were most parsimoniously explained by reference
to the roles of introduced predators (foxes and cats)
and herbivores (rabbits and stock). This explained
the continued persistence of the native species on
offshore islands from which the exotic species were
absent. In these cases, isolation is beneficial
(as Janzen 1983) and many reserves set in ‘seas of
altered habitat’ within mainland Australia have
proved to be insufficiently isolated to prevent their
invasion by exotic pests and pathogens.
Corridors or isolation?
Is it possible to offer guidance about the situations
when corridors may be beneficial and when not?
First, as Dawson (1994) notes, all-purpose corridors
do not exist, as each species has its own require-
ments for habitat, its own ability to move, and its
own behaviour. Corridors, like other landscape
components, therefore act as filters. He points out
that many rare and threatened species are unlikely
to benefit from corridors, because the corridor
would have to contain their presumably rare habi-
tat, i.e. rare species may require odd corridors.
Some corridors are essential in providing links
between preferred habitats for animals that under-
take regular seasonal migrations, e.g. some fish,
amphibians, reptiles, and for large mammals in
both the seasonal tropics and the Arctic. On the
other hand, for some populations, corridors may
act as ‘sinks’, drawing out individuals from the
main habitat area but not returning individuals to
supplement it, in which case they may do more
harm than good. In other cases, they may be fairly
neutral in their ecological cost–benefit, but perhaps
be quite expensive to purchase and set up if not
already existing in a landscape.
Simberloff et al. (1992) point out that numerous
corridor projects have been planned in the USA,
potentially costing millions of dollars, despite the
lack of data on which species might use the corri-
dors and to what effect. Once again, the cost–-
benefit equation for corridors may depend on
properties of the system involved, including scale
considerations and assessment of the major drivers
of species loss. In the longer term, it has been
argued that climate change is likely to drive sub-
stantial shifts in the distribution of species, and that
the resulting species migrations will be impeded by
the human sequestration of land to agriculture and
other purposes. Hence, on these grounds, it would
seem to be prudent to plan more or less continuous
habitat corridors that straddle major climatic/
elevational gradients where this is feasible (e.g.
Bush 1996, 2002).Reserve systems in the landscape
As we have seen, the debate about habitat corridors
has broadened from fairly simple theoretical begin-
nings to a consideration of how a whole range of
differing natural, semi-natural, and artificial
features are configured within a landscape. For
instance, countless thousands of birds are killed
each year through collision with motor vehicles
and with overhead power cables (Bevanger 1996;
Reijnenet al. 1996). Research has shown that deaths
through collisions with cables can be greatly
reduced by consideration of important flight paths
during construction, by design features of the
gantries, and by attaching a variety of objects to the
cables to enable birds to sight them (Alonso et al.
1994). Paved roads and tracks can play an impor-
tant role as corridors favouring the introduction of
alien plants (Arévalo et al. 2005) or animals
(Delgadoet al. 2001) into otherwise well-preserved
ecosystems, but also creating corridors of suitable
habitat that can in cases allow endemic species (e.g.
lizards) to disperse through inhospitable forest
matrix, enabling genetic exchange between other-
wise isolated populations (Delgado et al. in press).
On the other hand, wide roads and, especially,
fenced highways, may act to impede the move-
ments of large terrestrial animals, thus fragmenting
populations and interfering with migration. Such
information needs to be integrated into improved
management of whole landscapes (Spellerberg and
Gaywood 1993). In short, a landscape ecological
framework is needed.
Landscape components include both habitat
patches and the matrix in which the patches are
embedded. Modelling exercises typically favour
clumped configurations of reserves, treating the
matrix as a uniform ‘sea’. In practice, the optimalLANDSCAPE EFFECTS, ISOLATION, AND CORRIDORS 281