SPECIES EXTINCTION

Evolution is the interplay of the appearance of new life-forms and the disappearance of old ones. The appearance of new life-forms depends upon the availability of diverse habitats, relative stability of climatic regimes, and processes that allow genetic and behavioral modification to, in effect, isolate new life-forms from a common ancestor. When two populations can no longer interbreed, for whatever reason, they assume the status of separate species or subspecies. Historically, processes leading to the loss of life-forms have been related to changes in biophysical conditions, such as particularly rapid changes in climate. Of course it is seldom clear exactly what the causes of extinction are. Species extinction could arise from a combination of factors: changes in ecology; loss of critical habitat; pollution; overharvesting; or competition from an exotic (nonnative) species that is introduced into the ecosystem.

CAUSES AND CONSEQUENCES

The consequences of loss of species for ecosystem function depends upon the role played in the ecosystem by the species or group of species, and by the degree to which those roles are or could be assumed by other biotic components of the ecosystem. In general, the loss of a species in an ecosystem with naturally low species diversity is likely to have a larger consequence than the loss of a species in a more complex environment characterized by high species richness. There have been numerous theoretical and empirical investigations that have sought to validate this simple proposition. Unfortunately, owing to the natural complexity and diversity of ecosystems within the biosphere, no hard and fast rule on the consequences of the loss of species richness on ecosystem function has yet been accepted as above reproach.

What appears much clearer is the impact of human activity on species loss and accelerating species loss worldwide. There is a close parallel between early human migrations and the disappearance of large game species. Overexploitation of biological resources has continued to the present time, and numerous species have disappeared in regional environments as a consequence. For example the combination of overharvesting, pollution, and habitat loss (owing to shoreline restructuring) had much to do with the disappearance of the sturgeon, lake trout, and many other preferred species in the lower Great Lakes. (The introduction of exotic species, such as the sea lamprey, is also a factor.) Of greatest concern at the beginning of the twenty-first century is the potential for wholesale loss of species in some twenty-five hot spots around the world, as a consequence largely of habitat modifications (e.g., clearing tropical forests for agriculture, construction of large dams).

Biodiversity loss is one of the most consistent signs of ecosystem distress syndrome (EDS). Estimates range widely, but generally current estimates of species losses are ten-fold to one thousand-fold greater than historic levels, leading some to speculate that the earth is already entering a period of the sixth major extinction of life on the planet. This extinction, however, differs from the rest, in that the primary cause appears to be the effects of human activity. Humans appropriate more than 50 percent of global primary productivity. They have also altered the chemical composition of the atmosphere, triggering climate change, which in turn destabilizes ecological balance.

IMPLICATIONS FOR HUMAN HEALTH

Humans, as part of the web of life, are not immune to events that trigger the extinction of organisms on the profound scale that appears to be currently taking place; and the implications for human health are numerous and diverse.

Firstly, there are direct effects: for example, the loss of marine fisheries (over 70 percent of the major commercially fished marine stocks are overexploited and in decline), translates for many communities into a loss of reliable food supplies. This contributes to malnutrition—a rising problem, particularly in developing countries, where an estimated two billion people (approximately one-third of the global population) presently suffers from lack of adequate diet. Malnutrition reduces the longevity of a population both directly and indirectly by weakening the immune system, which renders the population more susceptible to diseases. Another direct impact is the loss of potential biological materials that are useful as medicines, both in traditional medicines and as ingredients in modern pharmacology. The loss of the inventory of biotic resources for medicinal purposes directly threatens human health. Finally, there is the loss of economic opportunity, and the loss of social cohesion that often accompanies a degrading environment. An impoverished socioeconomic condition generally is associated with a host of health threats, including substance abuse and violence.

Indirect effects are more difficult to pin down. Clearly, the loss of a significant portion of the species that inhabit a particular ecosystem has major implications for ecosystem functioning, including the provision of ecosystem services (such as production of food, regulation of hydrology, and pollination) that form part of the life-support system for humans and other species. Degradation of such environments through human activities, with a subsequent loss of species components, poses a host of threats to human health through the loss of critical ecosystem functions.

Loss of diversity of pathogens poses an entirely different set of issues. The history of European settlement in the Great Lakes is also a history of purposeful drainage of swamps and wetlands along the southwest shore of Lake Ontario in an effort (largely successful) to eliminate the mosquito vectors of malaria. In many parts of the world today, eradication of pathogens is part of a public health strategy which, in addition to the intended consequences, also degrades ecosystems. Worldwide, there has been a successful eradication of the smallpox virus. At the same time, the widespread use of antibiotics and pesticides is creating resistant strains (not new species) of pathogens and crop pests, thus contributing to an increase in genetic biodiversity. In this case, however, the increase in biodiversity is to the detriment of humans.

On balance, the impact of humans on biodiversity has resulted, through a variety of mechanisms and pathways, in increased human health burdens.

David J. Rapport

(see also: Biodiversity; Ecosystems; Environmental Determinants of Health )

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