GLOBAL WARMING
Many scientists agree that the earth is warming up due to the addition of anthropogenically produced greenhouse gases (CO2, CH4, NOx) to the atmosphere. Current estimates place the expected level of global warming over the next century to be between 1.0 and 4.5 oC. Recent ocean research suggests that the higher end estimate is closer to the truth based on increased heat storage in the oceans.
An increase of 3oC would make this the warmest period in the past 100,000 years, while an increase of 4oC would make the earth the warmest its been since the Eocene Epoch about 40,000,000 years ago.
While the absolute degree change would not make the planet warmer than it has been in the past, the rate at which that change takes place could have major effects on species diversity. Current estimates would put the rate of change at between 15 and 40 times the rate of past natural changes.
Changes is the global temperature patterns would trigger widespread alterations in the rainfall pattern, with some areas receiving a major increase in rainfall, while others experience major droughts. A long term drying trend is predicted for summers in midlatitude interior continental regions, with up to a 40% decrease in precipitation in the Great Plains.
Increased concentrations of CO2 may accelerate the growth of some plants at the expense of others which would destabilize natural communities. Greater warming is expected to occur in higher latitudes, suggesting that temperate and arctic species would be placed in greater danger than their tropical relatives. While continued melting of the ice caps would raise sea level, flooding coastal areas.
SHIFT OF SPECIES RANGES
The most obvious effect of global warmnimng will be the shift in species ranges, since species generally track climatic optima. There is, for instance, abundant fossil evidence from the Pleistocene for shifting plant and animal ranges due to climatic warming and cooling.
Species can shift their ranges both latitudinally (north to south) or through change in elevation (see Figure below). Remember, that a 3oC change in temperature can equally be achieved by a roughly 500m. change in elevation of a 250 km. change in latitude. In the example below species "A" will go extinct locally due to warming, while species "B" will suffer a severe range reduction leading to intensified intra-specific competition for space.
A somewhat similar case has recently been hypothesized for the Tropical Montane Cloud Forests in Costa Rica. In what is an otherwise pristine environment, the Cloud Forest appear to be losing their amphibian species. The "Lifting Cloud Base Hypothesis" seeks to explain the loss of the amphibians due to a subtle change in the climate. The Cloud Forest are often covered with clouds (hence the name). The cloud cover keeps the humidity high, to the point where many of the leafy plans are constantly dripping as if it had just rained. The upper part of the figure below illustrates the situation.
Warm, moist air rises from the ocean to the east, and as it hits the mountains undergoes orographic uplift. As the air rises, it cools and the moisture condenses to form clouds. The cloud level is controlled by temperature. In the lower illustration, gradual warming of the ocean surface waters has caused the altitude at which clouds form to rise, since the moist air is warmer than it had been. This results in a greater number of days per year during which the mountains are not cloud covered which lowers the average humidity of the environment. It is hypothesized that the slightly less humid environment has resulted in the loss of the amphibians which require high moisture levels. Further support for this hypothesis comes from the observation that animals characteristic of the dryer lowland environments at the foot of the mountains have extended their range higher into the mountains.
DISPERSAL RATE AND BARRIERS
Dispersal rate for a species depends on either the long-distance dispersal of colonists (e.g., seeds or migrating animals), or rapid iterative colonization of nearby habitat. Trees, for instance, can change the boundaries of their range by from 1 to 45 kilometers per century.
Species with intrinsically low colonization ability may well face extinction as the present habitat becomes unsuitable. Predicted shifts of up to 100's of kilometers per century are obviously beyond the dispersal ability of trees. Such species would face extinction as climatic zones shift.
Even species which can keep up with the rate of climatic zone shifts may face problems from barriers - any physical presence which prevents the species from continuing to migrate in the necessary direction. An excellent example of barriers possibly causing extinction comes from the documented shift in plant species during the last glaciation.
In the Tertiary there was a high diversity of plants with a circum-polar distribution (water shield, tulip tree, sweet gum, magnolia, hemlock, white cedar, etc.). Despite the fact that they survived the Ice Age in North America, they went extinct in Europe. The possible cause can be seen in the Figure below, a physiographic map of Europe. Where the central portion of North America is generally flat, with mountain belts running
along a roughly north-south trend, Europe has a number of east-west trending features that could act as barriers to the southward dispersal of plant species. In the map above, pick out the Pyrenees, Alps and the Caucasus mountain belts. Also consider the Mediterranean, Black and Caspian Seas, and how they would act as barriers to the migration of plants southwards.
COMMUNITY PROBLEMS
When we consider shifting climatic zones, we must also consider that species act as individuals and not as communities in some cases. In the figure below, species "A" and "B" would be intimately associated under current climatic conditions. However, species "B's" limited dispersal ability
results in a breakup of the community as the temperature zones shift northwards. As species shift their ranges independent of each other, this will result in competition with exotic species not previously encountered, which could lead to extinction.
Alternatively, where obligate relationships exist, the species with the lowest dispersal rate will determine the rate at which the community shifts. An example is when fast moving species are restricted by habitat (plants). For example deer have dispersal rates below 2 kilometers per year because they will not move out of their preferred forest habitat. Some tropical deep forest birds simply do not cross even small unforested areas. Hence, the slowest disperser may cause an entire community to extinction as climatic zones shift.
Habitat Destruction and Climate Change
Any factor that would decrease the probability of a species successfully colonizing a new habitat would increase the probability of destruction. For small isolated populations the question centers on how much of their range remains suitable, and can the species migrate?
In the figure above, in the upper illustration a reserve area has been marked out for a certain species. Areas outside the reserve undergo clear cutting and other forms of disturbance, leaving only isolated patches of the original habitat. However, the northwards shift in the range limit due to climatic change eventually makes the reserve unsuitable for the species. Its survival will now depend on its ability to migrate between small isolated patches in order to stay within its environmental range. If it cannot migrate, it will go extinct locally.