12.4. Ecosystems and Conservation
Until recent settlement, Australia and New Zealand were isolated for millions
of years, and their ecosystems have evolved to cope with unique climate and
biological circumstances (Kemp, 1981; Nix, 1981). Despite large year-to-year
climatic variability, many Australian terrestrial species have quite limited
ranges of long-term average climate, on the order or 1-2°C temperature
and 20% in rainfall (Hughes et al., 1996; Pouliquen-Young and Newman,
1999). Thus, these ecosystems are vulnerable to climatic change, as well as
invasion by exotic animals and plants.
Rapid land clearance and subsequent land-use change have been occurring as
a result of human activity over the past 500-1,000 years in New Zealand
(McGlone, 1989; Wilmshurst, 1997) and, in Australia, subsequent to Aboriginal
arrival tens of thousands of years agoand especially since European settlers
arrived 200 years ago. This has led to loss of biodiversity in many ecosystems
as well as loss of some ecosystems as a whole. One of the major impacts has
been an increase in weedy species in both countries. This is likely to continue
and be exacerbated by climate change. Land-use change also has led to fragmentation
of ecosystems and to salinization through rising water tables. These trends
can inhibit natural adaptation to climate change via the dispersal/migration
response. Systems therefore may be more vulnerable, and some might become extinct.
For example, Mitchell and Williams (1996) have noted that habitat that is climatically
suitable for the long-lived New Zealand kauri tree Agathis australis
under a 4°C warming scenario would be at least 150 km from the nearest extant
population. They suggest that survival of this species may require human intervention
and relocation. Similar problems have been identified by Pouliquen-Young and
Newman (1999) in relation to fragmented habitat for endangered species in the
southwest of western Australia.
Many of the region's wetlands, riverine environments, and coastal and
marine systems also are sensitive to climate variations and changes. A key issue
is the effect on Australia's coral reefs of greenhouse-related stresses
in addition to nonclimatic features such as overexploitation and increasing
pollution and turbidity of coastal waters from sediment loading, fertilizers,
pesticides, and herbicides (Larcombe et al., 1996).
12.4.2. Forests and Woodlands
In Australia, some 50% of the forest cover in existence at the time of European
settlement still exists, although about half of that has been logged (Graetz
et al., 1995; State of the Environment, 1996). Pressures on forests and
woodlands as a whole are likely to decrease as a result of recent legislation
relating to protection of forests in some Australian states, and as interest
in carbon sequestration increases. In New Zealand (Taylor and Smith, 1987),
25% of the original forest cover remains, with 77% in the conservation estate,
21% in private hands, and 2% state owned. Legal constraints on native wood production
mean that only about 4% currently is managed for production, and clear-felling
without replacement has virtually ceased.
The present temperature range of 25% of Australian Eucalyptus trees is less
than 1°C in mean annual temperature (Hughes et al., 1996). Similarly,
23% have ranges of mean annual rainfall of less than 20% variation. The actual
climate tolerances of many species are wider than the climate envelope they
currently occupy and may be affected by increasing CO2 concentrations,
which change photosynthetic rates and water-use efficiency (WUE) and may affect
the temperature response (Curtis, 1996). Such changes from increasing CO2
would be moderated by nutrient stress and other stressors that are prevalent
across Australian forests. Nevertheless, if present-day boundaries even approximately
reflect actual thermal or rainfall tolerances, substantial changes in Australian
native forests may be expected with climate change. Howden and Gorman (1999)
suggest that adaptive responses would include monitoring of key indicators,
flexibility in reserve allocation, increased reserve areas, and reduced fragmentation.
In a forested area in western Australia that is listed as one of 25 global
"biodiversity hotspots" for conservation priority by Myers et al.
