Impact of global warming on biodiversity

Key text

The pygmy possum might be snug, but those of us concerned for its future can't afford to be complacent. Perversely for an animal that inhabits one of the coldest environments on the Australian continent, one of the biggest threats to its survival – and that of many other Australian plants and animals – is global warming.

Global warming

The world is heating up. The average temperature of the Earth's surface increased by an estimated 0.6°C in the 20th century and, according to the most recent projections of the Intergovernmental Panel on Climate Change, could rise 1.4 to 5.8°C above the 1990 average by 2100. Much of this predicted increase is attributed by scientists to increasing concentrations of greenhouse gases such as carbon dioxide (CO₂) in the atmosphere.

The effects of such a temperature increase might include:

• more frequent extreme high maximum temperatures and less frequent extreme low minimum temperatures;

• a decrease in snow cover: satellite observations suggest that the area of the planet covered by snow has already declined by 10 per cent since the 1960s;

• an increase in the variability of climate, with changes in both the frequency and severity of extreme weather events;

• alterations to the distribution of certain infectious diseases; and

• rising sea levels.

In Australia the climate is expected to become significantly warmer. CSIRO scientists predict that temperatures over most of the continent will rise above 1990 levels by 0.4 to 2º C by 2030, and by 1 to 6º C by 2070. On a continent already as warm as Australia, such an increase could have major ecological impacts. The number of extreme rainfall events – such as those leading to flooding – is also expected to increase, even though most of the country is expected to become drier overall in the 21st century.

What will it mean for Australia's plants and animals?

Climate change could have dramatic effects on a wide range of Australian plants and animals. The threats to some particularly vulnerable species are described below.

Shifts in climatic envelopes

To estimate the effect of climate change on species, scientists use what they call a climatic envelope (sometimes also referred to as a bioclimatic envelope), which is the range of temperatures, rainfall and other climate-related parameters in which a species currently exists.

As the climate warms, the geographic location of climatic envelopes will shift significantly, possibly even to the extent that species can no longer survive in their current locations. Such species will need to follow their climatic envelopes by migrating to cooler and moister environments, usually uphill or southwards in the southern hemisphere. There is some evidence that plants and animals are already responding to warmer temperatures. The treeline (above which there are no trees) near Mount Hotham in the Victorian Alps has reportedly moved up in altitude by 40 metres in recent years.

In many cases, however, such migration might not be possible because of unsuitable soils and other unfavourable environmental parameters, geographical or human-made barriers and competition from species already in an area. The mountain pygmy possum is particularly vulnerable to a loss of habitat linked to global warming (Box 1: Giant problem for pygmy possum).

As human activities, particularly agriculture but also settlement and industrial development, have expanded over the last few centuries, natural vegetation – such as forests, grasslands and heathlands – has been cleared in large swathes. Once-extensive plant communities have been reduced in size and broken into smaller patches. This habitat reduction and fragmentation poses a problem because it limits the ability of many species to migrate to favourable conditions. Species on mountain-tops, islands and peninsulas will have a similar problem.

In general, those species with restricted climatic envelopes, small populations and limited ability to migrate are most likely to suffer in the face of rapid climate change. An estimated 25 per cent of Australian eucalypts, for example, have distributions that span less than 1°C of mean annual temperature, which is the average temperature for the entire year at a location. Even a relatively small increase in average temperature will shift the climatic envelopes of such species outside their current distribution.

Green ringtail possums

A number of species will be affected physiologically by global warming. There is evidence that some species are physiologically vulnerable to temperature spikes. For example, the green ringtail possum, an endemic species of Queensland's tropical rainforests, cannot control its body temperature when the ambient temperature rises above 30º C. An extended heatwave in north Queensland could kill off a large part of its population. Nor is that the only threat faced by green ringtail possums and other species in the country's tropical rainforests. The distribution of these forests is already severely limited by rainfall and temperature, and relatively small changes in either could have a dramatic effect.

