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Introduction
Dale R. McCullough
In recent years, the word âmetapopulationâ 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 speciesâ prerequisites of life) may vary somewhat in overall quality from place to place. Often such populations were termed âpanmictic,â 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 âcontinuousâ populations.
A metapopulation, by contrast, is discontinuous in distribution. It is distributed over spatially disjunct patches of suitable habitat âpatchesâ separated by intervening unsuitable habitat (âmatrixâ) 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 (âdispersalâ) is restricted. Furthermore, because many of the habitat patches may be small, consequently supporting small population sizes, extinction in given patches (âlocal extinctionâ) may be a common event compared to extinction of continuous populations. A metapopulationâs 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 âmetapopulationââthat 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. Levinsâ (1970) model, sometimes called a âclassical metapopulation,â closely mimicked the circumstances of these laboratory experiments. In Levinsâ 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.
Levinsâ (1970) model, like Huffaker et al.âs (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) modelâlandscapes and fragmentation seldom possess such symmetryâjust 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 conceptâs growth. In the traditional course of academic development, each step in the progression of an idea is clearly demarcated. Thus Smithâs original model is elaborated by Jonesâ model, and additional permutations are addressed by Johnsonâs 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 Hanskiâs (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 biodiversityâwhich, 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.
Figure 1.1. View eastward from Carpenter Mountain to Three Sisters Peak at the Cascade Mountains crest in western Oregon in 1958. Note the complete absence of clear-cut logging: the only disturbed area (in the hill in the middle foreground) is a fire scar.
Figure 1.2. View of the H. J. Andrews Experimental Forest on the western slope of the Cascade Mountains in Oregon showing recent clear-cut logging blocks in 1958.
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 dispersalâthe 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...