(2000), Pouliquen-Young and Newman (1999) used the BIOCLIM program (Busby, 1991)
to generate a climatic envelope from the present distribution of species. They
assessed the effects of three incremental temperature and rainfall scenarios
on three species of frogs, 15 species of endangered or threatened mammals, 92
varieties of the plant genus Dryandra, and 27 varieties of Acacia
in the southwest of western Australia. The scenarios were based on the spatial
pattern of change from the CSIRO RCM at 125-km resolution, scaled to the IS92
global scenarios. For plant species, suitability of soils also was considered.
The results indicate that most species would suffer dramatic decreases in range
with climate warming; all of the frog and mammal species studied would be restricted
to small areas or would disappear with 0.5°C global-average warming above
present annual averages, as would 28% of the Dryandra species and one
Acacia. At 2°C global average warming, 66% of the Dryandra
species, as well as all of the Acacia, would disappear. Adaptation opportunities
were considered minimal, with some gain from linking present conservation reserves
and reintroducing endangered species into a range of climatic zones.
Studies of the current distribution of New Zealand canopy trees in relation
to climate suggest that major range changes can be expected with warming (Whitehead
et al., 1992; Leathwick et al. 1996; Mitchell and Williams, 1996).
Trees in the highly diverse northern and lowland forests (e.g., Beilschmiedia
tawa) are likely to expand their ranges southward and to higher altitudes.
The extensive upland Nothofagus forests are likely to be invaded by broad-leafed
species. Few tree species are confined to cool southern climates; those that
are have a wide altitudinal range available for adjustment of their distribution,
so no extinctions are expected. Most concern centers on the ability of tree
species to achieve new distributions rapidly enough in a fragmented landscape,
as well as invasion of natural intact forests by exotic tree, shrub, and liana
species that are adapted to warm temperate or subtropical climates.
In Australia, rangelands are important for meat and wool production. In their
natural state, rangelands are adapted to relatively large short-term variations
in climatic conditions (mainly rainfall and temperature). However, they are
under stress from human activity, mostly as a result of animal production, introduced
animals such as rabbits, inappropriate management, and interactions between
all of these factors (Abel et al., 2000). These stresses, in combination
with climatic factors, have led to problems of land degradation, salinization,
and woody weed invasion and subsequent decreases in food production. In some
cases, native dominant species (mostly plants) have been replaced by exotic
species, leading to a decrease in population of many native animal species.
Woody weed invasion also has changed the fire regime through formation of "thickets"
that do not allow fires through, partly as a result of the fire resistance of
some species (Noble et al., 1996). Some Australian rangelands also are
vulnerable to salinization resulting from rising water tables from irrigation
and loss of native vegetation (see Section 12.3).
New Zealand rangelands are used predominantly for sheep grazing. Intensive
use of indigenous grasslands and shrublands on land cleared of trees in the
19th century has increased vulnerability to invasion, especially by woody weeds
(pine, broom, gorse, etc.) and herbaceous weeds (hawkweed, thistles, and subtropical
grasses). Weed invasions are unlikely to further increase the susceptibility
of the system to climatic disruptions but could themselves be accelerated by
warming or increased climatic variability. Fire is now strictly regulated, although
rangeland fires remain a serious problem, especially in ENSO drought years.
Rangelandsin particular, in drier areas of eastern South Islandhave
many problems, including animal and plant pests and declining profitability
of farming, leading to a decline in management and fertilizer inputs.
Increased CO2 is likely to have beneficial effects for native pastures, with
possible nitrogen limitation and increased subsoil drainage (Howden et al.,
1999d). Runoff and groundwater recharge also could increase (Krysanova et al.,
1999). This could lead to increased salinization problems in areas that are
susceptible. However, decreases in rainfall in excess of about 10% at the time
of CO2 doubling would dominate over the CO2 fertilization effect and lead to
a decline in pasture productivity. This is more likely in the latest climate
change scenarios that are based on coupled AOGCM results. Howden et al. (1999d)
conclude that a doubling of CO2 concentrations will result in only limited changes
in the distribution of C3 and C4 grasses and that such changes will be moderated
by warmer temperatures.