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The overall mobility of wildlife and the fact that they are physiologically
constrained by temperature and moisture make them effective indicators of climatic
changes (Root, 1988a; Parmesan et al., 2000). Evidence is presented below
that shows that many different taxa from around the world already are exhibiting
recognizable changes, such as poleward and elevational range shifts and changes
in the timing of events such as breeding (see Table
5-3). Many factors (e.g., habitat conversion, pollution) pressure animals
(see Figure 5-1). The information that follows indicates
that the changes from such pressures could result in patterns that differ from
those created by rapid climate change, which are created by the physiological
constraints of organisms in response to climatic variables. For example, the
pattern many species are showing of general poleward movement is not likely
to be created by habitat conversion because such conversion generally does not
occur less frequently along the poleward sides of many species ranges (e.g.,
along the northern boundaries in the northern hemisphere) than along those nearer
the equator. Consequently, the balance of evidence suggests that, for animals
that are exhibiting significant large-scale patterns of changes, the most consistent
explanation is recent climatic change. Thus, like the proverbial "canaries in
the coal mine," wildlife seem to be providing an important early indicator of
how ecosystems might respond to the discernible human impact on climate that
is contributing to its change (Santer et al., 1996).
Much of the early work on the effects of climate change on ecosystems focused
on vegetation. Animals (nondomestic animals or wildlife—these terms are
used interchangeably) are important members of ecosystems and are affected by
weather and climate (Andrewartha and Birch, 1954). Consequently, concerns about
the impacts that rapid climatic change may have on wildlife and the risks these
changes may impose on ecosystem services are assessed and summarized in this
section.
Recent estimates indicate that 25% (~1,125 species) of the world's mammals
and 12% (~1,150 species) of birds are at a significant risk of global extinction
(Stattersfield et al., 1998; UNEP, 2000). One indicator of the magnitude
of this problem is the speed at which species at risk are being identified.
For example, the number of birds considered at risk has increased by almost
400 since 1994, and current population sizes and trends suggest an additional
600-900 soon could be added to these lists (IUCN, 1994; UNEP, 2000). The number
of animals threatened with extinction varies by region (see Table
5-5). Global patterns of total diversity are reflected in the number of
species at risk in each region, in that areas with more total species are likely
to have more at risk. The number of threatened invertebrates in Table
5-5 is unrealistically low. The extinction rate of invertebrates in tropical
forests alone has been estimated at 27,000 yr-1, largely because
of habitat conversion (Wilson, 1992).
Extinction often is caused by a combination of pressures acting over time (Wilson,
1992). Three traits of species populations that contribute to endangerment status
are range size, distribution of suitable habitat within the range, and population
size. Species that are most at risk often have small ranges, inhabit a unique
type of habitat or one found in isolated areas (patchy in distributions), and/or
typically occur at low population densities (Rabinowitz, 1981; Rabinowitz et al., 1986). Using these criteria, signs that a species may be at risk include
shrinking range, decreased availability of habitat within the range, and local
or widespread population declines.
Species with restricted habitat requirements typically are most vulnerable
to extinction (Pimm et al., 1995), including many endemic species that
could be lost with loss of their habitat (Wyman, 1991; Bibby et al.,
1992; Stattersfield et al., 1998). For example, the Sundarban, the only
remaining habitat of Bengal tigers (Panthera tigris tigris) in Bangladesh, is
projected to decrease considerably in size as a result of rising sea levels;
Milliman et al. (1989) estimate a loss of 18% of the land by 2050 and
as much as 34% by 2100. For tigers and the many other species that inhabit these
forested wetland habitats, migration to higher ground probably would be blocked
by human habitation of adjacent lands (Seidensticker, 1987; ADB, 1994). Many
mountainous areas also have endemic species with narrow habitat requirements
(Dexter et al., 1995; Stattersfield et al., 1998). With warming,
habitats may be able to move up in elevation if the mountain is high enough.
If not, the habitat could be lost (Still et al., 1999). Some montane
species that are susceptible to this change include forest birds in Tanzania
(Seddon et al., 1999), Resplendent Quetzal (Pharomachrus mocinno) in
Central America (Hamilton, 1995), mountain gorilla (Gorilla gorilla beringei)
in Africa, and spectacled bear (Tremarctos ornatus) in the Andes (Hamilton,
1995, and references therein). Protecting species that currently are vulnerable,
endangered, or critically endangered (see Table 5-5)
requires measures that, in general, reverse the trend toward rarity. Without
management, there is high confidence that rapid climate change, in conjunction
with other pressures, probably will cause many species that currently are classified
as critically endangered to become extinct and several of those that are labeled
endangered or vulnerable to become much rarer, and thereby closer to extinction,
in the 21st century (Rabinowitz, 1981).
Concern that species will become rare or extinct is warranted because of the
goods and services provided by intact ecosystems and the species themselves.
Most of the goods and services provided by wildlife (e.g., pollination, natural
pest control; see Table 5-1) derive from their roles
within systems. Other valuable services are provided by species that contribute
to ecosystem stability or to ecosystem health and productivity. The recreational
value (e.g., sport hunting, wildlife viewing) of species is large in market
and nonmarket terms. Losses of species can lead to changes in the structure
and function of the affected ecosystems, as well as loss of revenue and aesthetics
(National Research Council, 1999).
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