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The Study of Habitat: A Historical and Philosophical Perspective
It is a good morning exercise for a research scientist to discard a pet hypothesis every day before breakfast. It keeps him young.
An animal's habitat is, in the most general sense, the place where it lives. All animals, except humans, can live in an area only if basic resources such as food, water, and cover are present and if the animals have adapted in ways that allow them to cope with the climatic extremes and the competitors and predators they encounter. Humans can live in areas even if these requirements are not met, because we can modify environments to suit our needs or desires and because we potentially have access to resources such as food or building materials from all over the world. For these reasons, humans occupy nearly all terrestrial surfaces of the earth, but other species of animals are restricted to particular kinds of places.
The distribution of animal species among environments and the forces that cause these distributions have frequently been the subjects of human interest, but for different reasons at different times. The primary purpose of this introductory chapter is to review some of the reasons why people study the habitats of animals and to outline how these reasons have changed over time. We also introduce the major concepts that will be addressed in this book.
Curiosity about Natural History
Throughout recorded history, humans, motivated by their curiosity, have observed and written about the habits of animals. The writings of naturalists were, for centuries, the only recorded sources of information about animal–habitat relationships. Aristotle was among the first and best of the early naturalists. He observed animals and wrote about a wide variety of subjects, including breeding behavior, diets, migration, and hibernation. Aristotle (384–322 BC) also noted where animals lived and occasionally speculated about the reasons why:
A number of fish also are found in sea-estuaries; such as the saupe, the gilthead, the red mullet, and, in point of fact, the greater part of the gregarious fishes.... Fish penetrate into the Euxine [estuary] for two reasons, and firstly for food. For the feeding is more abundant and better in quality owing to the amount of fresh river-water that discharges into the sea.... Furthermore, fish penetrate into this sea for the purpose of breeding; for there are recesses there favorable for spawning, and the fresh and exceptionally sweet water has an invigorating effect on the spawn. (Aristotle 344 BC)
Interest in natural history waned after Aristotle's death. Politics and world conquest were the focus of attention during the growth of the Roman Empire, and interest in religion and metaphysics suppressed creative observation of the natural world during the rise of Christendom (Beebe 1988). As a result, little new information was documented about animals and their habitats for nearly 1700 years after the death of Aristotle. Yet, as Klopfer and Ganzhorn (1985) noted, painters in the medieval and pre-Renaissance periods still showed an appreciation for the association of specific animals with particular features of the environment. "Fanciful renderings aside, peacocks do not appear in drawings of moors nor moorhens in wheatfields" (Klopfer and Ganzhorn 1985, 436). Similar appreciation is seen in artwork from India, China, Japan (e.g., Sumi paintings), and elsewhere during this period. Thus keen observers noticed relationships between animals and their habitats during the Dark Ages, but few of their observations were recorded.
The study of natural history was renewed in the seventeenth and eighteenth centuries. Most naturalists during this period, such as John Ray (1627–1705) and Carl Linnaeus (1707–1778), were interested primarily in naming and classifying organisms in the natural world (Eiseley 1961). Explorers made numerous expeditions into unexplored or unmapped lands during this period, often with the intent of locating new trade routes or identifying new resources. Naturalists usually accompanied these expeditions or traveled on their own, collecting and recording information about the plants and animals they observed. Many Europeans during this period also collected feathers, eggs, pelts, horns, and other parts of animals for "collection cabinets." Some cabinets were serious scientific efforts, but most were not. Nevertheless, new facts about the existence and distribution of animals worldwide were gathered during this time, and the resulting advances in knowledge generated considerable curiosity about the natural world.
During the nineteenth century, naturalists continued to describe the distribution of newly discovered plants and animals, but they also began to formulate ideas about how the natural world functions. Charles Darwin (1809–1882) was among the most prominent of these naturalists. His observations of the distributions of similar species were one set of facts among many that he marshaled to support his theory of evolution by natural selection (Darwin 1859). The work of Darwin is highlighted here, not only because he recorded many new facts about animals, but also (and more importantly) because the theory of evolution by natural selection forms the framework and foundation of the field of ecology.
