islands, thereby making such sites less suitable for,
or removing altogether, particular species
(McGuinness 1984; Simberloff and Levin 1985;
Dunstan and Fox 1996).
10.7 Relaxation and turnover—the evidence
If habitat islands behave according to the expecta-
tions of the EMIB, they should be supersaturated
with species immediately after system fragmenta-
tion. Subsequently, species numbers in the isolates
should ‘relax’ to a lower level (Fig. 10.5). Immi-
gration and extinction should continue to occur,
both during the relaxation period and subse-
quently, when the island has found its new, lower
equilibrium richness level. The time taken for relax-
ation to occur is called the ‘lag time’ and the antici-
pated eventual species loss is termed the extinction
debt(Ewers and Didham 2005). If these anticipated
effects are strong, even high-priority species—the
very species that you wish to conserve—may in
time disappear from a reserve. As we have noted,
however, the empirical evidence from a wide range
of real and habitat islands is that turnover tends to
be heterogeneous (i.e. structured). Some species
tend to be highly stable in their distribution across
habitatislands. Most turnover involves ‘ephemerals’,
species marginal to the habitat, or successional
change (Chapters 4–6). The metapopulation studies
examined above provide further insights into
turnover. They suggest a tendency for large
populations to persist, while smaller satellite popu-
lations may come and go, without jeopardizing the
metapopulation. Nonetheless, species do disappear
from particular habitat islands, and from entire
landscapes, and in cases the loss is a global one.
Earlier, we noted that many species losses are
attributable to deterministic causes, meaning hunt-
ing, habitat changes, and the like. The relaxation
effect, in contrast, is based on stochastic processes.
How strong is this island relaxation effect?
One of the best-known examples of relaxation on
ecological timescales is that of birds lost from Barro
Colorado island, in Panama. It is not an ideal study
in the present context in that, unlike most habitat
islands, it is actually now surrounded by water. The
island was formerly a hilltop in an area of continu-
ous terrestrial habitat, but it became a 15.7 km^2
island of lowland forest when the central section of
the Panama Canal Zone was flooded to make LakeRELAXATION AND TURNOVER—THE EVIDENCE 269Table 10.2Classes of edge-related changes triggered by the process of forest fragmentation, as informed by the
Minimum Critical Size of Ecosystem project. The first-order effects may lead to second-order and, in turn, third-order
knock-on effects (modified from: Lovejoy et al. 1986)
Class Description of change
Abiotic Temperature
Relative humidity
Penetration of light
Exposure to wind
Biological
First-order Elevated tree mortality (standing dead trees)
Treefalls on windward margin
Leaf-fall
Increased plant growth near margins
Depressed bird populations near margins
Crowding effects on refugee birds
Second-order Increased insect populations (e.g. light-loving butterflies)
Third-order Disturbance of forest interior butterflies, but increased population of light-loving species
Enhanced survival of insectivorous species at increased densities (e.g. tamarins)*
*Does not apply to birds (Stouffer and Bierregaard 1995).