Sharing the Planet: Can Humans and Nature Coexist?

• Author: Tuxill, John
• Position: Effects of urban growth

The fate of birds, mammals, fish, and other species depends on humans finding ways to reduce the size of their impact on the planet.

LIKE THE DINOSAURS 65,000,000 years ago, humanity finds itself in the midst of a mass extinction, a global evolutionary convulsion with few parallels in the entire history of life. Unlike the dinosaurs, though, humans are not simply the contemporaries of a mass extinction--they are the mason for it.

The loss of species touches everyone, no matter where or how they live. Earth's endowment of species provides humanity with food, fiber, and many other products and "natural services" for which there is no substitute.

About 25% of drugs prescribed in the U.S. include chemical compounds derived from wild organisms, and billions of people worldwide rely on plant- and animal-based traditional medicine for their primary health care. Biodiversity provides a wealth of genes essential for maintaining the vigor of crops and livestock. It provides pollination services, mostly in the form of insects, without which we could not feed ourselves. Frogs, fish, and birds provide natural pest control; mussels and other aquatic organisms cleanse our water supplies; and plants and microorganisms create our soils.

Birds, mammals, reptiles, amphibians, and fish constitute the vertebrate animals, distinguished from invertebrates by an internal skeleton and a spinal column--a type of anatomy that permits, among other things, complex neural development and high metabolic rates.

Vertebrates combined total about 50,000 species and can be found in virtually all environments on Earth, from the frozen expanses of Antarctica to scorching deserts and deep ocean abysses. By virtue of the attention they receive from researchers, vertebrates can serve as ecological bellwethers for the multitude of small, obscure organisms that remain undescribed and unknown. Since vertebrates tend to be relatively large and to occupy the top rungs in food chains, habitats healthy enough to maintain a full complement of native vertebrates will have a good chance of retaining the invertebrates, plants, fungi, and other small or more obscure organisms found there. Conversely, ecological degradation often can be read most clearly in native vertebrate population trends.

Disappearing birds

Estimates are that at least two out of every three bird species are in decline worldwide. Four percent--403 species-- are endangered.

The most threatened major groups include rails and cranes, parrots, terrestrial game birds (pheasants, partridges, grouse, and guans), and pelagic seabirds (albatrosses, petrels, and shearwaters). About one-quarter of the species in each of these groups is endangered. While just nine percent of songbirds are threatened, they still contribute the single largest group of endangered species because they are far and away the most species-rich bird order.

The leading culprits in the decline of birds are a familiar set of interrelated factors, all linked to human activity: habitat alteration, overhunting, exotic species invasions, and chemical pollution of the environment. Habitat loss is by far the leading factor--at least three-quarters of all threatened bird species are in trouble because of the transformation and fragmentation of forests, wetlands, grasslands, and other unique habitats by human activities, including intensive agriculture, heavy livestock grazing, commercial forestry, and suburban sprawl.

In some cases, habitat alteration is intensive and large-scale, as when an internationally funded development project converts large areas of native forest to plantation crops, or a large dam drowns a unique river basin. In other instances, habitat is eroded gradually over time, as when a native grassland is fragmented into smaller and smaller patches by farming communities expanding under a growing population.

High concentrations of gravely endangered birds are found on oceanic islands worldwide. Birds endemic to insular habitats account for almost one-third of all threatened species and 84% of all historically known extinctions. Since island birds often are concentrated in just a handful of populations, if one such group is wiped out by a temporary catastrophe such as a drought, the birds usually have few population sources from which they can recolonize the formerly occupied habitat. Equally important is that many island birds have evolved in isolation for thousands or even millions of years. Such species are particularly vulnerable to human hunting, as well as predation and competition from non-native, invasive species. (Invasives are highly adaptable animals and plants that spread outside their native ecological ranges--usually with intentional or inadvertent human help--and thrive in human-disturbed habitats.)

No island birds have been more decimated than those of Hawaii. Virtually all of its original 90-odd bird species were found nowhere else in the world. Barely one-third of the species remain alive today, and two-thirds of these continue to be threatened with extinction. The degree of ecological disruption is so great that all lowland Hawaiian songbirds are non-native species introduced by humans.

An equally disturbing trend is population declines in more widespread species, particularly those that migrate seasonally between breeding and wintering grounds. Long-term population declines are tied to a host of contributing hazards. Habitat loss squeezes species on both breeding and wintering grounds, as well as at key stopping points--such as rich tidal estuaries for shorebirds--along their migratory...