the fluvial geomorphologist may be a means of
understanding species distributions within the
landscape (Fig. 10.10), and of understanding move-
ments of species between disjunct habitat patches
(Fig. 10.11). Studies of features such as hedgerows
have not always found them to be as effective in
connecting up woodlands as was hoped, with some
species moving as well through surrounding fields
and others simply not moving well along the
hedges. However, habitat corridors may be impor-
tant in particular cases.
A useful illustration of how corridors can be ben-
eficial comes from the study by Saunders and
Hobbs (1989) of Carnaby’s cockatoo (Calyptorhyncus
funereus latirostrus) from the Western Australian
wheat belt, an area of 140 000 km^2 in the south-west
of the state, 90% of which has been cleared for
agriculture. The Carnaby’s cockatoo is one of
Australia’s largest parrots and was once the most
widely distributed cockatoo in the region. It breeds
in hollows in eucalypt trees and eats seeds and
insect larvae from plants in the sand-plain heath. It
congregates in flocks, both to nest and forage.
Individuals may live 17 years and breeding status is
not attained until at least 4 years of age, each pair
rearing a single offspring at a time. The widespread
clearance of the land has removed extensive areas of
the vegetation type in which they feed, replacing it
with annual crops that are useless to the birds. In
recent developments, wide verges of native vegeta-
tion have been left uncleared along the roads. These
act to channel the cockatoos to other areas where
food is available.
Cockatoos have not survived in areas of earlier
clearances, carried out without these connecting
strips, as once they have run out of a patch of
acceptable habitat it takes a long time for the flock
to find another patch of native vegetation. The
more patchy the vegetation, the less successful the
birds are at supplying adequate food to rear their
nestlings. Furthermore, the narrow road verges of
these early clearances result in a higher incidence of
road deaths. The example illustrates that the degree
and nature of connectivity of different landscape
elements—in this case of the breeding and feeding
habitats, and of the vehicular hazards—are critical
to the survival of this species in these newly
fragmented landscapes. Given the relatively long
life cycle of these birds, and their flocking behav-
iour, failure can have a sudden and wholesale
expression in the bird’s disappearance. The big
reduction in area is fairly recent, and the cockatoo is
not yet in equilibrium with the new regime. It is
still on a downward path and is viewed as a species
in danger.The benefits of isolation
Although much of the literature is concerned with
maintaining population movements, there are also
contexts in which isolationof populations of a target
species might be desirable (Simberloff et al. 1992),
e.g. where there is a threat from a disease organism.
Young (1994) evaluated 96 studies of natural
die-offs of large mammal populations, defined as
cases where numbers crashed by 25% of the
individuals or more. He noted that populationsLANDSCAPE EFFECTS, ISOLATION, AND CORRIDORS 279Figure 10.10Association of different-sized carnivorous mammal
species with stream order and typical food particle size in accordance
with the stream-continuum concept. (Redrawn from Harris 1984,
Fig. 9.8.).