Conservation planning in a changing world
193
answers to questions like SLOSS are likely to exhibit
scale dependency. For example, large mammals, with
their large home ranges or territories and their vulnerability to human hunting, are likely to require vastly
larger reserves than are needed to capture viable populations of butterfl ies. The former are also likely to have
less specifi c habitat requirements than the latter, so
reserve systems designed for one are likely to be suboptimal for the other.
Another aspect of scale is the range of sizes and
inter - reserve distances. Imagine that you were distributing reserves across East Lincolnshire (a low - lying
and rather fl at part of the UK). The range in reserve
sizes and distances involved in these generally agricultural landscapes would be small, and one would not
expect to encounter different species pools. Imagine, by
way of contrast, that you were distributing reserves
across a large - scale biogeographical gradient such as
the Mexican transition zone, i.e. the transition zone
between the Nearctic and Neotropical regions. Here we
might be contemplating rather larger reserves and
potentially very large distances between them, and different reserves might contain contrasting proportions
of species ultimately derived from different biogeographical regions/provinces.
The falsifi cation of the assumption of a single species
pool that is inherent in the ETIB undermines the application of this theory to resolve SLOSS at such a scale.
Instead, we may turn to the direct analyses of species ’
distributional data discussed in the previous two chapters for reserve network planning in such a context.
Hence, although some authors have attempted to
apply the island analogy on these very coarse scales
(e.g. Brooks et al. , 1997 , 2002 ), as we demonstrate
below, applied island biogeography is essentially a
framework for application at local to landscape scales
(Whittaker & Fern á ndez - Palacios, 2007 ).
The SLOSS debate illustrates that although ETIB provides a basic conceptual model for understanding
habitat fragmentation, apart from a number of broad
generalizations that are largely ecological ‘ good sense ’
anyway (e.g. many large refuges hold important species
that small ones do not, ecologically heterogeneous
refuges tend to hold more than homogeneous ones,
habitat connectivity can often be benefi cial in terms of
species richness), generating policy - relevant guidance
from a broad - brush macroecological theory is not
straightforward.
In this chapter, we review some of the more interesting themes within applied island biogeography, starting with the most basic general question: is it realistic
species. Hence, decisions about corridors should be case -
specifi c (see Whittaker & Fern á ndez - Palacios, 2007 )
1
.
One of the most hotly discussed conservation topics
of the 1970s and 1980s was the so - called SLOSS
debate, which posed the question: ‘ Given the opportunity to put a fi xed percentage of land into conservation
use, is it better to opt for a Single Large Or Several Small
reserves? ’ At one extreme is the creation of a single
large reserve; the alternative is to opt for several smaller
reserves that amount to the same area but which are
scattered across the landscape.
The answer to the SLOSS question is by no means
simple. Crucially, it depends on the slope of the species –
area curve, the proportion of common species in the
small reserves and the gradient of colonizing abilities
among species in the available pool of species. Indeed,
both theoretical analyses and empirical evidence
suggest that, in some circumstances, several small
reserves may contain more species than a single large
one. This is due to compensating advantages such as:
greater overall representation of rare habitats; more
effective representation of differing biogeographical
elements across a region; competitive effects involving
there being different ‘ winners ’ in different patches; less
effective spread of disease and exotic species; and more
habitat for edge species (e.g. Soul é & Simberloff, 1986 ;
Zimmerman & Bierregaard, 1986 ; Godefroid &
Koedam, 2003 ).
Moreover, the initial debate over SLOSS generally
overlooked the complexity of species diversity dynamics. Factors such as the minimum viable population
(MVP) for rare/ecologically important species, the
minimum area needed in order to sustain an MVP and
the minimum dynamic area to maintain the ecosystem
integrity must also be considered in questions concerning nature conservation (Soul é & Simberloff, 1986 ;
Shafer, 1990 ; Wu & Vankat, 1995 ). It is now generally
accepted that the species – area relationship and the
equilibrium theory of island biogeography – in part an
attempt to explain systematic variations in the form of
the species – area relationship – are unable to provide
fi nal resolution to the SLOSS question.
There are several broad explanations for the diffi -
culty of extracting generalities from the study of
habitat islands, and perhaps the most important is that
1 While in traditional biogeography the term corridor is given to a connection that allows essentially free passage of a particular biota (cf.
Chapter 7 ; and see also: Lomolino et al. , 2006 ), research within the
present frame of reference on ecological habitat corridors can refer to
very narrow connecting features, which may be highly selective in
terms of the species that can move along them.
