As discussed in Section 19.2, change in climate already appears to be affecting many biological systems. Continued climate changes can threaten a large number of unique biological systems. This section identifies specific characteristics of some of the most unique and threatened systems, which explain why many are at risk from climate change. In addition, some specific examples of unique and threatened biological systems are presented. Many others also are threatened by climate change; these are discussed in detail in other chapters of this report. Examples of natural systems that may be threatened include montane ecosystems that are restricted to upper 200-300 m of mountainous areas, prairie wetlands, remnant native grasslands, coldwater and some coolwater fish habitat, ecosystems that overlie permafrost, and ice-edge ecosystems that provide habitat for polar bears and penguins. Examples of species that may be threatened by changes in climate include forest birds in Tanzania, the resplendent quetzal in Central America, the mountain gorilla in Africa, amphibians that are endemic to the cloud forests of the neotropics, and the spectacled bear of the Andes.

19.3.3.1. Risks to Species and Ecosystems

Laboratory and field studies have demonstrated that climate plays a strong role in limiting the ranges of species and ecosystems. Species already are responding to changes in regional climate, with altered population sizes and breeding times or flowering dates that occur earlier in the season (see Chapter 5). These responses suggest that many unique species will undergo complex changes with a few degrees of warming, which could lead to extinction in many locations. Such species can be found across various regions (see Table 19-1). Other chapters in this report list many examples (see Table 19-2). However, projecting possible responses of wild animal and plant species is extremely difficult for most species because there are many possible biological interactions and confounding factors, such as habitat destruction and invasive species.

Species that make up a natural community, however, most likely will not shift together (Davis, 1986; Overpeck et al., 1994; Root, 2000). This could break apart established natural communities and create newly evolving assemblages. Depending on the magnitude and duration of the environmental disturbance, some or all individuals of a given species may shift out of an area. This, in turn, can cause a local (or even the overall) population size to decline. Superimposed on these potential changes are those caused by land-use change, which frequently fragments populations into patches throughout their ranges.

Table 19-2: Threatened and unique entities identified in WGII TAR.
Chapter	Entity
4. Water Resources	- Endorheic lakes: Caspian and Aral Seas, Lake Balkash, Lake Chad, Lake Titicaca, Great Salt Lake - Glaciers (in general, no particular reference)
5. Ecosystems and Their Services	- Some butterfly species in United States and Europe - Leadbetters's possum in Australia - Cape Floral Kingdom, South Africa
6. Coastal Zones and Marine Ecosystems	- Coral reefs
7. Human Settlements	- Coastal settlements along North Sea coast in northwest Europe, the Seychelles, parts of Micronesia, Gulf Coast of United States and Mexico, Nile delta, and Bay of Bengal
10. Africa	- Cape Floral Kingdom and Succulent Karoo
11. Asia	- Biodiversity of Lake Baikal - Glaciers in the Tianshan, Hindukush Himalayas; permafrost in Tibet - Mangroves
12. Australia and New Zealand	- Alpine ecosystems, snow and glaciers in New Zealand, wetlands in Kakadu National Park, Queensland fruit fly - Indigenous communities
13. Europe	- Snowpack and permafrost in the mountains
14. Latin America	- Mountain glaciers
15. North America	- Mountain glaciers - Sardine population - Indigenous communities
16. Polar Regions (Arctic and Antarctic)	- Indigenous communities
17. Small Island States	- Mangroves and seagrass beds - Coral reefs

Species with wide nonpatchy ranges, rapid dispersal mechanisms, and a large population normally are not in danger of extinction [e.g., European house sparrow (Passer domesticus) and many weedy plant species]. Those with narrow patchy ranges and small populations frequently are endangered and may require management for survival [e.g., most crane species (Gruidea spp.)]. In summary, species tend to become rarer when ranges shift from wide to narrow, available habitat becomes patchier, and population size declines (Huntley et al., 1997). Indeed, a species is likely to become extinct if it is forced into a narrow patchy range and its population declines—a probability that is enhanced when environmental disturbances such as climate change, along with companion transient changes, occur.

Even when conservation management of rare species is effective, survival still may be problematic because in a small population, genetically similar individuals may breed, which decreases genetic variability. This, in turn, may reduce adaptability to stresses, thereby further lowering population size and decreasing the types of habitat within which the species could survive. Environmental catastrophes such as hurricanes, oil spills, extreme temperatures, and drought can trigger the extinction of even well-managed rare species. The only way to reduce the risk of extinction brought about by catastrophes is to increase population sizes and maintain corridors between isolated populations.

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