Life-history theory and management - Ecological Applications: Toward a Sustainable World - page 73

from the worst effects of habitat disruption - a pattern analogous to that for
Angermeier's (1995) fi sh.
Habitat fragmentation is another major consequence of human behavior - what
remains of previously extensive habitat has become increasingly fragmented into
isolated pockets. This has consequences for many but not all species and it is rea-
sonable to ask whether vulnerability to habitat fragmentation can be linked to par-
ticular life-history traits. Henle et al. (2004) addressed this question by analyzing
accumulated information for diverse plant and animal taxa, both from controlled
fragmentation experiments and the many uncontrolled 'natural' experiments docu-
mented around the world. Habitat specialists might be expected to have a higher
extinction risk because the chance that their niche will be represented in a remain-
ing fragment should be smaller than for generalists. Studies on highly endangered
species like the red-cockaded woodpecker ( Picoides borealis ) (Haig et al., 1993) show
that the absence of suitable microhabitat is a principle factor determining their
sensitivity to forest fragmentation. This kind of habitat specialization also turns out
to be a risk factor, in relation to habitat fragmentation, for forest trees, birds,
mammals and reptiles.

3.4.2 When big isn't
best - r/K theory,
harvesting, grazing
and pollution

A pattern that has repeatedly emerged is that extinction risk tends to be highest for
species with a large body size. Figure 3.5 illustrates this for Australian marsupials
that have gone extinct in the last 200 years or are currently considered endangered.
Again the human pressures have been diverse, including hunting, habitat degrada-
tion and habitat destruction. Some climatic regions (arid compared to mesic zone)
and some taxa (e.g. potoroos, bettongs, bandicoots and bilbies) have experienced
higher extinction/endangerment rates than others, but the strongest relationship is
between body size and risk of extinction (Cardillo & Bonham, 2001). Recall that
body size is part of the life-history syndrome (essentially r / K ) that associates large
size, late maturity and small reproductive allocation (Box 3.1). In fact, you can blame
the vulnerability of larger species on their relatively low reproductive rate - they

Fig. 3.5 Body-size
frequency distribution
of the Australian
terrestrial marsupial
fauna including 25
species that have gone
extinct in the last 200
years (black). Sixteen
species currently
considered endangered
are shown in dark grey.
(After Cardillo &
Bonham, 2001.)

40

Other species
Endangered
Extinct

35

30

25

20

15

10

5

0
0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6 3.9 4.2 4.5 4.8

Log 10 body weight (g)