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 in. Hence, if a spotted owl (Strix occidentalis) is seen in a young, second-growth forest, for example, it is concluded that spotted owls do not need old growth to survive. Being observed in a habitat and maintaining a viable population in a habitat, however, are two very different things.

The spectrum of habitats over which a species is observed varies widely in quality. Not uncommonly, an individual organism—especially of a mobile species such as most birds—wanders into an area that is clearly unfit for survival; its days in that area are numbered. A loon landing in a wet parking lot that it mistakes for a lake would be an example of this situation. In many other cases more subtle differences in habitat quality and corresponding responses of populations require intensive, scientific study to discern. In some areas—called sources—habitat quality is high, the rate of reproduction exceeds the rate of mortality, and the population grows or exports individuals to other areas. In some of these other sites—called sinks—mortality exceeds reproduction, and a population can persist only if immigrants frequently disperse in from nearby source populations. Very slight (to human observers) distinctions in habitat structure, prey populations, predators, pathogens, competitors, disturbances, and other factors may distinguish a source from a sink. The spatial configuration of sources and sinks across a landscape may ultimately determine the survival of the entire population or metapopulation.

Species-specific area requirements also determine habitat suitability. A site that appears ideal for a species in terms of habitat structure may fail to support a population, or even an individual, if it is too small. Many songbird species are "area-sensitive" and usually breed only in tracts of forest or grassland many times larger than the size of their territories (Whitcomb et al. 1981; Robbins et al. 1989; Herkert 1994; Vickery et al. 1994). The probability of finding breeding pairs or populations of area-sensitive species generally increases with the size of the habitat patch. The animals with the largest area requirements are generally large, mammalian carnivores—for this and other reasons, they are absent from many regions of North America today. For example, in the Rocky Mountains, individual, annual home ranges are on the order of 150 km2 for black bears, more than 400 km2 for mountain lions and wolverines, and nearly 900 km2 for grizzly bears. The gray wolf, a social predator, uses from 250 to >2,000 km2 per pack territory. For long-term persistence of populations of these species, wild areas on the order of 100,000 km2 or larger seem to be required (Noss et al. 1996 and references therein). Because no single reserve in North America is this large, conservation proposals for these species must employ networks of reserves connected by regional-scale corridors. But even for species much less demanding than large carnivores, habitat needs often must be considered in terms of the constellation of patches of potentially suitable, potentially connected habitat across a large landscape, rather than site by site. Single sites or a collection of disconnected sites will often be insufficient for long-term survival.

Because of factors such as those just summarized, habitat-based conservation is more complicated than it might initially seem. It gets even more complex when we begin considering the needs of many species at once, along with the ecological processes that keep their habitats in suitable condition. The science behind the conservation of communities and ecosystems is still in its infancy, though it is developing rapidly and has made substantial progress in recent years. Despite the challenges, there is no alternative to habitat-based conservation, short of saving species only in captivity—and this is really no alternative at all.

The Problem of Habitat Alteration

Habitat-based conservation is logical and consistent with accepted scientific principles. But there is a more urgent reason to focus on habitats and ecosystems in conservation planning: natural habitats are disappearing rapidly. Scientists agree that habitat alteration is the greatest threat to species and ecosystems in the United States and worldwide (Ehrlich and Ehrlich 1981; Diamond 1984; Wilcox and Murphy 1985; Wilson 1985; Ehrlich and Wilson 1991; Soulé 1991; Noss and Cooperrider 1994; Noss et al. 1995). Habitat alteration comprises not only the direct, physical conversion of a natural area to an unnatural habitat (habitat destruction)—for example, rainforest converted to cattle pasture or a wetland replaced by a parking garage—but also includes breaking a large, contiguous patch of habitat into smaller patches (habitat fragmentation) and changes in the composition, structure, or function of an ecosystem(habitat degradation) (Noss et al. 1995; Noss and Peters 1995). These processes can take place on several spatial scales, from the "internal" degradation or fragmentation of a small patch of relatively uniform habitat, to the fragmentation and homogenization of entire landscapes and regions (Noss and Csuti 1994).

