
biogeography are richer than this. This richness of
ideas and information needs to be understood and
integrated into teaching for both pure and applied
purposes. Moreover, there is also a need for a
renewed research effort within this area of conser-
vation biogeography, in the search for improved
scientific guidance.
10.14 Summary
The island analogy can be extended to patches of
habitats within continents. The conversion of more-
or-less wild habitats to other uses is fragmenting,
isolating, and reducing ‘wild’ areas across much of
the globe. The implications of this insularization are
examined in this chapter from the scale of individ-
ual populations up to whole landscapes.
The minimum viable population (MVP) is the
smallest number of individuals required to ensure
long-term population persistence. We do not know
how big MVPs should be. We do know that MVPs
vary from species to species, and that the effective
population size is generally smaller than the actual
population size, i.e. not all individuals, or all adults,
are involved in breeding. Population loss may be
due to stochastic demographic and/or genetic
effects, or to environmental disturbance and change.
Stochastic demographic effects may have been over-
estimated. Genetic effects are potentially complex,
and vary between taxa. The significance of environ-
mental change or catastrophe is probably crucial to
population viability, but can be difficult to model.
MVPs therefore need to be established separately for
different types of species, and management regimes
must be responsive to changing circumstances.
The area required by a MVP is termed the mini-
mum viable area, and may be estimated from
knowledge of home range size, although this
approach assumes no population exchange with
other isolates. Another approach to area (or habitat,
or isolation) requirements is to use incidence func-
tions, although the patterns revealed may be con-
founded by multicausality, and may fail to predict
changes that follow within fragmented landscapes.
Moreover, recent work suggests that area require-
ments for species presence in fragments may vary
significantly even within the same region.
Where geographically separated groups are
interconnected by patterns of extinction and recolo-
nization they constitute a population of popula-
tions, or a metapopulation. Here, the idea is that
particular patches have their own internal popula-
tion dynamics, but when they crash to local extinc-
tion they are repopulated from another patch
within the metapopulation. Such a scenario alters
the projections of population viability considerably.
In many cases, however, habitat patches appear to
describe a source–sink rather than a mutual sup-
port system. In such circumstances, a large patch
acts as an effectively permanent population, with
smaller sink habitats around it being re-supplied
from the source population. The sink habitats may
have little relevance to overall persistence-time of
the metapopulation.
A heated debate developed around the SLOSS
‘principles’, which represented an attempt to
answer the question of whether it was better to
advocate Single Large or Several Small reserves of
the same overall area, based on the assumptions of
the EMIB. Unfortunately, island theory provides no
resolution to this debate. In practice, the optimal
reserve configuration for one type of organism,
landscape, or scale of study system, may not be so288 ISLAND THEORY AND CONSERVATION