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Metapopulations and Wildlife Conservation
By Dale Richard McCullough, Jonathan Ballou, Bradley Stith
ISLAND PRESSCopyright © 1996 Island Press
All rights reserved.
Dale R. McCullough
In recent years, the word "metapopulation" has found its way into the discourse about wildlife conservation and management. The concept is not particularly new, but its relevance has grown with the changes wrought on natural environments by expansion of modern human civilization. This book considers the ideas concerning metapopulations and explores their usefulness to applied programs to conserve wildlife in a rapidly changing world.
Typically a population is thought of as an interacting collection of animals of the same species occupying a defined geographic area. The boundaries of this area might be set by a number of different criteria. In various cases, the area chosen may be local distributional limits, the entire range for narrowly distributed species, geographic units in which movement and interaction of the animals are greater within than between units, or simply a land scale amenable to administrative efficiency. In traditional usage, movements and interactions by individuals were relatively continuous over the population area, even though the habitat (the physical and biological environment that satisfies the species' prerequisites of life) may vary somewhat in overall quality from place to place. Often such populations were termed "panmictic," which means completely mixed in terms of genetics. Most populations of vertebrates have age, sex, and social structure, however, and do not mix freely. Breeding does not occur at random even in the case of very small populations. For the sake of consistency in this book, such traditional populations will be referred to as "continuous" populations.
A metapopulation, by contrast, is discontinuous in distribution. It is distributed over spatially disjunct patches of suitable habitat "patches" separated by intervening unsuitable habitat ("matrix") in which the animals cannot survive. Because of the risk of mortality in crossing hostile conditions of the matrix, movement of animals between the patches is not routine. Consequently, movement between patches ("dispersal") is restricted. Furthermore, because many of the habitat patches may be small, consequently supporting small population sizes, extinction in given patches ("local extinction") may be a common event compared to extinction of continuous populations. A metapopulation's persistence depends on the combined dynamics of extinction within given patches and recolonization among patches by dispersal. So long as the rate of recolonization exceeds the rate of extinction, the metapopulation can persist even though no given subpopulation in a patch may survive continuously over time.
Levins (1969, 1970) first coined the term "metapopulation"—that is, a population of populations. His interests related to the control of insect pests in a patchy environment. Prior work on subdivided populations had been done by people like Huffaker et al. (1963), who studied mite populations on food patches in a laboratory setting. In these experiments, food patches were arranged in uniform x and y arrays. Levins' (1970) model, sometimes called a "classical metapopulation," closely mimicked the circumstances of these laboratory experiments. In Levins' model, habitat patches separated by nonhabitat space were all of equal size, quality, and spacing, and organisms had equal probabilities of dispersing from one patch to any other patch in the array. The only matter of concern was the presence or absence of the individuals on each patch: if they were present, the assumption was that they would quickly increase to carrying capacity. Levins asked what rates of recolonization by random dispersal were necessary to balance random rates of extinction by patch so that the metapopulation would persist through time.
Levins' (1970) model, like Huffaker et al.'s (1963) experiments, had an elegant simplicity that illuminated the concept of how a constellation of partially isolated patches in space could yield overall stability to a system that was inherently chaotic at the level of the individual patch. This kernel of an idea, after lying largely dormant for nearly 20 years, has recently burgeoned into our current appreciation of metapopulation dynamics. Although some insects and plant populations conformed sufficiently to continue a small interest in the metapopulation idea, it suffered from being a solution in want of a problem. As pointed out by Harrison (1991, 1994) and others, the simplified assumptions of the Levins (1970) model do not apply very well to the complexity of most populations in nature. The problem that reawakened the metapopulation idea was the rapid and rampant fragmentation of once continuous habitats by the spread of human activity. As the remnant patches of habitat receded to a size too small to support populations without incurring local extinction, biologists quickly realized that hope for future perpetuation of many species rested upon maintaining many habitat patches and having animals disperse among them. The problem of fragmentation demanded a solution, and metapopulation ideas floating on the fringes of biological thinking were brought into central focus.
Given that few populations fit the assumptions of the Levins (1970) model—landscapes and fragmentation seldom possess such symmetry—just what is a metapopulation? Why do we use the term in a way that clearly violates the intent of the originator? The first and simplest response is, who cares? We are facing a crisis of loss of habitat, loss of species, and loss of biodiversity unprecedented in human experience in its degree and global scale. It may rival the great upheavals of paleontological history in that the rules of the game for survival are being irreversibly changed. Biologists hoping to arrest, or at least slow, this process want fixes, and they want them now. Metapopulation dynamics, as broadly and loosely conceived, offer one such fix. Whether it will work, ultimately, only time will tell.
