CLIMATE CHANGE AS A THREAT TO BIODIVERSITY
Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 15: pages 417-421
A changing global climate threatens species and ecosystems. The distribution of species (biogeography) is largely determined by climate, as is the distribution of ecosystems and plant vegetation zones (biomes). Climate change may simply shift these distributions, but often, barriers and human presence will provide no opportunity for distributional shifts. For these reasons, some species and ecosystems are likely to be eliminated by climate change.
Climate and the Distribution of Species & Ecosystems:
Each location on the earths surface has a characteristic pattern of temperature and rainfall. Individual species are adapted to the range of temperature and precipitation they normally experience. The distribution of ecosystems is closely related to the distribution of climates (transparency). Any change in climate will profoundly influence the distribution of species and ecosystems.
Predicted Future Climates:
General circulation models (GCMs) predict future climate scenarios. Further work is needed to improve their realism. For example, they operate at a fairly coarse scale (500 km grid), are better at predicting temperature than rainfall, and have some problems with the representation of clouds (a potential negative feedback or climate stabilizer). Nevertheless, the several models agree in general on a number of predictions.
The rate of change in climate is an important concern. This global warming is going to happen very quickly in the lifespan and life cycle of organisms -- 100 years is only a few multiples of the generation time of long-lived organisms. Evolutionary adaptation seems unlikely. Until recently, it was said that the rate of human-induced climate change was unprecedented. Recent resulted (reported in fall 1997) suggest that a warming event at the end of the last ice age may have caused very sharp transitions, occurring in decades or less, so the earth probably has experienced such rapid transitions before. (And perhaps climate change is the explanation for the sudden, post-glacial extinction of large mammals.)
Effects of Climate Change on Species & Ecosystems
If significant climate change occurs over the next 50 to 100 years, as seems highly plausible, many natural populations of wild organisms will be unable to exist within their natural ranges. Changes in temperature and precipitation, and resultant changes in vegetation and habitat, are likely to seriously affect the suitability of the locales where species are presently found. Thus, climate change is an additional factor threatening the survival of species.
How much will a given temperature rise affect the biota ? We can estimate this in a couple of ways. Analysis of shifts in the biota during the glacial and interglacial periods established that a 2 C change in average earth temperature can have big effects. During the middle Holocene, temperatures in eastern North America were 2 C warmer than at present. Manatees swam off the New Jersey shore, tapirs and peccaries foraged in Pennsylvania, and Cape Cod had a forest like modern-day North Carolina.
One can also look at the changes in average temperature that accompany a given change in latitude or elevation. In the eastern deciduous forests of North America, a 2-6 C warming will force species to move 500-1000 km northwards. A 3 C warming roughly corresponds to the difference today between Ann Arbor and the northern tip of Lower Michigan, at the UM Biostation. Given a 100-year time frame, species will have to move 5 to 10 km/year to disperse to favorable climes. The Engleman spruce probably is average in its natural dispersal capabilities, and is thought to move between 1 and 20 km per century. 100 km per century would be a high dispersal rate. Present and prohected distributions of some North American tree species are shown in a trasnparency.
There is another solution -- "put the seeds in an envelope and mail them to someone 500 km north". It probably is true that human intervention can solve a lot of the issues pertaining to physical transport. However, consider the following:
Not all biogeography is north-south. In mountainous regions, vegetation and biotic zones change with altitude. In general, one achieves a 3 C cooling by travelling 500 M upwards in elevation, so global warming should shift vegetational zones uphill. For the uppermost zones, of course, there is nowhere left to go. Many mountain tops of the American west contain "glacial relicts" species that are found very far to the north, but exist on mountain tops because the climate is suitable. These almost certainly will be lost.
Not all dispersal routes are north-south. For fishes of the central west, the opportunity to swim north is largely unavailable (transparency).
Insights from Paleoecology
A good deal is known of the changes in the distributions of organisms during the quaternary, which is the most recent few million years of earth history, and especially of the last tens of thousands of years. Plant distributions are especially well known since the retreat of the glaciers, because the sediments of lakes left by the glaciers provide a chronology of fossil pollen. From this evidence, maps of plant species shifts can be reconstructed with considerable accuracy for the past 18,000 years.
Davis states that four generalizations can be drawn from this evidence:
Deforestation and Climate Change
Much of todays discussion of climate change focuses on warming. This is because GCMs dont do well at predicting changes in precipitation, and less thought has been given the likely consequences. However, the expected changes include more intense storms, and some areas becoming wetter while others become drier. This will surely have consequences for species and ecosystems.
In addition, we tend to view climate change as the result of rising CO2 levels. Large-scale deforestation can substantially change local climate. For example, a model was used to explore the effects of deforestation on the climate of the Amazon region. When forest is replaced by degraded grass pastures, there is a significant increase in surface land temperature, and a decrease in evapotranspiration and precipitation. The climate becomes hotter and drier, and thus less hospitable to the reestablishment of tropical forests.
The model results are strongly dependent upon the interaction between the forest and the hydrologic (water) cycle. Much of the rainfall in moist forest regions is internally generated. Plants take up water through their root tips, and lose this water through their leaf surfaces during the gas exchange necessary for obtaining CO2 for photosynthesis. This process of taking water in through the roots and losing it though the leaves provides the uphill circulation mechanism of plants. It also returns enormous amounts of water to the atmosphere, providing the basis for subsequent precipitation. Thus, water that is internally cycled within rain forest regions, will be lost as runoff if the forest is removed. Subsequent changes in climate may make it impossible for a rain forest to be regenerated.
Transparencies: 1. Climates of some major biomes 2. Biomes and climate 3. Obstacles to population migration 4. Fish dispersal routes 5. Biological reserve and shifting climate 6. Changes in species associations 7. Changes in vegetation types. 8. Mountain zonation 8. Rapid climate change.
Climate change and the maintence of conservation values in terrestrial ecosystems: a good article on effects of climate change.
Changing ecoregions: brought to you by the Sierra Club, be sure to click on the various areas listed throughout the world that are experiencing ecological change.