Conservation biologists and managers involved in conserving biodiversity are often forced to make decisions without adequate empirical data. Ecological theory provides some structure for those decisions. Both the theory of island biogeography and the study of metapopulation dynamics can provide insight into the conservation of organisms on patchy, fragmented landscapes. The theories are not dissimilar, but rather form points on a continuum. Island biogeography theory is based on a situation in which a mainland source population not threatened with extinction exists; metapopulation dynamics deal with situations in which the source population has been reduced to a set of small semi-isolated populations. This paper explores the applications of both theories to conservation practice. There are no easy solutions, but the theories provide a framework for conservation planning.
The relatively new science of conservation biology has been described as a "crisis discipline" because its practitioners often find themselves in situations in which they must make decisions without sufficient empirical data or adequate analysis of theoretical models (Soulé 1985). Preserving rare and endangered species becomes increasingly difficult as the next century draws nearer: human population growth continues at an exponential rate; more land is cleared or altered for human use; and every year more species move one step closer to extinction (Cox 1997). Conservation biologists frequently participate in planning for species protection through the design of nature reserves and national parks. Two theories have been suggested as a basis for the design of nature reserves: the theory of island biogeography (MacArthur and Wilson 1967) and metapopulation dynamics (Hanski and Gilpin 1991). Can ecological theory provide a usable framework for the conservation of biodiversity?
return to table of contentsThe theory of island biogeography (MacArthur and Wilson 1967) describes the relationship between the number of species found on an island and the area of the island (MacArthur and Wilson 1967). The relationship between the number of species and area is called a species area curve and represented by the following equation: S = CAz , where S is the number of species of a given taxon; A is the island's area; C is a parameter which depends on the taxon and the biogeographic region of the island; and z is a constant which varies little between taxa, but which will increase as habitat heterogeneity increases (MacArthur and Wilson 1967). The species-area relationship is interesting, but the key piece of island biogeography theory as it applies to conservation biology is the suggestion that island biotas reach and maintain an equilibrium number of species [figure 1] (MacArthur and Wilson 1967). Rates of immigration will decrease over time as more species become established; rates of extinction will increase over time: when more species are present, more have the chance of going extinct (MacArthur and Wilson 1967).
Why is this idea important in conservation biology? The fragmentation
of natural habitats results in smaller patches surrounded by uninhabitable or
hostile human environment (McCullough 1996). If an island, in a
biogeographic sense, can be described as "... a self-contained region whose
species originate entirely by immigration from outside the region"
(Rosenzweig 1995), then remnant patches of habitat, national parks,
and nature reserves can be considered islands. In a 1975 paper, Jared
Diamond suggests that island biogeography theory has a great deal to say
to conservation planners. The four key implications of the theory
are as follows:
Point one is easily seen as the species-area relationship. Point two is directly related to the equilibrium between immigration and extinction. When a reserve is set aside as the rest of the habitat is cleared or otherwise altered, the reserve at first contains most - but not all - of the species originally native to that habitat. Since the area has been reduced, the remaining fragment of habitat should not be able to support the same total number of species; many species will then go extinct, until the new equilibrium is reached (Diamond 1975). Determining which species or taxa are likely to survive is a two-pronged question: species will go extinct in a reserve that may be colonized from another reserve, or species will go extinct in an isolated reserve with no chance of colonization from another site (Diamond 1975). The risk of total extinction of a species in a reserve is related first of all to that species' ability to disperse from another site to recolonize a reserve and to whether or not there is a source population for colonizers.
Diamond goes on to suggest principles for reserve design, based on his first three arguments [figure 2] (Diamond 1975). A large reserve is better than a small reserve because it can hold more species at equilibrium and extinction rates will be lower. If it is not possible to have one large reserve in an area with homogeneous habitat, then individual reserves should be as large as possible and located near each other to maximize the potential for dispersion. In the same vein, reserves should be placed equidistant from each other rather than linearly, and, whenever possible, should be connected by corridors of the same habitat (Diamond 1975).
In January of 1976 a contrasting view of the application of MacArthur and Wilson's theory appeared in Science. In this paper, Simberloff and Abele argue that "...the proof of the underlying theory has not been so broad that conservation applications ought clearly to follow" (Simberloff and Abele 1976). Pointing to the species-area equation, Simberloff and Abele argue that rather than planning one large reserve to maximize species diversity, one should plan multiple smaller reserves, comprising the total area of the larger reserve. They argue that, based on the species-area relationship, a greater number of species will actually be preserved in a series of smaller reserves. The strategy of planning several small reserves also minimizes the risk of stochastically caused extinctions: in a large reserve, the spread of disease or effect of a weather related catastrophe could conceivably decimate a population. The same event in a small reserve might have a similar effect in that reserve, but the same species might survive and prosper in another reserve not affected by the disaster (Simberloff and Abele 1976).
