Over Exploitation

Many of the best documented cases of individual species being driven to extinction or near-extinction by humans are those of over- exploitation.

The passenger pigeon--a species that resembled the smaller, mourning dove--was in the early 1800s the most abundant bird in North America, and so plentiful that migrating flocks of a billion or more individuals would darken the skies of parts of the eastern U.S. for days at a time. By the end of the last century it had been hunted to the brink of extinction, and in September of 1914, in a Cincinnati zoo, the passenger pigeon disappeared forever with the death of the last remaining bird. The American bison, or buffalo, of the Great Plains was also nearly hunted out of existence in the same century, and its larger, woods-dwelling relative was driven to extinction.

As many as a quarter of all the bird species in the world may have similarly vanished in the course of the last 1000 years with the expansion of human populations through the islands of the South Pacific. The spread of early people through the New World, about 10,000 years ago, was probably responsible for the extinction of many of the large mammals that were originally here: now-extinct mammoths, sloths, and cave bears are known to have been hunted by those who first walked through North and South America. The same impact was felt by large mammals in Australia, New Zealand, and Madagascar. The current and rapid loss of tropical hardwoods in many regions due to high commercial demand, low rates of successful replacement, and the long periods of growth necessary to produce new, marketable resources has raised concern about over- exploitation of some species, such as rosewood, although none of the trees are known to have been driven to extinction.

Over-exploitation is also a major factor in reducing the natural biodiversity of marine fisheries through major reductions in populations, although again, no extinctions have been documented. During the last two decades, the world has seen the collapse of a number of marine fisheries. Some of these have recovered, but others, such as the cod and haddock fisheries in the North Atlantic, have not. Even for those that recover, the consequences of the original over-exploitation on population dynamics and genetic diversity are now only poorly understood. What is often apparent is a systematic decrease in the size, and hence age, of the individuals that are harvested. The selective loss of larger fish has significant impacts on those that remain. If fertility is strongly related to body size, as is the case for many fish species, over-exploitation not only reduces the abundance of a species, but it may also make recovery more difficult in systematically removing the most fecund individuals. The ensuing consequences for overall ecosystem functioning and biodiversity are as yet not well understood.

Whole ecosystems can also be affected by over-exploitation. For example, a reduction in organic carbon and nutrients, including phosphorus or nitrogen, as may occur in intensively farmed areas, decreases the fertility of soils. When losses are severe, the resulting depletion can lead to either more intensified use by adding more fertilizers and then herbicides and pesticides to control weeds and pests (in the cycle noted above), or to abandonment. If abandoned, the land will probably not recover its original component of plant and animal species because of the depleted nutrients. Through this chain of happenings, an over-exploitation of the soil for agricultural gain can have long term, negative impacts on the biodiversity of the region.

Alien introductions
Introductions and invasions of alien species of plants and animals is a long-recognized problem, as detailed in an earlier issue of CONSEQUENCES. We have only limited ability to predict quantitatively the results of any particular intruder, including its capability of establishing a permanent, reproducing population. What is certain is that some areas are by nature more susceptible. Continental forests are reasonably resistant to newly introduced tree species, except in cases where they have been disturbed by heavy cutting or partial clearing. Native meadows and prairies, when disturbed, have also proven particularly susceptible to intruders, as is the case for the many grasslands around the world that have been converted to pasture or cultivated land. For example, many of the now common grasses in the intermontane western U.S. and southwestern Canada are transported Eurasian weeds. These species were able to invade and become established because the original perennial tussock grasses were unable to support the intensification of grazing from large-scale cattle ranching.

Freshwater lakes and streams have little immunity to invading species. Alien plants or animals seem able to establish reproducing populations relatively easily, and the new species often have significant impacts on biotic composition, and on a variety of ecosystem processes. Two examples of the kind of changes that can result from even well-meant introductions are the purposeful introduction of game fish to many lakes and streams throughout the world that replaced native varieties, and the ecological havoc that followed the introduction of the Nile perch in Lake Victoria in 1960 to benefit commercial fishing. In less than thirty years, the appetite of the Nile perch, whose food is smaller fish, led to the extinction of about thirty species of fish that were native to the lake.

In terrestrial ecosystems, the largest changes occur when the intruder brings quite different traits from those of native species. The best documented example is that of the introduction of the exotic tree, Myrica faga, into Hawaii, which has resulted in large changes in ecosystem dynamics. The significant difference, in this case, was the ability of the introduced tree--a legume like peas and beans and clover--to convert atmospheric nitrogen to ammonia, a characteristic not previously present in those ecosystems. This ability of the introduced tree increases the nitrogen content of soils, and thus alters the raw materials on which many other plant species depend.

Introduced species with characteristics that are not qualitatively different from those of native species, can through force of numbers have large and long-lasting effects on them. About 100 European starlings were released in New York City in 1890-91 by a collector bent on establishing all the birds mentioned in the writings of William Shakespeare. The result, evident throughout the country today, is a diminished number of many native American songbirds, through competition for nest-sites, in which the aggressive and now very abundant starling has been extremely successful