Conceptual foundations

CONCEPTUALFOUNDATIONS

The three chapters in this section explore the scope of the metapopulation concept and its applications. Hanski and Simberloff sketch the history of metapopulation studies and the range of approaches, both theoretical and empirical, that have been used in single-species studies. Harrison and Taylor assess critically the pertinence of the metapopulation approach to field studies and expand their review to multispecies situations. Wiens more directly connects the metapopulation concept to the complexities of real landscapes. Hanski and Simberloff outline in some detail the use (and misuse) of the metapopulation concept in conservation, where an apparent paradigm shift has occurred from the dynamic theory of island biogeography to the metapopulation theories. The gradual unfolding and evolution of the metapopulation concept from the pioneering studies of Sewall Wright, Andrewartha and Birch, Huffaker, Den Boer, Ehrlich, Gadgil, and Levins have been narrated previously and are summarized here by Hanski and Simberloff and by Harrison and Taylor. Harrison and Taylor make the interesting point that the origin of the metapopulation idea is different in single-species and in mul-

tispecies studies. Single-species studies have tended to emphasize the benefits of migration in leading to the establishment of new populations and thereby compensating for extinctions in small habitat patches. In the multispecies metapopulation scenarios, the key issue has been the locally unstable interaction among competitors and between a prey and its predator. Habitat fragmentation can be beneficial in creating the possibility for asynchronuous fluctuations, which can enhance regional stability. A high rate of migration may eliminate such asynchrony, and is hence potentially harmful for regional persistence in multispecies metapopulations. Multispecies metapopulation theory is further discussed by Nee, May, and Hassell and by Holt in Part II. In the predecessor of this volume, Metapopulation Dynamics: Empirical and Theoretical Investigations (Gilpin and Hanski, 1991), metapopulation dynamics was seen to imply significant turnover of local populations, local extinctions, and colonizations. This notion follows directly from Levins's original concept of a metapopulation as a population of populations, analogous to a population of individuals with finite lifetimes. This narrow classical view of metapopulations has now become superceded by a broader view, where any assemblage of discrete local populations with migration among them is considered to be a metapopulation, regardless of the rate of population turnover. (In a nonequilibrium metapopulation declining toward extinction even among-population migration is not a necessary criterion, but a system with no turnover and no migration would not classify as a metapopulation.) There are important questions to be asked about the role of migration in (local) population dynamics, and these questions are most naturally asked in a metapopulation (regional) context. Metapopulation dynamics in the narrow sense, with significant population turnover, is of course included in metapopulation dynamics in the broad sense. The realization that natural populations exemplify many kinds of spatial population structures has stimulated a terminology, originally due to Susan Harrison, and including entries such as patchy populations (not really metapopulations), classical (Levins) metapopulations, mainland-island metapopulations, source-sink metapopulations, and nonequilibrium metapopulations. These concepts and types of metapopulation structures are discussed by Hanski and Simberloff and by Harrison and Taylor. The danger here is that too much emphasis is given to classification, definition of ideal types, which in itself

does not guarantee any better understanding of the ecology, genetics, and evolution of metapopulations. What matters is what works. Does the "metapopulation approach" help answer important questions about spatially structured populations? Does it provide us with scientific insight to the problems in which we are interested? All this being said, there still is a need to be concerned with the type of spatial structure of populations in any empirical study and in an application of the metapopulation concept and models to real populations. One should avoid the temptation of pigeonholing every population with some form of patchiness as a "metapopulation," as Harrison and Taylor warn. In the worst case, this may obscure what is important and draw attention to elements that are less critical. Unfortunately, there are no easy answers here; one simply has to know the species and one has to understand the interactions between the populations and their environment. Metapopulation biology may be a multifaceted subject, but there is one common element that characterizes this approach to population biology. The metapopulation approach is based on the notion that space is not only discrete but that there is a binary distinction between suitable and unsuitable habitat types. If this does not fit one's idea of a particular environment, one is probably better off in using some approach other than the metapopulation approach. An important reason for the appeal of the metapopulation concept comes from our subjective conviction that natural lansdscapes truly are, for many species, patchworks of one or several habitat types. Though the metapopulation view of nature is complex enough, it appears to be hopelessly simplified in comparison of how landscape ecologists view reality. Wiens in his chapter lists four components that characterize landscape ecology: variation in patch quality, variation in the quality of the surrounding environment, boundary effects, and how the landscape affects patch connectivity. Wiens is correct in suggesting that most of these elements are by and large missing from metapopulation models, which are typically focused on idealized habitat patches in a featureless landscape. Recent studies of Andr6n and Green (cited by Wiens) appear to suggest that where the suitable habitat fragments for some species cover only a relatively small fraction of total area (let us call these LC landscapes, for low coverage), patch area and isolation effects tend to be significant; but where much of the area is covered by more or less suitable habitat (HC landscapes, for high coverage), other factors, such as exactly how individuals move in acom-

plex landscape, begin to dominate. Now, it so happens that the classical metapopulation concept implicitly assumes a LC landscape, hence the tradition of representing habitat patches as dots on maps, rather than drawing them as realistic habitat fragments. There appears to be a real difference between the two traditions here, as they have been largely concerned with either LC landscapes (metapopulation ecology) or HC landscapes (landscape ecology). As Wiens stresses, it is imperative for the practical application of both metapopulation biology and landscape ecology in conservation and planning that more common ground is established by developing appropriate theory and designing appropriate field studies. Some necessary constituents of a more unified approach seem relatively easy to achieve. For instance, it should not be too difficult to correct among-patch distances by taking into account how the features of the intervening landscape affect individual movement behavior. On the other hand, when considering HC landscapes, patch models are likely to be inadequate anyway. Metapopulation theory may well remain a useful practical tool for LC landscapes, with relatively small and isolated fragments of suitable habitat, but the "reserve mentality" that this approach implies should give away, as Wiens argues, to "mosaic management" of the environment in HC landscapes. Today, we do not yet have a conceptual and practical synthesis of metapopulation biology and landscape ecology, but no doubt the time will come when we will.