Curiosity about Ecological Relationships
In the early 1900s, curiosity about how animals interact with their environment provided the impetus for numerous investigations into what are now called ecological relationships. Interest in these relationships initially led to detailed descriptions of the distribution of animals along environmental gradients or among plant communities. Merriam (1890), for example, identified the changes that occur in plant and animal species on an elevational gradient, and Adams (1908) studied changes in bird species that accompany plant succession. Biologists living in this period postulated that climatic conditions and availability of food and sites to breed were the primary factors determining the distributions of animals they observed (see Grinnell 1917a).
Biologists in the early to mid-1900s, however, recognized that the distribution of some animals could not be explained solely on the basis of climate and essential resources. David Lack (1933) was apparently the first to propose that some animals (in this case, birds) recognize features of appropriate environments, and that these features are the triggers that induce animals to select a place to live. Areas without these features, according to Lack, generally will not be inhabited, even though they might contain all the necessary resources for survival. Lack's ideas gave birth to the concept of habitat selection and stimulated considerable research on animal–habitat relationships during the next 60 years.
Svardson (1949) developed a general conceptual model of habitat selection, and Hilden (1965) later expressed similar ideas. Their models characterized habitat selection as a two-stage process in which organisms first use general features of the landscape to select broadly from among different environments, and then respond to subtler habitat characteristics to choose a specific place to live. Svardson (1949) also suggested that factors other than those associated with the structure of the environment influence selection. For example, whether an animal stays or leaves a particular place could be influenced by conspecifics (Butler 1980),interspecific competitors (Werner and Hall 1979), and predators (Werner et al. 1983), as well as by features of the environment that are directly or indirectly related to resources needed for survival and reproduction. Habitat selection, therefore, has come to be recognized as a complicated process involving several levels of discrimination and spatial scales and a number of potentially interacting factors. Study of these factors and the behaviors involved in habitat selection has resulted in a wealth of information about why we find animals where we do (see Stauffer  for an overview of the recent history of habitat studies).
The distribution of animals is also intimately tied to the concept of niche. This concept has been defined in multiple ways over time (see e.g., Schoener 1989; Griesemer 1992; Pianka 1994 for historical overviews) and continues to be the subject of much discussion. Grinnell (1917b) formally introduced the term when he was attempting to identify the reasons for the distribution of a single species of bird. His assessments included spatial considerations (e.g., reasons for a close association with a vegetation type), dietary dimensions, and constraints placed by the need to avoid predators (Schoener 1989). Thus, in this view, the niche included both positional and functional roles in the community. Elton (1927) later described the niche as the status of an animal in the community and focused on trophic position and diet. Views of the niche articulated by Grinnell and Elton are often contrasted, but Schoener (1989) argued that they had much in common, including the idea that a niche denotes a "place" in the community, dietary considerations, and predator-avoiding traits. Hutchinson (1957) articulated the multivariate nature of the causes of animal distribution in his presentation of the n- dimensional niche. In this view, niche dimensions are represented by multiple environmental gradients. A given species (or population) can exist in only a subset of the conditions defined by all the gradients (its potential or fundamental niche) but may be further restricted in distribution (its realized niche) by predators and competitors. Odum (1959) viewed the niche as the position or status of an organism in an ecosystem resulting from its behavioral and morphological adaptations. His idea of the niche was dependent on both where an organism lives and what it does, but he separated, to some degree, habitat from niche with the analogy that an organism's "address" is its habitat and its "profession" is its niche. More recent ideas about the niche (e.g., MacArthur and Levins 1967; Levins 1968; Schoener 1974) consider niche axes as resources (i.e., those important for an animal) and niche as the combination of several "utilization distributions" along those axes. The point of our brief review of the concept of the niche is to illustrate that, although the term can be viewed in a variety of ways, most concepts include elements that are traditionally considered part of habitat. Thus studies designed to describe or define an animal's niche (of which there have been many) almost always elucidate animal–habitat relationships as well.
