Conservation planning in a changing world
211
the predictive ability of patch area and isolation for
species occupancy. Surprisingly, both area and isolation performed poorly as predictors. Prugh et al . (2008)
concluded that is the type of land cover separating
patches that most strongly affects the sensitivity of
species to patch area and isolation. Thus, although
patch size and isolation are indeed important for the
occupancy of many species, they fi nd, as do Watson
et al . (2005) in their study, that it is crucial to take
account of the properties of the intervening matrix.
Hence, a key conclusion of this work is that improving the quality of matrix may result in ‘ higher conservation returns than manipulating the size and
confi guration of remnant patches for many of the
species that persist in the aftermath of habitat destruction ’ (Prugh et al. , 2008 , p. 20,770).
8.3.2 Metapopulation d ynamics
Plant and animal species are typically patchily distributed; indeed all species ’ ranges involve discontinuities,
and especially so at fi ner scales of analysis. It is frequently possible to discern that within a landscape, a
particular species occupies geographically separated
patches that are interconnected by occasional movements of individuals and gametes. The name for this
network of local populations is a metapopulation .
The fi rst metapopulation models were constructed
by Richard Levins in papers published in 1969 and
1970 (Gotelli, 1991 ). The basic idea can be understood
as follows: imagine that you have a collection of populations, each existing on patches of suitable habitat.
Each patch is separated from other nearby habitat
patches by unsuitable terrain. Although these separate
populations each have their own essentially independent dynamics, as soon as one crashes to a low level, or
indeed disappears, that patch will provide relatively
uncontested space for ‘ surplus ’ individuals from one of
the nearby patches, which will soon colonize the now -
unpopulated patch.
For example, Lei and Hanski (1997) studied metapopulation structure in a threatened species of butterfl y,
Melitaea cinxia , and its specialist parasitoid, Cotesia
melitaearum , in a large network of small habitat
patches. They observed that the incidence of the parasitoid in host populations was positively correlated
with the size of the host population and the area of the
habitat patch. C. melitaearum is thus expected to have
a substantial risk of extinction from patches in which
cautions that the relationship was based on analyses
for species more or less restricted to the habitat patches
considered, and that we should not consider as granted
a similar relationship for generalist species.
If incidence functions really refl ect key controlling
variables, then they might be of great value in designing reserve networks but, if they are found to be inconsistent across the range or through time, they will need
more careful interpretation. Empirical work suggests
that, in practice, they do vary in both time and space.
In illustration, Hinsley et al . ’ s (1996) study of 31 woodland bird species in 151 woods in a lowland arable
landscape in eastern England over three consecutive
years has shown that the incidence functions vary
through time in relation to density - independent mortality (extremes of weather conditions). Additionally,
Hinsley et al . (1996) showed that specialist species
were more likely to disappear from small woods after
severe winter weather than were generalists, and that
they could take more than a year to recolonize. We may
interpret these patterns as refl ecting underlying metapopulation dynamic processes, discussed in the following section.
An illustration that incidence functions might vary
across the range of a species comes from another study
of woodland birds, this time undertaken in Australia
and based on data from three different landscapes
located quite near one another (and thus within the
same biogeographical context and climate regime).
The study, by Watson et al . (2005) , demonstrated that
area - and isolation - based incidence functions differed
signifi cantly, seemingly as a function of differences in
properties of the landscape matrix within which the
woodlands were embedded. The three landscapes were
an urban area, a peri - urban area and a rural (agricultural) landscape.
Interestingly, it was evident that while some species
were able to occupy smaller woodlands within the
rural landscape, others actually showed a higher incidence in small woods in the urban area (within the city
of Canberra itself). This provides some indication of the
diffi culty of designing a reserve network system optimized for all members of the community of interest
(see also Magle et al. , 2009 ).