A second answer is found in the manner of the concept's growth. In the traditional course of academic development, each step in the progression of an idea is clearly demarcated. Thus Smith's original model is elaborated by Jones' model, and additional permutations are addressed by Johnson's model, much like a biblical listing of who begat whom. But when the right idea arrives at the right time to fill a deeply felt need, it can take on a life of its own, evolving in a larger context outside the rules of priority and progression. It is as if a virus escaped from the laboratory and infected a multitude of people, mutating as it spread. Possession passes to the users, and the idea evolves according to their needs. To those in the arena, the nuances of an abstract model are the luxury of academicians.
In view of the continuing evolution of the metapopulation idea, it perhaps is neither desirable nor possible to give a rigorous definition to the term. For the purposes of the conservation and management audience this book addresses, a working definition will do. The definition must include populations that are already functioning as metapopulations, but it must remain cognizant also of the processes that are converting continuous populations to metapopulations. By definition a continuous population is not a metapopulation, nor is a spatially segregated population with high dispersal that eliminates the possibility of local patch extinction. Consequently, the two keys to the metapopulation idea are, first, a spatially discrete distribution and, second, a non-trivial probability of extinction in at least one or more of the local patches.
Purists may find this rather loose definition unsatisfactory, but it is the needs of applied ecologists that this book addresses. Some ambiguity is needed to admit the vagaries of nature across space and dynamic processes over time. For example, the fine distinction of a population as continuous versus metapopulation is of little importance if it can be seen that the processes operating on that population are inexorably shifting it from the former to the latter. For applied ecologists, it is the outcome that matters. A precise scheme that describes nicely but conserves nothing is of little help. Consequently, the definitions that will categorize populations as continuous or metapopulations must be contained within a broader delineation of the processes that shift populations to metapopulation status. The case histories in this book, therefore, include not only populations that clearly function as metapopulations, but others that illustrate the transition from continuous to metapopulation. Grizzly bear populations (Chapter 14 in this volume), for example, vary from natural metapopulations in the absence of anthropogenic influences, to extensive continuous populations, to metapopulations created by habitat fragmentation, depending on which part of their geographic distribution one chooses to examine.
Theoretical explorations of metapopulation dynamics, elucidated by computer models, are far ahead of empirical studies on the reality of metapopulation behavior in nature. Gilpin and Hanski's (1991) predominantly theoretical treatment of metapopulation concepts was extremely mathematical and abstract and not readily accessible to many of the wildlife conservators and land managers on the ground.
Some wildlife populations are natural metapopulations, although in the past they were not viewed in that manner. The metapopulation concept has given theoretical structure to the dynamics of such species, and technological developments have contributed greatly to our knowledge. Tools such as radiotelemetry to study dispersal, powerful DNA techniques to illuminate genetic structure, GPS instruments to locate accurately, and desktop computers and GIS software to handle voluminous quantities of data open up whole new avenues of inquiry. One set of natural metapopulations includes species adapted to a unique habitat disjunct in distribution. Examples include hyraxes (Heterohyrax brucei) and klipspringers (Oreotragus oreotragus) on rock outcrops in the Serengeti Plain; marmots (Marmota spp.) and pikas (Ochotona princeps) of mountain meadows and talus slopes in the western United States; an array of species occupying relict habitats on mountaintops in the Great Basin ranges; and gulls, terns, and other nesting birds on islands. A second set of metapopulations includes secondary-successional species occupying disturbed areas in forests or other extensive habitats. In fact, naturally occurring metapopulations are rather common. We have long recognized the biology unique to these species but lacked a unifying concept with which to view them collectively. The recent adaptation of the metapopulation concept has given that unity.
The greatest concern, however, is not with naturally occurring metapopulations but with the rapid conversion of what were originally continuous populations to metapopulations by fragmentation of vast expanses of natural habitats through human activity. The northern spotted owl (Strix occidentalis caurina) is a celebrated case. Initially it occupied the undisturbed old-growth forests of the Pacific Northwest (Figure 1.1). When clear-cut logging first began in the H. J. Andrews Experimental Forest (Figure 1.2), on the western slope of the Cascade Range in Oregon in the 1950s, no one worried about the fate of spotted owls. Indeed, such cuttings were seen as diversifying the landscape and increasing biodiversity—which, in fact, they did. It was only years later, after logging had removed much of the original old-growth forest, that the plight of the spotted owl and associated species was fully realized (Thomas et al. 1990; Chapters 7 and 8 in this volume).
