19.3.3.2. Biodiversity Hot Spots
Biodiversity "hot spots" are areas that feature exceptional concentrations
of species, including many endemic species. Unfortunately, many such hot spots
also experience large habitat losses. In addition to a hot spot's economic,
social, and cultural significance to local people, the uniqueness of its biodiversity
and its high share of global biodiversity give the hot spot a global value.
Thus, biodiversity hot spots qualify as unique and threatened entities.
Myers et al. (2000) define a hot spot as an area featuring a biogeographic
unit that contains at least 0.5% of the world's 300,000 vascular plant
species as endemics and has lost 70% or more of its primary vegetation. Table
19-3 shows that two-thirds of the hot spots listed in Myers et al. (2000)
are in the tropics, some of which have the highest percentage of global plants
(6.7%) and as much as 28% of area of habitat with primary vegetation. Arctic
and boreal biomes, however—which are devoid of hot spots—will have
the greatest changes in temperature and precipitation by 2100, whereas the exposure
of nearly all hot spots to a global change of 4°C and/or 30% of precipitation
is ranked only 3 (on a 1 to 5 scale proposed by Sala et al., 2000). With respect
to biome-specific exposure, climate is expected to warm most dramatically at
high latitudes, change least in the tropics, and show intermediate changes in
other biomes. Indeed, Table 19-3 shows that the tropical
hot spots are least vulnerable to climate change and elevated CO2 (0.12 and
0.10, respectively, on a scale of 0 to 1), whereas the eight Mediterranean and
savanna hot spots are at least twice as vulnerable (0.24 and 0.30 for climate
change and elevated CO2, respectively—Sala et al., 2000).
The Cape Floral Kingdom (also called the Cape Floristic Province) and the adjacent
succulent Karoo in South Africa are examples of Mediterranean and savanna biodiversity
hot spots that very much qualify as unique and threatened entities. The Cape
Floral Kingdom is sixth in the world in plant richness of species (5,682 endemic
species—Cowling and Hilton-Taylor, 1997). These hotspots are vulnerable
for the following reasons:
- Their mountains have no permanent snow cover to which high montane species
can retreat as climate warms.
- Montane endemic plants already are concentrated near the peaks, with little
or no possibility for altitudinal expansion.
- Endemics are concentrated in the southwestern corner of Africa, with no
possibility for latitudinal shifts farther south (except for the extreme southern
tip of the continent, which is intensively farmed).
- Increased frequency of fires and drought will affect many short-lived and
fire-sensitive species; seedlings that germinate after fires will be exposed
to successively more extreme climate conditions.
The succulent Karoo flora may be effectively lost with a mean annual temperature
increase of 3-4°C (Rutherford et al., 1999), owing to changing fire
regimes, loss of specialist pollinators, and increased frequency of drought.
Tropic hot spots that are not as sensitive as the Cape Floral Kingdom also will
be seriously affected if other anthropogenic drivers act synergistically (Sala
et al., 2000). Thus, although the hot spot analysis (Myers et al., 2000) indicates
that much of the problem of current and projected mass extinction could be countered
by protection of the 25 hot spots, the ability of these hot spots to be sources
of biodiversity is threatened by climate change.
19.3.3.3. Ecotones
Ecotones are transition areas between adjacent but different environments:
habitats, ecosystems, landscapes, biomes, or ecoclimatic regions (Risser, 1993).
Ecotones that are unique entities in the context of climate change are transition
zones between ecoclimatic regions. Ecotones have narrow spatial extent, a steep
ecological gradient and hence high species richness (Risser, 1993), a unique
species combination, genetically unique populations (Lesica and Allendorf, 1994),
and high intra-species genetic diversity (Safriel et al., 1994).
| Table 19-3: Sensitivity of biodiversity hot spots
(Myers et al., 2000; Sala et al., 2000). |
 |
| Biome |
Number of
Hotspots
Biome
|
% of
Global
Plants
(range)
|
% of
Remaining
Habitats with
Primary
Vegetation
(range)
|
Impact by 2100
(of a large change
in driver, scale 1-5)
|
Effect by 2100
(expected change in
driver x impact, scale 0-1)
|
|
of climate
change
|
of elevated
CO2
|
of climate
change
|
of elevated
CO2
|
|
| Tropical forests |
15 |
0.5-6.7 |
3-28 |
3 |
1 |
0.12 |
0.10 |
Mediterranean
|
5 |
0.7-4.3 |
5-30 |
3 |
2 |
0.24 |
0.20 |
Savanna, grassland
|
3 |
0.6-1.5 |
20-27 |
3 |
3 |
0.23 |
0.30 |
| North temperate forest |
2 |
0.5-1.2 |
8-10 |
2 |
1.5 |
0.17 |
0.15 |
|
Ecotones affect distant and larger areas: They regulate interactions between
biomes by modifying flows between them (Johnston, 1993; Risser, 1993); they
generate evolutionary diversity (Lesica and Allendorf, 1994); and they serve
as repositories of genetic diversity to be used for rehabilitation of ecosystems
in adjacent ecoclimatic regions if and when these ecosystems lose species because
of climate change (Volis et al., 1998; Kark et al., 1999). Conservation of ecotone
biodiversity therefore is an adaptation. Finally, although ecological changes in
response to climate change will occur everywhere, the signals will be detectable
first in ecotones (Neilson, 1993). This sensitivity makes them indicators that
provide early warning for other regions (Risser, 1993).
Although ecotones are unique in provision of climate change-related services,
they are threatened. Conservation traditionally is aimed at "prime"
core areas of biomes rather than ecotones. Even conservation efforts that are
directed at ecotones may not suffice, however: 47-77% of the areas of biosphere
reserves are predicted to experience change in ecosystem types, compared to
only 39-55% of the total global terrestrial area that will undergo such
changes (Leemans and Halpin, 1992; Halpin, 1997).
An example of a threatened ecotone is the desert/nondesert ecoclimatic transition
zone—the semi-arid drylands sandwiched between arid and the dry subhumid
drylands (Middleton and Thomas, 1997). Semi-arid drylands are prone to desertification,
expressed as irreversible loss of soil productivity because of topsoil erosion
(see Section 11.2.1.4). Already affected by extreme soil
degradation are 67 Mha of semi-arid drylands (2.9% of global semi-arid area)—nearly
as much as affected dry-subhumid drylands (28 Mha, 2.2%) and arid drylands (43
Mha, 2.7%—Middleton and Thomas, 1997). This degradation is destroying the
habitats of the biodiversity assets of these ecotones, including those to be
conserved as an adaptation to climate change (Safriel, 1999a,b).
Climate change is expected to exacerbate desertification (see Section
11.2.1.4; Schlesinger et al., 1990; Middleton and Thomas, 1997). Reduced
precipitation and increased evapotranspiration will change ecotones' spatial
features (e.g., coalescence of patches at one side and increased fragmentation
at the other—Neilson, 1993). Furthermore, overexploitation of vegetation
that is typical in semi-arid drylands (UNDP, 1998; ICCD, 1999), in synergy with
climate change, will further increase habitat loss and hence loss of biodiversity,
ecosystem services, and the potential for adaptation. Similar synergies between
climate change effects and other anthropogenic impacts are projected for alpine
ecotones (Rusek, 1993). To conclude, ecotones between biomes and within climatic
transition areas are unique entities; they are important for monitoring climate
change and for adapting to climate change, yet they are highly threatened by
climate change interacting with other anthropogenic stresses.
