Gatun in 1914. Of about 208 bird species estimatedto
have been breeding on Barro Colorado island
immediately following isolation in the 1920s and
1930s, 45 had gone by 1970 (Wilson and Willis
1975). Of these, some 13 have been attributed to
‘relaxation’.
The other losses could be attributed to particular
forms of ecological change. At the time of the analy-
sis, much of the forest was less than a century old,
following abandonment of farming activity before
- Many of the birds lost were typical of second
growth or forest-edge, suggesting a probable
successional mechanism as forest regeneration
reduced the availability of these more open habi-
tats. Some were ground nesters, which may have
been eliminated by their terrestrial mammalian
predators. The latter became abundant because of
the disappearance of top carnivores (such as the
puma) with large area requirements (Diamond
1984). This effect, of increasing numbers of smaller
omnivores and predators in the absence of large
ones, has been termed mesopredator release(Soulé
et al. 1988), and it has now been documented in
several other systems (Fig. 10.7; Crooks and Soulé
1999; Laurance 2002). Some of the birds lost were
members of the guild of ant followers, which have
been found to be vulnerable to fragmentation
elsewhere(below). In a simple sense, Barro Colorado
island provides evidence of relaxation, but the
stochastic signal is seemingly much smaller than
that produced by changing habitat and food-web
relationships, and it might therefore be asked
‘which is the signal and which the noise in the
system?’ (cf. Sauer 1969; Lynch and Johnson 1974;
Simberloff 1976).
That species numbers and composition may
change as a consequence of fragmentation is not in
dispute. However, it is remarkably difficult to find
good quantitative data for the island relaxation
effect for systems of habitat islands (Simberloff and
Levin 1985; and see review in Shafer 1990). Most
studies, including that for Barro Colorado island,
lack precise information about the species composi-
tion before fragmentation (e.g. Souléet al. 1988).
The Barro Colorado island study is also fairly typi-
cal in the losses recorded being mostly ‘determinis-
tic’ in nature. The predictions of species loss
through ‘relaxation’ derive from the EMIB, but the-
oretically relaxation may also affect metapopula-
tions. Hanski et al. (1996) speculate that the pace of
fragmentation in many regions has been so rapid
that ‘scores of rare and endangered species may
already be “living dead”, committed to extinction
because extinction is the equilibrium toward which
their metapopulations are moving in the present
fragmented landscapes’ (p. 527). However, their
findings are heavily dependent on models of the
data of a single species of butterfly, the Glanville
fritillary (Melitaea cinxia). The effect identified
appeared to be far from instantaneous, and might
turn out in practice to be a relatively weak effect in
relation to some of the other processes discussed in
this chapter.
A classic illustration both of genuine reasons for
concern and of the uncertainty involved in projec-
tions is provided by the tropical moist forests of the
Atlantic seaboard of Brazil, which have been
reduced over the past few centuries to only about
7% of their estimated former cover (Ribon et al.
2003). Based on the ‘rules’ derived from island
theory, some 50% of species are expected to go
extinct. To date, no extinctions have been docu-
mented with certainty (Whitmore and Sayer 1992;270 ISLAND THEORY AND CONSERVATION
Fragment
areaFragment age
(time since
isolation)+
+––
–
–
Figure 10.7Model of the combined effects of trophic cascades and
island biogeographical processes on top predators (for example,
coyote), mesopredators (domestic cat) and prey (scrub-breeding birds)
in a fragmented system. Direction of the interaction is indicated with
a plus or minus. (From Crooks and Soulé 1999.)