It is curious that over the years we have made a mental transposition of the matrix and patches. Originally the matrix was viewed as the extensive climax habitats found by Europeans on arrival on the North American continent. The patches were the disturbed areas caused by fire, storm, and flood. As humans have increased disturbance through logging, cultivation, drainage, planting, and spraying, the amount of climax habitat has declined (Figure 1.3). Thus the remaining undisturbed habitat has become the patches and the extensive disturbed areas are now the matrix. In fact, these categorizations do not adequately express the complexity of either the original (which included aboriginal human effects) or the modern human-altered landscapes. Both are better viewed as complex mosaics.
Nonetheless, abstractions that simplify (as in Figure 1.3) clarify the paradigm of metapopulation dynamics. Why did the wildlife field not think of the original landscape in metapopulation terms? Certainly disturbed patches were recognized as such, and a cardinal principle was the importance of creating edge to favor game species by producing disturbed patches in climax habitats. Fragmentation, for most of the history of the wildlife field, was viewed as good, not bad. But like the mineral, vitamin, or medicine that in small doses is beneficial but in large amounts is poisonous, too much fragmentation creates a different set of problems.
Probably the main reason disturbance-adapted animals were not thought of in terms of metapopulation ideas was their high capacity for dispersal—the consequence of a long evolutionary history of being adapted to finding and colonizing a shifting distribution of disturbed areas caused almost randomly across the landscape by meteorological phenomena. And if dispersals are high enough, discontinuously distributed subpopulations function as a continuous population because the matrix does not constitute a significant barrier. Although disturbance species were well adapted to dispersal across a matrix of climax habitat, rapid fragmentation of climax habitat is forcing a myriad of animal species that formerly were in continuously connected habitats to function as metapopulations in the currently fragmented habitats. Because subpopulations are small, they become more prone to extinction from both natural and anthropogenic causes. The problem is exacerbated by the array of predators and competitors that are favored in the altered habitat, and if the patches are small, their size is not sufficient to buffer the patch from intrusion. Thus, predators and competitors not only lower the likelihood of successful dispersal between habitat patches but also increase the likelihood of extinction within habitat patches. In the absence of successful dispersal, subpopulations will gradually become locally extinct patch by patch until the last subpopulation, and the species, passes into extinction. Unfortunately, these "interior" species, for example, the denizens of the deep forest, are often not good dispersers because they avoid open areas, which they did not have to cross in the extensive pristine forest.
Although fragmentation is the most prevalent cause of continuous populations being reduced to metapopulations, other processes can create metapopulations. The second category includes deterioration of habitat quality without major conversion, which can have the same effect of isolating subpopulations to a set of the most suitable remaining patches of habitat in the degraded environment. These isolates, too, must function as metapopulations in order to persist.
A third category of human-induced metapopulation structure is caused by overexploitation, which extirpates a species population from much of its range. Reduction in population numbers commonly results in confinement of the species to isolated areas that are of higher quality (thus better able to replenish the local population), inaccessible, possessed of more escape cover, or otherwise protected.
A fourth category pertains to recovered species. Decline in population size and isolation to patches often leads to establishment of reserves to protect the wildlife species and, if that fails, implementation of captive breeding programs. Such parks may function much like habitat "islands" in a sea of unsuitable habitat (or developed area) where dispersing animals are killed intentionally or accidentally. This situation has prompted the application of island biogeography theory to landscape planning. But island biogeography theory alone is not sufficient for planning and managing such situations. Because recovered species often are reestablished by artificially transplanting them to widely isolated patches of suitable habitat or areas protected from exploitation, they become functional metapopulations. If habitat patches and transplant sites are isolated, humans may have to accomplish dispersal artificially in order for the species to persist.
The ultimate force driving this process is the inexorable increase in the human population, which shows no sign of abating. Planet Earth will continue to be further fenced, plowed, grazed, logged, mined, and paved for the foreseeable future. It is the inevitability of this fate that has led to the development of the field of conservation biology, variously called a "crisis discipline" or a "rescue science" for salvaging biodiversity. Natural habitats in the future will progressively become reduced in area and more fragmented and isolated. An increasing number of animal populations will be altered from continuous populations to metapopulations.
Excerpted from Metapopulations and Wildlife Conservation by Dale Richard McCullough, Jonathan Ballou, Bradley Stith. Copyright © 1996 Island Press. Excerpted by permission of ISLAND PRESS.
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