Coral bleaching

Warmer sea surface temperatures are blamed for an increase in a phenomenon called coral bleaching, which is a whitening of coral caused when the coral expels a single-celled, symbiotic alga called zooxanthellae. This alga usually lives within the tissues of the corals and, among other things, gives them its spectacular range of colours. Zooxanthellae are expelled when the coral is under stress from environmental factors such as abnormally high water temperatures or pollution. Since the zooxanthellae help coral in nutrient production, their loss can affect coral growth and make coral more vulnerable to disease. Major bleaching events took place on the Great Barrier Reef in 1998 and 2002, causing a significant die-off of corals in some locations.

Increases in extreme events

Predicted changes in the intensity, frequency and extent of disturbances such as fire, cyclone, drought and flood will place existing vegetation under stress and favour species able to rapidly colonise denuded areas. In many cases this will mean the spread of 'weed' species and major changes in the distribution and abundance of many indigenous species.

Changes in rainfall

Australia is a dry continent. Its plants and animals are mostly well adapted to drought and have developed a wide range of strategies for coping with the country's climatic extremes. The marginal nature of the environment, however, means that even minor changes in rainfall patterns could have major impacts on wildlife. The Murray-Darling Basin (Australia's largest water catchment) and southwest Western Australia are already threatened by salinity and other environmental problems. Predicted decreased rainfall and consequent lower river flows in both regions would have a major impact on aquatic biota. Freshwater wetlands such as the Macquarie Marshes in the central west of New South Wales – and the frogs, waterbirds, turtles and other aquatic life dependent on them – are also at risk because of a change in water quality and quantity.

Rises in concentrations of carbon dioxide

The basic ingredients of photosynthesis are carbon dioxide and water. Increased carbon dioxide in the atmosphere causes increased growth rates in many plant species. This is good news for farmers, but only if this carbon dioxide 'fertilisation' effect is matched by adequate soil moisture and other nutrients. Leaf-eating animals like koalas may not be so lucky: increased concentrations of carbon dioxide could diminish the nutritional value of foliage. Rising levels of atmospheric carbon dioxide could also decrease the calcification rates of corals, meaning that reefs damaged by bleaching or other agents would recover more slowly.

Sea-level rise

In most climate-change models, sea levels are predicted to rise by 9 to 88 centimetres by 2100, due to the thermal expansion of the oceans and the melting of polar ice-caps. Coupled with the effects of storm surges, which are expected to be of a greater magnitude in a warmer world, CSIRO scientists predict that the Australian coastline could retreat inland by 4.5 to 88 metres by 2100. Coastal ecosystems, such as mangrove forests and low-lying freshwater wetlands in Kakadu National Park, could be severely affected.

What would rapid species extinction mean for Australia?

Global warming is predicted to take place faster in the next century than at any time for at least the last 10,000 years. Coupled with other factors, such as continued land-clearing, this could mean the extinction of species at a rate even greater than when the dinosaurs disappeared about 65 million years ago. Some species not under immediate threat of extinction might nonetheless suffer decreases in population size, diminishing intra-species' genetic diversity (and therefore face increased vulnerability).

Does it really matter if many species go extinct? The world would certainly be a less interesting place with less biodiversity, but would it affect us?

A diversity of species increases the ability of ecosystems to do things like hold soils together, maintain soil fertility, deliver clean water to streams and rivers, cycle nutrients, pollinate plants (including crops), and buffer against pests and diseases – these are sometimes called 'ecosystem functions' or 'ecosystem services'. A loss of species could reduce this ability, particularly if environmental conditions are changing rapidly at the same time. It is therefore possible that as the climate changes and as species are eliminated from an area we will see a change in some ecosystem functions; this could mean more land degradation, changes in agricultural productivity and a reduction in the quality of water delivered to human populations.

Adapting to change

Scientists agree that human-induced global warming is happening, and that the world will continue to warm for some time even if greenhouse gas emissions are somehow curbed. Some species, particularly insects, might be able to adapt to changing conditions or evolve in response to global warming (see Box 2: Responses to global warming). But for many, especially those that are already rare and have limited climatic envelopes, global warming could pose an insurmountable challenge.

