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the forest edges than in the interior because of
higher densities of nest predators—e.g. blue jay
(Cyanocitta cristata), weasel (Mustela erminea), and
racoon (Procyon lotor)—around forest edges.
Wilcove et al. (1986) estimated that as a result,
reserves of less than 100 ha will not support viable
populations of forest songbirds. A few studies of
birds and invertebrates have suggested that some
species prefer edge habitats for breeding even
though mortality rates at edges may be higher, a
phenomenon termed an ecological trap(Ewers and
Didham 2005). Paton (1994) has undertaken a
critical review of studies of nest predation in edge
habitats. He concluded that, in general, there does
appear to be evidence for a depression of breeding
success, due either to enhanced predation or
through parasitism. However, these forms of edge
effect usually operate only within about 50 m of an
edge, a narrower belt than some have claimed (e.g.
Terborgh 1992). Nonetheless, such an effect implies
that the effective reserve area may be less than that
described by the perimeter of the isolate.
Edge effects may be taxon or even system
specific. For instance, Burkey (1993) undertook an
experimental study of egg and seed predation with
distance from the edge of a patch of Belizian rain
forest. Egg predation rates were higher in a 100 m
edge zone, but, conversely, seed predation rates

were found to be higher 500 m into the forest than

30 m and 100 m from the edge. In contrast to

Burkey’s egg predation data, Delgado et al. (2005)

report higher predation pressure under closed

canopy than within edge habitats in their study of

artificial nest predation by ship rat (Rattus rattus) in

laurel forest on Tenerife. This illustrates that the

relationship between a reserve and its surrounding

matrix is not subject to easy generalization. There

are species that share both zones, and just as there

are matrix species which may impact negatively

upon core reserve species, there may also be reserve

species (dependent on it for breeding and cover)

which exploit resources in the matrix. The concepts

of source and sink populations may again be useful

in this context, as the maintenance of maximal

diversity across a landscape depends on sufficient

source or core habitat for species of each major

habitat. The identification of edge effects provides a

part of such an analysis.

10.11 Landscape effects, isolation, and corridors

The benefits of wildlife corridors

The premise of much of the literature discussed in

this chapter is that habitat connectivity is beneficial

to long-term survival, as it enables gene flow

within populations and metapopulations. Habitat

connectivity might be achieved by having stepping

stones, or corridors, of suitable habitat linking

larger reserves together. In practice, habitat corri-

dors act as differential filters, enabling the move-

ment of some species but being of little value to

others. As Spellerberg and Gaywood (1993) point

out, we are not merely concerned with forest penin-

sulas or hedgerows. All sorts of linear landscape

features, such as rivers, roads, and railways, may

act as conduits for the movement of particular

species. Equally, they may represent barriers or

hazards for others (Reijnen et al. 1996). Harris (1984)

points out that landscapes with great topographic

relief channel their kinetic energy via dendritic trib-

utaries, main channels, and distributaries, and that

these in turn influence the landscape template in

characteristic ways. The stream-order concept of

278 ISLAND THEORY AND CONSERVATION

Area (Log, ha)

0.1 1 10 100 1000

Species Richness

0

10

20

30

40

50

60

70

R^2 = 0.88

R^2 = 0.92

R^2 = 0.78

Figure 10.9Bird species richness–area relationships for littoral
forests, southeastern Madagascar. Closed circles (and bold, solid line):
all bird species; open circles: forest dependent bird species, fitted by
linear regression (solid line), and by break-point regression (broken
line), demonstrating a small-island effect. (From Watson et al. 2004b.)