Recently, Prugh et al . (2008) compiled occupancy
data for 1,015 bird, mammal, reptile, amphibian and
invertebrate populations from 89 case studies, including in total 12,370 habitat patches that were embedded within unsuitable matrix of land cover on six
continents. Using incidence functions, they evaluated
211
the predictive ability of patch area and isolation for
species occupancy. Surprisingly, both area and isolation performed poorly as predictors. Prugh et al . (2008)
concluded that is the type of land cover separating
patches that most strongly affects the sensitivity of
species to patch area and isolation. Thus, although
patch size and isolation are indeed important for the
occupancy of many species, they fi nd, as do Watson
et al . (2005) in their study, that it is crucial to take
account of the properties of the intervening matrix.
Hence, a key conclusion of this work is that improving the quality of matrix may result in ‘ higher conservation returns than manipulating the size and
confi guration of remnant patches for many of the
species that persist in the aftermath of habitat destruction ’ (Prugh et al. , 2008 , p. 20,770).
8.3.2 Metapopulation d ynamics
Plant and animal species are typically patchily distributed; indeed all species ’ ranges involve discontinuities,
and especially so at fi ner scales of analysis. It is frequently possible to discern that within a landscape, a
particular species occupies geographically separated
patches that are interconnected by occasional movements of individuals and gametes. The name for this
network of local populations is a metapopulation .
The fi rst metapopulation models were constructed
by Richard Levins in papers published in 1969 and
1970 (Gotelli, 1991 ). The basic idea can be understood
as follows: imagine that you have a collection of populations, each existing on patches of suitable habitat.
Each patch is separated from other nearby habitat
patches by unsuitable terrain. Although these separate
populations each have their own essentially independent dynamics, as soon as one crashes to a low level, or
indeed disappears, that patch will provide relatively
uncontested space for ‘ surplus ’ individuals from one of
the nearby patches, which will soon colonize the now -
unpopulated patch.
For example, Lei and Hanski (1997) studied metapopulation structure in a threatened species of butterfl y,
Melitaea cinxia , and its specialist parasitoid, Cotesia
melitaearum , in a large network of small habitat
patches. They observed that the incidence of the parasitoid in host populations was positively correlated
with the size of the host population and the area of the
habitat patch. C. melitaearum is thus expected to have
a substantial risk of extinction from patches in which
cautions that the relationship was based on analyses
for species more or less restricted to the habitat patches
considered, and that we should not consider as granted
a similar relationship for generalist species.
If incidence functions really refl ect key controlling
variables, then they might be of great value in designing reserve networks but, if they are found to be inconsistent across the range or through time, they will need
more careful interpretation. Empirical work suggests
that, in practice, they do vary in both time and space.
In illustration, Hinsley et al . ’ s (1996) study of 31 woodland bird species in 151 woods in a lowland arable
landscape in eastern England over three consecutive
years has shown that the incidence functions vary
through time in relation to density - independent mortality (extremes of weather conditions). Additionally,
Hinsley et al . (1996) showed that specialist species
were more likely to disappear from small woods after
severe winter weather than were generalists, and that
they could take more than a year to recolonize. We may
interpret these patterns as refl ecting underlying metapopulation dynamic processes, discussed in the following section.
An illustration that incidence functions might vary
across the range of a species comes from another study
of woodland birds, this time undertaken in Australia
and based on data from three different landscapes
located quite near one another (and thus within the
same biogeographical context and climate regime).
The study, by Watson et al . (2005) , demonstrated that
area - and isolation - based incidence functions differed
signifi cantly, seemingly as a function of differences in
properties of the landscape matrix within which the
woodlands were embedded. The three landscapes were
an urban area, a peri - urban area and a rural (agricultural) landscape.
Interestingly, it was evident that while some species
were able to occupy smaller woodlands within the
rural landscape, others actually showed a higher incidence in small woods in the urban area (within the city
of Canberra itself). This provides some indication of the
diffi culty of designing a reserve network system optimized for all members of the community of interest
(see also Magle et al. , 2009 ).
Recently, Prugh et al . (2008) compiled occupancy
data for 1,015 bird, mammal, reptile, amphibian and
invertebrate populations from 89 case studies, including in total 12,370 habitat patches that were embedded within unsuitable matrix of land cover on six
continents. Using incidence functions, they evaluated