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.

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