Thomas (1994) usefully reminds us that there are
many reasons for species colonization of a patch,
and they are not all island effects. He suggests six
circumstances in which natural colonizations by
butterflies are observed:
●succession following disturbance
●where new ‘permanent’ habitat is created close to
existing populations
●introductions of species outside former range
margins
●regional increases in range
●turnover-prone peripheral patches (which act in
essence as ‘sinks’ and are therefore unimportant to
overall persistence)
●seasonal spread.
Thomas regards butterflies as persisting in
regions where they are able to track the environ-
ment, and becoming extinct if they fail to keep up
with the shifting habitat mosaic, or if the shift in
habitat regionally is against them. Metapopulation
models therefore need to be superimposed on an
environmental mosaic, which in many cases will
itself be spatially dynamic. ‘An environmental
mosaic perspective shifts the emphasis on to tran-
sient dynamics and away from the equilibrium
(balance) concept of metapopulation dynamics, for
which there is little evidence in nature’ (Thomas
1994, p. 376). The possibility of incorporating
patchy disturbance phenomena into metapopula-
tion models has since been developed by Hess
(1996).
Value of the metapopulation concept
As Gotelli (1991) concludes, the empirical support
for classic metapopulation modelsis fairly thin on
the ground and tests are liable to be difficult.
Metapopulations are difficult to replicate, and the
timescale of their dynamics (many generations of
the organisms involved) may be of the order of
decades. In addition, it is readily appreciated that
populations are subdivided on many scales and so
the delimitation of the local population is often sub-
jective. Hence, to achieve greater realism, meta-
population simulations have to be extended from
the original, simplistic models, to allow for the
differing degrees of population connectivity, and
differing forms of interpatch relationship, to be
found in real systems (Harrison 1994; Hanski 1996;
Ehrlich and Hanski 2004). Fahrig and Merriam
(1994) identify the following factors as potentially
significant:●differences among the patch populations in terms
of habitat area and quality
●spatial relationships among landscape elements
●dispersal characteristics of the organism of interest
●temporal changes in the landscape structure.These factors can be difficult to capture and may
require rather different variables to be assessed for
different species.
In modelling exercises there is typically just such
a trade-off between greater realism and improved
model fit on the one hand and generality on the
other. In this respect metapopulation models are
no better or worse than most other ecological
models. The value of the metapopulation concept
is that it has led conservation biologists to give
greater attention to the spatial structure and
temporal interdependency of networks of local
populations, which is a clear step forward from
considering them as essentially independent
entities.10.5 Reserve configuration—the ‘Single Large or Several Small’ (SLOSS) debate
Given a finite total area that can be set aside for con-
servation as a natural landscape is being converted
to other uses, what configuration of reserves should
conservationists advocate? At one extreme is the
creation of a single large reserve; the alternative is
to opt for several smaller reserves amounting to the
same area, but scattered across the landscape. This
question, reduced to the acronym SLOSS, was cast
in terms of the assumptions and predictions of the
EMIB (e.g. Diamond and May 1981). As one of the
main aims of conservation is to maximize diversity
within a fragmented landscape, might not island
ecological theory, which focuses on species num-
bers, provide the answer as to the optimal configu-
ration of fragments?THE ‘SINGLE LARGE OF SEVERAL SMALL’ (SLOSS) DEBATE 263