The papers by Diamond and Simberloff and Abele sparked a heated debate: who had the answer? Was it better to plan for one large reserve, or was the alternative approach better? The controversy over single, large reserves or several small reserves - subsequently referred to as the SLOSS debate - was fought out in the ecological literature throughout the next decade. No one has reached a conclusion about which approach is better in theory, and the debate itself has been criticized to some degree. The underlying question in the SLOSS controversy is, "All things being equal, which approach is better?" In practice, conservationists rarely have a choice between sites in which all variables are equal (Soberón 1992, Murphy and Wilcox 1986). Additionally, some authors have observed that each side in the SLOSS debate is trying to achieve a different result: those who argue in favor of single large reserves focus on minimizing extinction rates, while those arguing for several small reserves are concerned with maximizing species diversity (Burkey 1989, Murphy and Wilcox 1986). While these goals are not necessarily mutually exclusive, the approach of a conservation planner will vary, depending on the purpose of the project.
return to table of contentsSome researchers suggest that perhaps island biogeography theory is not the best model for conservation planners to follow. In the last decade there has been increasing interest in the application of metapopulation studies to conservation. A metapopulation is most simply described as a population of populations (Hanski and Gilpin 1991). While habitat remnants can be viewed as islands of usable habitat for a species, metapopulation structure describes the species' remnant populations: what was once a population connected by at least potential continuous interaction between individuals - a panmictic population - has been changed into a group of populations connected only by dispersing individuals (McCullough 1996, Hanski and Wilcox 1991, Doak and Mills 1994).
Through the development of theoretical models, Hanski and colleagues have been able to demonstrate that metapopulation dynamics are related to extinction and colonization rates. The incidence function model describes the relationship between the probability of local extinction and patch size; it also demonstrates that the probability of recolonization of an empty patch is dependent both on the isolation of that patch from occupied patches and the size of the occupied patches (Wahlberg et al 1996). Wahlberg et al were able to predict the occurrence of an endangered butterfly in habitat patches in Finland using this model. They were also able to simulate the effects of patch destruction on the species' long-term survival, finding that destruction of centralized patches would result in the collapse of the species, while the destruction of peripheral patches was not significant. Wahlberg et al conclude that the model is a useful tool for predicting the occurrence of similar taxa in fragmented landscapes, but warn that the model may not be applicable to species which do not follow a strict metapopulation model, such as populations which regularly interbreed although they exist in patchy habitat (Wahlberg et al 1996).
When is it appropriate to use metapopulation dynamics as a framework for conservation? Conservationists or reserve planners should analyze the following criteria:
Some real-life success in managing an endangered population based on metapopulation dynamics has been documented. The management plan for the northern spotted owl (Strix occidentalis) is a case in point. Because human activity continues to irrevocably alter the owls' habitat, the plan is focused on maintaining the population as a metapopulation rather than a continuous population: large pieces of suitable habitat in close proximity to one another will hopefully allow for dispersal and recolonization of patches when necessary (Doak and Mills 1994). In order for this approach to be successful, of course, there must be sufficient life-history data for the species involved - will it be able to exist as a metapopulation? Not all species can. Plant species may be unable to disperse over great distances because of physiological constraints; insects and small mammals may be at great risk from predators in altered habitat between patches.
The key distinction between the application of island biogeography theory and metapopulation dynamics is the distinction between discrete land patches - islands - and discrete populations on a landscape. Unlike islands, patches of habitat generally do not have sharply defined boundaries but rather edges in which habitat shifts through a gradient of desirable to undesirable (Wiens 1996). Habitat fragmentation is essentially a disruption of the continuity of a landscape. It occurs at a variety of scales and therefore has different effects, depending upon the scale at which the disruption is perceived (Wiens 1996). A large mammalian predator, for example, may be able to travel from one forested patch to another through agricultural areas with little effect. A small mammal may not be able to make the same trip because of increased vulnerability to predators or hostile environmental conditions.
Metapopulation dynamics may provide a more "realistic" model for conservation of species in fragmented habitat. Reserves are frequently surrounded by less suitable habitat, isolating the species found within their boundaries. In addition, recovery plans for threatened species often involve the reintroduction of animals to parks or protected areas, artificially creating metapopulation structures (McCullough 1996). Corridors between reserves and buffer zones around reserves may facilitate dispersal for some organisms (Wiens 1996), but in many cases managers will have to intervene by transporting animals into other habitat patches (McCullough 1996).
Though there are no easy solutions for conservationists and reserve planners on the horizon, both metapopulation dynamics and island biogeography theory provide some helpful guidelines. The two theories are closely linked by the processes of colonization and extinction; the fundamental difference between the two is that island biogeography theory includes a mainland source of colonists not threatened by extinction while metapopulation theory assumes the contrary: there is no larger whole, only fragments (Hanski and Gilpin 1991, Quammen 1996). It is interesting to note that the participants in the SLOSS debate advocating the design of several small reserves are describing a situation much like the spatial distribution of local populations in a metapopulation structure. Returning to Jared Diamond's original recommendations for reserve design, one again finds suggestions which relate directly to metapopulation structure: in cases where one large undisturbed area is not possible, he advocates establishing a series of reserves which are spatially arranged in a manner which maximizes the potential for dispersal between patches.
In real life, most populations probably lie somewhere in the continuum between the two types of models. It is wise to remember as well that in real life, the survival of any given species is also dependent upon the survival of other organisms within its community. The next step for both island biogeography theory and metapopulation studies is to incorporate community interactions, such as predator-prey relationships. Wrangling over theoretical models can detract from the business at hand, which is stopping or at least slowing the process of human-caused extinctions. Ecological theory will continue to evolve and develop; planning for reserves and parks should continue to include the best of current theory.
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