In Australia, action plans that have been prepared for a number of endangered species try to address the possible impacts of global warming. For example, the recovery plan for the mountain pygmy possum prepared by the NSW National Parks and Wildlife Service includes the development of a model to illustrate habitat suitability under current snow conditions and to identify key refugia for the possum under the predicted impacts of global warming. The action plan prepared by the government of the Australian Capital Territory for the northern corroboree frog, includes a commitment to a coordinated research program on the actual and potential effects of global warming on the species.

Related site: National Biodiversity and Climate Change Action Plan Describes actions we can take to reduce the negative impacts on biodiversity from climate change. (Department of the Environment and Water Resources, Australia)

At the national level, the Natural Resource Management Ministerial Council has prepared a national biodiversity and climate change action plan for the period 2004-2007. The plan, which was developed in consultation with scientists, conservationists and national, state and local governments, contains seven objectives, along with actions that should be taken to achieve the objectives. At this early stage of development, many of these actions are aimed at improving our understanding of the impacts of global warming on biodiversity, while others are general or strategic in nature.

Some of the impacts of global warming may be sudden, but in many cases societies will have some years to adapt their management of biodiversity as conditions change. Increasing our understanding of the effects of climate change on biodiversity, and developing practical ways of mitigating such effects, are critical to limit the damage. Even so, the dangers are great – for humans as well as our native plants and animals. Not only mountain pygmy possums stand to lose their security blanket.

Related Nova topics:

Warmer and sicker? Global warming and human health

Getting into hot water – global warming and rising sea levels

Coral bleaching – will global warming kill the reefs?

The Southern Ocean and global climate

Enhanced greenhouse effect – a hot international issue

Carbon currency – the credits and debits of carbon emissions trading

Conservation genetics – molecular detectives at work

Australia's threatened species

Box 1: Giant problem for pygmy possum

However, with global warming, the extent of snow cover in Australia is predicted to decline dramatically. Even today it comprises only 0.15 per cent of the continent, an area that could decrease by 39 to 96 per cent by 2070. An average temperature rise of only 3ºC would entirely eliminate snow from all Australian mountains. Scientists predict that a loss of snow cover would increase winter mortality of the species, because of decreased insulation and a reduction in the availability of suitable habitat. Moreover, the breeding grounds of one of the possum’s most important food sources, the Bogong moth, could be affected by increased drought due to global warming. The net effect would almost certainly be an increase in the vulnerability of the mountain pygmy possum to extinction. Already there is evidence that annual snow cover is decreasing; for example, a strongly decreasing trend has been detected at the Spencers Creek snowfield between 1959 and 1999.

Other alpine species are at risk: one study estimated that nearly half the 190 alpine plant species on Mt Kosciuszko are vulnerable to global warming.

Related sites

• Mountain pygmy-possum – Burramys parvus (Australian Government Department of the Environment and Water Resources)

• Possum dreaming (Australian Broadcasting Corporation)

Box 2: Responses to global warming

Evolutionary responses

But Australian scientists have detected what they think is an evolutionary response to rapid climate change amongst the fruitfly Drosophila – a species that has often been used in genetic experiments. This insect carries a gene called Adh; a variation of this gene, called Adh^s , is thought to help the insect survive arid conditions. Usually Adh^s is more common in northern Australia, which is hotter and drier, but scientists at La Trobe University in Melbourne have discovered that the distribution of the gene has moved 400 kilometres to the south – presumably in response to rising temperatures and decreasing rainfall.

Behavioural responses

Scientists who study the relationship between the seasons and biological phenomena have looked at long-term records of the indicators of change from one season to another, such as temperature, rainfalls and the number of hours of sunlight. They found that global warming has changed the timing of the seasons – spring arrives earlier and autumn lasts longer – and that wildlife is adapting to the change by altering its behaviour. Scientists found that a number of plants are consistently forming buds and flowering earlier in spring, and that the migration and breeding times of birds has also changed.

Related sites

• Climate change – ecosystems (World Wildlife Fund)

• Changing times (Australian Broadcasting Corporation)

• Climate change alters genes on the fly (BioEd Online, USA)

• Global warming shows up in fly genes (1 September 2006, Scientific American)

Activities

• On borrowed time – a resource with four inquiry-based teaching and learning units and two decision-making interactives which allow students to reflect, consider and make decisions relating to Australia's biodiversity.