7. Adaptability as measured by
migration and distribution


Climate Change and Biodiversity Conservation



Central to resolving the question of how to define "natural adaptation" is the question of which climate change impacts species and natural communities can adapt to, and how fast. Natural adaptation can take a number of forms, including acclimation, evolution, or migration. There is rather poor information on the first two, but it is possible to make some rough quantifications for migration rates. Palaeoecological evidence points to maximum migration rates for a number of tree species in the northern hemisphere; some of these are presented in Figure 1.


Figure 1





The achievement of the migration rates given in Figure 1 depends on factors apart from temperature change. Human land­use patterns and the increasingly fragmented nature of temperate forests now present formidable barriers to natural migration. One study of temperature tolerances of more than 15,000 native plant species in the US and Canada has shown that a temperature rise of 3°C would place 7-11 per cent of them outside their thermal tolerance, and therefore vulnerable to extinction, unless they could migrate to cooler areas.


Migration is an issue for fauna as well as for flora. For example, although most birds are extremely mobile, some species will not cross open clearings, sometimes even as small as tree­fall gaps. Others are associated with specific vegetation species or formations, such as Kirtland's warbler and Jack pine. Alternatively, tree migration combined with increased storms may reduce the availability of large tree holes used as breeding sites by species such as the red­cockaded woodpecker of US old­growth forests, or Australia's rare Major Mitchell cockatoo. Annual migrants, such as Bewick's swan, depend on the availability of specific habitats for `refuelling' stops on their long journeys, often requiring optimum conditions for building up fat reserves prior to setting off. Others, such as crossbill, may migrate in synchrony with the availability of transient food sources.


Several species of shorebird, such as godwit and whimbrel, rely on relatively few coastal refuelling stops on the Atlantic and Pacific flyways. Alternative sites with sufficient predictability of timing and supply of food probably do not exist. Coastal inundation or other climate impacts could seriously affect these species. In the US, populations of shorebirds such as semi­palmated sandpipers, knot, and sanderling depend on synchronous timing of horseshoe crab spawning in the Delaware Bay, and on insect emergence at their arrival in the Arctic tundra. Both these events are probably significantly climate controlled, although scientists do not yet fully understand the relationship.




A 3o C temperature rise could wipe out more than 10% of North America's native plants



Freshwater fish, despite being highly mobile, and in many cases migratory as well, are limited by the physical distribution of rivers and lakes. Most fish have limited tolerances for changes in water temperature, and where water warms as a result of climate change, there will be very significant threats for species that are unable to find their way to cooler waters. A recent study by the US Environmental Protection Agency suggested that cold water fish could lose between 50 and 100 per cent of suitable habitat in 20 states. Amongst the states likely to lose all suitable habitat would be Maine, Massachusetts, Indiana, and Nebraska. Amongst the species most at risk would be the already threatened populations of cutthroat trout, brook trout, Chinook and chum salmon.


As can be seen from the foregoing discussion, warming at the rate implied by a CO2 doubling by around the middle of the next century will prevent many ecosystems from adapting to climate change. Thus, CO2 concentrations would have to be stabilized at a level well below an equivalent doubling from pre­industrial levels in order to have a reasonable likelihood of preventing dangerous interference with the climate system, and preventing ecological harm.






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Copyright 1996, The World Wide Fund For Nature