Predictions based on scenarios of climate change suggest increased variability and unpredictability in productivity and community composition. Any changes that might occur in the location and seasonal patterns of rainfall and in ENSO characteristics would result in significant impacts and vulnerability (IPCC 1996, WG II, Section 2.7.2 and Boxes 2.2 and 2.8). Although migration to more suitable climatic regimes is an option for some biota (Mitchell and Williams, 1996), ecosystems are not expected to shift en masse. Differential rates of migration and survival of different species will inevitably change the abundance and distribution of species, community structure, and possibly ecosystem function at any given location. Altered species interactions-for example, predation, parasitism, and competition-also are likely and may eliminate formerly successful species even if the climate remains within their physiological tolerances (Peters, 1992). Ecosystems whose "keystone" species are particularly sensitive to climate will be more at risk. (A keystone species is one that has a central servicing role affecting many other organisms and whose demise is likely to result in the loss of a number of species and lead to major changes in ecosystem function.) Some species will be slow to migrate because of factors such as slow reproduction rate or limited seed dispersal mechanisms (IPCC 1996, WG II, Section 1.3.5); species with better capacity for dispersal and establishment, including weeds, will have an advantage. Disruption in forest composition is most likely to occur where fragmentation of the forest reduces the potential for dispersal of new, more suitable species (Whitehead et al., 1992). Although many species will be able to adapt, climate change is expected to reduce biodiversity in individual ecosystems overall (IPCC 1996, WG II, Section 1.3.6).
Rising temperatures can be accommodated by moves toward higher elevations (if the terrain allows) because air temperature decreases by about 1°C for every 100-200 m increase in elevation (IPCC 1996, WG II, Section 22.214.171.124), or toward higher latitudes because temperatures generally decline as one moves poleward, especially in mid-latitudes. However, elevational migration is not an option for most of Australia's predominantly flat expanses, and latitudinal migration is a limited option in parts of tropical Australia where the latitudinal variation of temperature is small and poleward migration is limited by desert or land-use change; here the biota will increasingly experience temperatures never previously encountered. In contrast to North America and Eurasia, the oceanic boundary of southern Australia will restrict the poleward migration of terrestrial biota. Small offshore islands and mountain tops provide limited migration options, and escarpments, deserts, agricultural land, and urban areas present physical barriers to migration. Conversely, changes in climate thresholds, such as frost, may remove an existing limitation to species survival and performance. The survival of vulnerable species in refugia, particularly during drought, and then expansion into adjacent areas (e.g., Morton et al., 1995) suggest that migration will be even more limited if there are increases in the frequency or intensity of climatic extremes such as drought.
More than half of the region comprises the arid and semi-arid ecosystems of grasslands, shrublands, and savanna. These areas exhibit a high degree of spatial variation, from small-scale variation in water and soils to larger-scale variation in climate patterns. In such rangeland ecosystems, the amount and timing of rainfall and nutrient limitations are the major determinants of plant community composition, distribution, and productivity (Mott et al., 1985; IPCC 1996, WG II, Section 2.1). Possible increases in rainfall intensity with climate change would increase the proportion of rain that runs off particular landscape elements and runs onto others, resulting in changes in the temporal and spatial functioning of rangelands (Stafford Smith et al., 1994). In general, rangelands are not in equilibrium but fluctuate between states according to rainfall, fire, grazing, and other factors (Westoby et al., 1989; IPCC 1996, WG II, Section 2.1). They have adapted to the naturally high variation of rainfall associated with their low mean rainfall and ENSO-related interannual variability. Should ecosystem productivity and composition become more variable and unpredictable-as has been suggested by some climate change studies-then land management issues, such as the type of grazing activity, the use of fire for woody weed control, and pest animal management, will become more important (Stafford Smith et al., 1994).
The responses of animals to climate change will be partly determined by the response of co-occurring plants and habitat. For example, migratory species may be affected by reduction of suitable habitats along their migration routes. Thermal stress, which is particularly evident in animals in a variety of Australian ecosystems (Nix, 1982), may affect the geographic range and the reproductive biology of species. Indeed, contractions in core climatic habitat were shown for more than 80% of threatened vertebrate species in Australia under three climate change scenarios based on CSIRO (1992) (Dexter et al., 1995). Although some animal species may be physically capable of moving great distances, behavioral traits may restrict dispersal options (Peters, 1992). As with plants, animals that are less adaptable, less mobile, or physically restricted (such as fish in lakes) may decline or become extinct.
Forest and woodland are reported to cover 19% of Australia and 28% of New Zealand (WRI, 1996). Indigenous forests occupy nearly a quarter of New Zealand, mainly under conservation control and often in mountain lands. The slowly maturing trees of indigenous forests will be more vulnerable than other plants to long-term external change.
Alpine ecosystems in Australia, which occupy only a small area, have been identified as being particularly susceptible to climate change (Busby, 1988; Nias, 1992; Brereton et al., 1995). Upslope areas required to cope with the predicted rise in temperatures may not be available for many of these ecosystems. In small alpine streams, any increase in water temperatures and reduction in water flow could be stressful for alpine stream animals. Model studies for Australian mountain vegetation show that there is potential for the expansion of woody vegetation and shrub communities, as well as rises in treelines (IPCC 1996, WG II, Section 126.96.36.199).
Pests, weeds, and diseases play significant roles in Australian and New Zealand ecosystems (and agriculture). The geographical distribution and severity of their impacts on host plants and ecosystems could be dramatically changed by the combination of changes in climate, atmospheric composition, and habitat (Sutherst et al., 1996). For example, any changes in the timing and intensity of plant moisture stress will bring changes in the relative advantage of different types of herbivorous insects.
Fire plays an important role in the composition, function, and dynamics of many Australian ecosystems (IPCC 1996, WG II, Section 1.5.1). For example, the importance of the length of inter-fire interval has been repeatedly demonstrated (Christensen and Kimber, 1975; Burgman and Lamont, 1992; Gill and Bradstock, 1995; Morrison et al., 1995; Keith, 1996). It has been strongly suggested that the risk of fire may increase as the climate changes (IPCC 1996, WG II, Section 1.3.1) because of factors such as higher temperatures and water stress-though there remains much speculation about future fire regimes owing to the uncertainty associated with the many climatic and ecosystem variables involved (Williams and Gill, 1995). If fires become more frequent under climate change (Beer and Williams, 1995; Pittock et al., 1997), species composition and structure could be altered, at least in southern Australia. Changes in other components of the fire regime, such as intensity (which could be affected by changes in fuel loading) and season, likewise could affect species diversity. Any increased fire occurrence also could present increased risks to people and infrastructure in forested urban fringe areas of southeastern Australia.
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