SPECIES AND HABITATS

The U.S. Endangered Species Act of 1973, one of the strongest and most influential environmental statutes worldwide, was inspired by the recognition that human activities are driving species to extinction. Public concern about the environment in the early 1970s was centered on pollution as a threat to human health, and pollution remains the most visible of environmental issues to Americans. The "human environment" and human well-being take precedence over other species and the total environment in virtually all legislation. Our culture is decidedly anthropocentric, and we worry little about the future beyond our lifetimes. But the Endangered Species Act (ESA) is a bit different from other environmental statutes. Those who crafted the Endangered Species Act saw value in other living things and understood that human destruction of habitat threatened the existence of many species. The purpose of the Act was stated clearly: "to provide a means whereby the ecosystems upon which endangered species and threatened species depend may be conserved."

Despite its virtues the ESA is far from perfect from a biological perspective. The implied theme of saving species through the conservation of ecosystems or habitats—which might provide for truly proactive conservation—was not well developed in the ESA. In fact, the term ecosystem does not appear in the Act after the preliminary statement of purposes. Ecosystem conservation was an idea far ahead of its time in 1973 and remains to be firmly encoded in law today. Not only does the United States lack a national strategy to conserve biodiversity and sustain healthy ecosystems, but legal constraints on destroying habitat on either public or private land are extremely limited. Aside from wetlands regulations, zoning ordinances, and an assortment of local statutes, there are few restrictions on what private landowners can do with their lands. As we review in this book, the ESA generally prohibits destruction of the habitat of animal species listed under the Act, and this prohibition applies on private as well as federal lands. There are ways to get around this rule, however, and the mitigation required of landowners for destroying habitat of listed species often has been meager. But the bigger problem over the long term is that referred to above: the ESA, and environmental policies generally, have not encouraged proactive actions that might preclude the need to list species as endangered or threatened. Such actions fall mainly within the realm of habitat or ecosystem conservation, the subject of this book.

The meager attention to ecosystems and habitats in conservation policy is not all that surprising. These concepts are poorly understood by the general public, and even biologists seldom agree on what they mean in detail. Among many conservationists the feeling seems to be that saving species is hard enough—don't bother us with the complexity of ecosystems! But there are signs of increased interest in the idea of habitat-based conservation among scientists, legal scholars, lawmakers, and citizens on all sides of the issues surrounding endangered species. People are beginning to realize that conflicts can be avoided, or at least reduced, by fulfilling the needs of many species at once through the broad-scale conservation of habitats, and that such actions may keep some species off the endangered species list, thus reducing the regulatory burden for private landowners. To encourage this interest in habitat-based conservation and channel it along scientifically defensible lines, we have written this book.

Why Worry about Habitats?

Sustaining healthy habitats and ecosystems as a way of maintaining viable populations and preventing extinctions makes sense from a scientific standpoint (the Note from the Authors explains our use of habitat, ecosystem, and related terms). Simply put, if we want to save species we must protect a sufficient quality and quantity of habitats. This understanding did not arise overnight, but developed over many years of observation and research by scientists and others. The earliest humans must have observed that not only they but also other animals require food, water, and shelter to survive. Later, naturalists began to expand this body of knowledge by taking note of the particular habitat conditions under which species of plants and animals are found in Nature. Although for centuries the formal science of biology was preoccupied with naming and describing new species, some of the more perceptive individuals became intimately familiar with the habitats in which species were found and how species lived their lives. This detailed, natural historic information was central to the development, in the middle and late nineteenth century, of the science of ecology.

Ecology has always been a habitat-centered science. In 1840 the German chemist Justus von Liebig formulated the "law of the minimum," which stated that each kind of plant requires some minimal quantity of nutrients, water, or other materials to survive (Liebig 1840). Liebig's law was later extended to animals and, in 1913, it was elaborated into Victor Shelford's "law of tolerance," with the recognition that each species lives within certain bounds of temperature, humidity, soil texture and chemistry, and other factors (Shelford 1913). The concept of the ecological niche was a continuation of this line of thinking. To ecologists, the niche comprised the complete set of habitat requirements of a species—that is, the upper and lower limits of all environmental variables within which the species could survive. Many ecologists included interspecific interactions, position in the food web, and other community-level details in their characterizations of niches (Smith 1974). Although niche theory is no longer in vogue in ecology, all biologists recognize the inseparability of species and habitat, not only in terms of living requirements but also in terms of natural selection (the process of differential survival and reproduction of individuals) and the continual interchange of matter and energy between organisms and their surroundings. A distinct boundary between an organism and its environment is illusory—in a fundamental sense they are one. The centrality of the species-habitat connection in biology helps explain the confusion and outrage expressed by scientists and others over a 1994 Circuit Court of Appeals decision (later overturned by the Supreme Court) that destruction of the habitat of an ESA-listed species does not constitute "taking" of the species (Noss and Murphy 1995).

To many laypersons the habitat requirements of a species begin and end with the kind of environment you find the species...