4.3.1.3. Terrestrial Ecosystems
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 5.2.3.3), 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
5.2.3.4).
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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