Ann Arbor, Michigan
Ecosystems are being damaged by human-induced disruption of the dynamics between and among species. Such ecosystem impacts can have a domino or cascading effect within a system. In some cases, an extinction may not appear to be catastrophic and this gives the public a false sense of security, i.e. the passenger pigeon went extinct, and so far we have seen no apparent major ramifications for the human population. There are other examples that have had major impacts for humans.
An observed increase in Prairie Canid populations, attributable to humans drastically reducing wolf populations, provides a good example of unexpected cascading effects. In the natural system, wolves control coyotes, and coyotes control red foxes; so if you suppress wolf populations, coyote populations explode; if you suppress coyotes, populations of red foxes explode. So what have humans done by suppressing the wolf? We have caused a population explosion among red foxes. Why is this a problem? Down the food chain, red foxes hunt water fowl. Currently, water fowl abundance continues to fall; duck populations in red-fox areas have been decimated (Meanwhile, humans are spending lots of money to try to protect water fowl -- so humans can hunt them -- but the reintroduction of wolves or coyotes would probably be more effective). Also, skunk and raccoon populations are exploding because there are no wolves to control them. Rabies from raccoons is now a significant threat to humans in some parts of the country (e. g., U.S. east coast). So by eliminating wolves, we have altered the balance of an entire ecosystem. These are "surprises."
Another example is a case involving the dodo bird, which was brought to extinction by humans hunting it as a food source. In that ecosystem, the seed husk of a predominant local tree is so tough that the seeds canÕt sprout. The dodo bird would eat the fruit of the tree, swallow the seed and use rocks in its gizzard to sand down the seed husk. In this way, the dodo bird ensured the propagation of the trees. So by killing off the bird, humans also killed off the tree. This is how effects can cascade through an ecosystem. A third example is the replacement of US grasslands with agricultural lands. This has caused a decline in grassland birds. The use of fertilizers, pesticides and herbicides on agricultural land further damages the bird populations.
Chemicals in the environment do affect human beings. Pesticides in ground water are now a fact of life, contrary to the long-held, popular myth that ground water could be entirely purified by filtering through soil. There is bird embryo deformation as a result of our use of agricultural chemicals. There is also increasing incidence of cancer in fish as a result of these chemicals. Birds and fish that are mainly bottom feeders are most affected by the build up of chemical residues. Theodora Colburn and others are investigating connections between environmental chemicals (such as PCBs) that may mimic estrogens and the reductions in sperm count in human and other animals, as well as other dysfunctions of the reproductive system.
On the positive side, laws enacted to protect ptarmigan and other species have been largely successful and some species are making a comeback. In such cases, people foresaw potential extinctions, and intervened in time. Dead birds used to be fashionable on womenÕs clothing and hats but the publication of data on the tons of bird feathers used and the numbers of birds being devastated turned that fashion around and stopped the bird slaughter.
Terry Root is studying impacts of temperature changes from global warming (especially winter night impacts) on wintering birds in North America. Using National Audubon Society Christmas Bird Count data from 1900 to the present, she is examining the relationship of environmental factors (such as temperature, frost, vegetation, and humidity) to the distribution of birds. Bird ranges are strongly correlated with isotherms. A large percentage of species have their northern boundary affected by temperature, frost, and/or vegetation. Those species that are limited by temperature will move as the climate changes; those limited by vegetation will not be able to move so quickly. This will disrupt communities of birds, leading to potentially catastrophic results. The synergistic effects of a combination of these effects with habitat fragmentation caused by increased development and agriculture will likely exacerbate this problem. What will happen when land-use changes split up predators and prey? An additional factor is the introduction of non- native species and their impacts on native ones. Through the tearing apart of communities, land use changes, and introduction of alien species, many potential surprises may emerge.
Root has also been studying the effect of climate on passerine birds. Her data show that the northern boundary metabolic rate is 2.5 times the basal metabolic rate for all passerine birds limited by temperature. A small scale study showed that temperature is the critical variable for passerines. Results from both 1990 and 1991 data show that birds limited by temperature do move with changes in temperature. In short, global warming will prompt the immediate migration of some types of birds. In effect, this is the beginning of an ecological climate model. Additional work is needed to incorporate birds limited by vegetation, land use patterns, community disruption, relations between species, human impacts, etc., to develop a more complete model.
On the subject of the appropriate scale for biological/global change research, Root believes that small scale experiments are needed to understand the internal dynamics of an ecosystem, but that one cannot necessarily extrapolate these results out to the large scale. Large scale research helps to pinpoint which species should be studied at the small scale. A good research plan therefore begins with large scale studies to find associations and define which species to study at the small scale; it will probably then be necessary to revisit the larger scale. Most biology experiments are conducted on areas smaller than a tennis court. Such work is important but inadequate for understanding regional- and global-scale processes.
The interactions between biotic and abiotic effects also merit study. The findings of small- and large-scale biological research must be studied and coupled with the research of climatologists working on larger scales. Impact assessment, policy assessment, and policy choice feed back into anthropogenic disturbances.