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Summary: In responding to climate change, Australasia's biota may face a greater rate of long-term change than ever before. They also must respond in a highly altered landscape fragmented by urban and agricultural development. There is ample evidence for significant potential impacts. Alterations in soil characteristics, water and nutrient cycling, plant productivity, species interactions (competition, predation, parasitism, etc.), and composition and function of ecosystems are highly likely responses to increases in atmospheric CO2 concentration and temperature and to shifts in rainfall regimes. These changes would be exacerbated by any increases in fire occurrence and insect outbreaks.
Aquatic systems will be affected by the disproportionately large responses in runoff, riverflow and associated nutrients, wastes and sediments that are likely from changes in rainfall and rainfall intensity and by sea-level rise in estuaries, mangroves, and other low-lying coastal areas. Australia's Great Barrier Reef and other coral reefs are vulnerable to temperature-induced bleaching and death of corals, in addition to sea-level rise and weather changes. However, there is evidence that the growth of coral reef biota may be sufficient to adapt to sea-level rise. Our knowledge of climate change impacts on aquatic and marine ecosystems is relatively limited.
Prediction of climate change effects is very difficult because of the complexity of ecosystem dynamics. Although Australasia's biota and ecosystems are adapted to the region's high climate variability (exemplified in arid and ENSO-affected areas), it is unclear whether this will provide any natural adaptation advantage. Many species will be able to adapt through altered ecosystem relationships or migration, but such possibilities may not exist in some cases, and reduction of species diversity is highly likely. Climate change will add to existing problems such as land degradation, weed infestations, and pest animals and generally will increase the difficulties and uncertainty involved in managing these problems.
The primary human adaptation option is land-use management-for example, by modification of animal stocking rates in rangelands, control of pests and weeds, changed forestry practices, and plantings along waterways. Research, monitoring, and prediction, both climatic and ecological, will be necessary foundations to human adaptive responses. Active manipulation of species generally will not be feasible in the region's extensive natural or lightly managed ecosystems, except for rare and endangered species or commercially valuable species. In summary, it must be concluded that some of the region's ecosystems are very vulnerable to climate change.
Climate is a primary influence not only on the individual plant, animal, and soil components of an ecosystem but also on water and nutrient availability and cycling within the ecosystem, on fire and other disturbances, and on the dynamics of species interactions. Changes in climate therefore affect ecosystems both by directly altering an area's suitability to the physiological requirements of individual species and by altering the nature of ecosystem dynamics and species interactions (Peters and Darling, 1985). In addition, biota face an environment in which the rising atmospheric CO2 concentration also will directly affect plants and soils.
The rate of climatic change may exceed any that the biota have previously experienced (IPCC 1996, WG II, Chapter A and Section 4.3.3). This rate of change poses a potentially major threat to ecosystem structure and function and possibly to the ability of evolutionary processes, such as natural selection, to keep pace (Peters and Darling, 1985). Although many of the biota and ecosystems in the region have adapted to high climate variability (exemplified in the region's arid and ENSO-affected areas), it is unclear whether this will provide any advantage in adapting to the projected changes in climate.
Furthermore, in contrast to the case of climate change over geological time scales, today the region's biota must respond in a landscape that has been highly modified by agricultural and urban development and introduced species (Peters, 1992). Considerable fragmentation of habitat has occurred in Australasia's forests, temperate woodlands, and rangelands. In the short term, land-use changes such as vegetation clearance are likely to have a much greater bearing on the maintenance of conservation values than the direct effects of climate change on biodiversity (Saunders and Hobbs, 1992). In the longer term, however, climate change impacts are likely to become increasingly evident, especially where other processes have increased ecosystem vulnerability (Williams et al., 1994).
Australasia's isolated evolutionary history has led to a very high level of endemism (plants and animals found only in the region). For example, 77% of mammals, 41% of birds, and 93% of plant species are endemic (see Annex D). As one of the 12 recognized "mega-diversity" countries (and the only one that is an OECD member), Australia has a particular stewardship responsibility toward an unusually large fraction of the world's biodiversity. Many of New Zealand's endemic bird species are endangered. Species confined to limited areas or habitat, such as Australia's endangered Mountain Pygmy Possum (Burramys parvus)-which is only found in the alpine and subalpine regions of southeast Australia (Dexter et al., 1995)-may be especially vulnerable to climate change.
Certain ecosystems have particular importance to the region's indigenous people, both for use as traditional sources of food and materials and for their cultural and spiritual significance. Selected climate change impacts on Australian Aborigines and New Zealand Maori are considered in Sections 4.3.3.1 and 4.3.4.2 respectively.
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