Hunting Animals for Food and Sport
The earliest humans relied, in part, on killing animals for survival, and they undoubtedly recognized and exploited the patterns of association between the animals they hunted and the kinds of places where these animals were most abundant. Use of fire by Native Americans altered the ecosystems in which they lived (Botkin 1990) and influenced (probably intentionally) the number of animals they hunted. Similarly, people who later made an "economic" living by trapping and hunting, or could afford the luxury of hunting for sport, knew where to find animals and probably speculated accurately about the habitat features that influenced the abundance of game species. Marco Polo reported, for example, that in the Mongol Empire in Asia, Kublai Khan (AD 1215–1294) increased the number of quail and partridge available to him for falconry by planting patches of food, distributing grain during the winter, and controlling cover (Leopold 1933). This advanced system of habitat management suggests a general understanding of the habitat requirements of target game species, but it is unlikely that the information was obtained through organized studies of habitat use. Also, the men who hunted and trapped for subsistence or sport rarely recorded their knowledge about habitats for posterity.
Not until people began to attempt to apply biology systematically to the management of game as a "crop" in the early 1900s did they realize that "science had accumulated more knowledge of how to distinguish one species from another than of the habits, requirements, and inter-relationships of living population" (Leopold 1933, 20). The absence of information about habitat requirements of most animals and the desire to increase game populations by manipulating the environment stimulated detailed investigations of the habitats and life histories of game species. H. L. Stoddard's work on bobwhite quail (Colinus virginianus ), published in 1931, and Errington and Hammerstrom's work on pheasants, published in 1937, exemplify early efforts of this kind.
Studies similar to Stoddard's have been conducted on most game animals in North America from 1930 through the present day (e.g. Bellrose 1976; Wallmo 1981; Thomas and Toweill 1982), but many of these studies only summarize general habitat associations and do not identify critical habitat components. Since the early 1980s, the number of hunters has increased while undeveloped land available for managing wild animal populations has decreased. The need to manage populations more intensively is therefore great, and detailed knowledge of habitat requirements is essential for this task. Studies of the habitat requirements of game animals continue to be conducted, as one can easily see by reviewing recent scientific journals on wildlife management.
Public Interest and Environmental Laws
Human activities have dramatically disturbed natural environments in North America and throughout the world. These disturbances have been associated primarily with the rapid increase in the size of the human population and the exploitation of natural resources, including wild animals, for human use. Interest in wild animals by the general public also increased during this period, and concern about the negative effects of human activities on animal populations and other aspects of the natural environment eventually led to the passage of laws in the United States that were designed to aid management of wild animals or reduce environmental degradation. The following summary pertains to U.S. history; it is beyond the scope of the text to review public interest and environmental law in other nations.
Public interest early in the century focused on "game" animals, and some laws passed in the 1930s reflected this interest. The Migratory Bird Hunting Stamp Act of 1934 and the Pittman-Robertson Federal Aid in Wildlife Restoration Act of 1937, for example, primarily taxed sportsmen and provided funds for management of waterfowl and other hunted species (see table 1.1). As noted in the previous section, information needed for management of these species stimulated efforts to describe and quantify their habitats.
An increase in environmental awareness during the 1960s and 1970s broadened the scope of the kinds of animals about which the general public was concerned. Animal species not hunted for sport and without any other apparent economic utility were also perceived as having value. (The ethical rationales underlying these values are discussed in the next section.) Among the laws passed during this period were the National Environmental Policy Act (1969), the Endangered Species Conservation Act (1973), the Federal Land Policy and Management Act (1976), and the National Forest Management Act (1976) (Bean 1977; see also table 1.1). Legislators designed these laws, in part, to ensure that all wildlife species and other natural resources were considered in the planning and execution of human activities on public lands. Knowledge of the habitats of animal species is obviously required before the effects of an environmental disturbance can be fully evaluated, before a refuge for an endangered species can be designed, or before animal habitats can be maintained on lands managed under a multiple-use philosophy. Biologists responded to the need for information about habitat requirements by studying, often for the first time, numerous species of "nongame" animals and by developing models to help predict the effects of environmental changes on animal populations (e.g., Verner et al. 1986).
Excerpted from Wildlife-Habitat Relationships by Michael L. Morrison, Bruce G. Marcot R. William Mannan. Copyright © 2006 Michael L. Morrison, Bruce G. Marcot, R. William Mannan. Excerpted by permission of ISLAND PRESS.
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