193
answers to questions like SLOSS are likely to exhibit
scale dependency. For example, large mammals, with
their large home ranges or territories and their vulnerability to human hunting, are likely to require vastly
larger reserves than are needed to capture viable populations of butterfl ies. The former are also likely to have
less specifi c habitat requirements than the latter, so
reserve systems designed for one are likely to be suboptimal for the other.
Another aspect of scale is the range of sizes and
inter - reserve distances. Imagine that you were distributing reserves across East Lincolnshire (a low - lying
and rather fl at part of the UK). The range in reserve
sizes and distances involved in these generally agricultural landscapes would be small, and one would not
expect to encounter different species pools. Imagine, by
way of contrast, that you were distributing reserves
across a large - scale biogeographical gradient such as
the Mexican transition zone, i.e. the transition zone
between the Nearctic and Neotropical regions. Here we
might be contemplating rather larger reserves and
potentially very large distances between them, and different reserves might contain contrasting proportions
of species ultimately derived from different biogeographical regions/provinces.
The falsifi cation of the assumption of a single species
pool that is inherent in the ETIB undermines the application of this theory to resolve SLOSS at such a scale.
Instead, we may turn to the direct analyses of species ’
distributional data discussed in the previous two chapters for reserve network planning in such a context.
Hence, although some authors have attempted to
apply the island analogy on these very coarse scales
(e.g. Brooks et al. , 1997 , 2002 ), as we demonstrate
below, applied island biogeography is essentially a
framework for application at local to landscape scales
(Whittaker & Fern á ndez - Palacios, 2007 ).
The SLOSS debate illustrates that although ETIB provides a basic conceptual model for understanding
habitat fragmentation, apart from a number of broad
generalizations that are largely ecological ‘ good sense ’
anyway (e.g. many large refuges hold important species
that small ones do not, ecologically heterogeneous
refuges tend to hold more than homogeneous ones,
habitat connectivity can often be benefi cial in terms of
species richness), generating policy - relevant guidance
from a broad - brush macroecological theory is not
straightforward.
In this chapter, we review some of the more interesting themes within applied island biogeography, starting with the most basic general question: is it realistic
species. Hence, decisions about corridors should be case -
specifi c (see Whittaker & Fern á ndez - Palacios, 2007 )
1
.
One of the most hotly discussed conservation topics
of the 1970s and 1980s was the so - called SLOSS
debate, which posed the question: ‘ Given the opportunity to put a fi xed percentage of land into conservation
use, is it better to opt for a Single Large Or Several Small
reserves? ’ At one extreme is the creation of a single
large reserve; the alternative is to opt for several smaller
reserves that amount to the same area but which are
scattered across the landscape.
The answer to the SLOSS question is by no means
simple. Crucially, it depends on the slope of the species –
area curve, the proportion of common species in the
small reserves and the gradient of colonizing abilities
among species in the available pool of species. Indeed,
both theoretical analyses and empirical evidence
suggest that, in some circumstances, several small
reserves may contain more species than a single large
one. This is due to compensating advantages such as:
greater overall representation of rare habitats; more
effective representation of differing biogeographical
elements across a region; competitive effects involving
there being different ‘ winners ’ in different patches; less
effective spread of disease and exotic species; and more
habitat for edge species (e.g. Soul é & Simberloff, 1986 ;
Zimmerman & Bierregaard, 1986 ; Godefroid &
Koedam, 2003 ).
Moreover, the initial debate over SLOSS generally
overlooked the complexity of species diversity dynamics. Factors such as the minimum viable population
(MVP) for rare/ecologically important species, the
minimum area needed in order to sustain an MVP and
the minimum dynamic area to maintain the ecosystem
integrity must also be considered in questions concerning nature conservation (Soul é & Simberloff, 1986 ;
Shafer, 1990 ; Wu & Vankat, 1995 ). It is now generally
accepted that the species – area relationship and the
equilibrium theory of island biogeography – in part an
attempt to explain systematic variations in the form of
the species – area relationship – are unable to provide
fi nal resolution to the SLOSS question.
There are several broad explanations for the diffi -
culty of extracting generalities from the study of
habitat islands, and perhaps the most important is that
1 While in traditional biogeography the term corridor is given to a connection that allows essentially free passage of a particular biota (cf.
Chapter 7 ; and see also: Lomolino et al. , 2006 ), research within the
present frame of reference on ecological habitat corridors can refer to
very narrow connecting features, which may be highly selective in
terms of the species that can move along them.