Habitat alteration may be caused by agriculture, logging, mining, urban development, fire suppression or other changes in the natural disturbance regime, alteration of stream flows because of dams or diversions, disturbance by off-road vehicles, heavy grazing by livestock, pollution, introduction and invasion of nonnative species—which also have biological effects, such as predation and competition—or other factors. Note that most of this destruction is not malicious; it is an unintended consequence of normal, basically legitimate economic activity (habitat alteration by golf course and ski resort development, off-road vehicle use, and other recreational activities is an exception to this rule). The connection between human land use and habitat alteration is difficult to break.

Fundamentally, human activities need not result in severe and irreversible losses of biodiversity. We evolved on this Earth and, at least in some places and times, had an apparently sustainable relationship with the rest of Nature. Perhaps this relationship can be renewed. This realization is a basic premise—or hope—of conservation planning and ecosystem management. We know that, in many cases, relatively minor changes in how we extract resources, design developments and highway networks, or manage habitats, can make tremendous differences in terms of impacts on species and communities. These changes must be made. But with a growing population and economy, the possibility of a sustainable relationship between humans and Nature becomes increasingly remote. With so many of us, consuming so much, our collective impact is staggering. Barring major changes in the scale and intensity of human activity, the inevitable consequence of human land use is habitat destruction, fragmentation, and degradation, which in turn result in a decline in populations of species that are unable to adapt to the new habitat or its new occupants. Population declines are usually first evident on a local scale, but ultimately widespread habitat alteration results in endangerment and extinction of species. This story has been played out for at least hundreds of years (Noss and Cooperrider 1994). We expect it to continue but are committed to finding ways to lessen the damage. This is perhaps the best conservationists can ever do.

Listing of species under the ESA is one, often belated, sign that habitat alteration has gone too far. As of 31 December 1996, the U.S. Fish and Wildlife Service (FWS) and National Marine Fisheries Service listed 1,051 species (437 animals [which includes four species with dual status], 614 plants) in the United States as endangered or threatened under the ESA. Habitat alteration is considered the leading factor in the endangerment of species in the United States (Flather et al. 1994; Wilcove et al. 1996; fig. 1.1). More specific studies have come to similar conclusions. For example, direct habitat destruction was a contributing factor in the disappearance of three-quarters of the 27 species and 13 subspecies of freshwater fishes that have gone extinct in North America over the past century (Miller et al. 1989) and, along with introductions of nonnative species, remains the leading threat to fish species and aquatic biodiversity in general (Williams et al. 1989, Allan and Flecker 1993).

In part because habitat alteration has not abated, the list of endangered and threatened species has grown steadily since the ESA was first passed, except for a period between March 1995 and April 1996, when Congress imposed a moratorium on species listings. The list of candidate species (those being considered for listing) also increased rapidly from the inception of the ESA until July 1995, when it shrank by nearly 4,000 species with the FWS's elimination of Category 2 (C2), those candidates for which sufficient information to decide whether listing is warranted is not currently available. The decision to eliminate these species from consideration for listing was based not on scientific information but apparently on the perceived political liability of having such a large list of candidates. At the time of this writing the candidate species list contains only those species for which sufficient information to list is in the hands of the FWS, and includes 182 Category 1 (C1) species and an additional 238 species proposed for listing (i.e., well into the listing process). The burden of proof for listing is on citizens who petition the government to add species to the list, and few government funds are available for status surveys.

Because getting a species listed is influenced by politics (a classic example is the northern spotted owl, Strix occidentalis caurina, whose listing was delayed for several years due to factors that a federal judge determined had nothing to do with biology), a better indication of the status of species can be found in lists produced by independent scientific and conservation organizations. In particular, The Nature Conservancy ranks species in terms of their rarity and vulnerability at global and state scales. In the United States, 1,339 species (160 vertebrates, 166 invertebrates, and 1,013 vascular plants) are considered critically imperiled globally (G1); 1,831 species (157 vertebrates, 157 invertebrates, and 1,517 vascular plants) are considered imperiled globally (G2); and an additional 3,076 species (256 vertebrates, 264 invertebrates, and 2,556 vascular plants) are considered vulnerable globally (unpublished Natural Heritage Central Database, January 1996). These figures—together with others such as the number of species that have already gone extinct in the United States since European settlement—give a more accurate picture of the status of biodiversity than the official list of endangered and threatened species.