Considering the Size of Habitat Areas

The issue of the size of protected areas is closely linked to concerns for identifying and protecting critical habitats. Not only do existing protected areas not represent the full range of habitats, but most protected areas are too small to sustain the biodiversity that they are intended to protect. According to the World Commission of Protected Areas, as of 2000, there were 30,000 protected areas in the world covering 13.25 million square kilometers, and 59 percent of these areas are smaller than 100 hectares. As a general rule, larger species, especially carnivores with high metabolic demands (for example, cougars, grizzly bears, and harpy eagles), require large areas to sustain the prey populations upon which they depend. Large areas also include and sustain greater diversity and redundancy of habitats, providing for a greater amount of biodiversity and the continuation of more natural ecosystem processes. Consequently, most conservationists conclude that large, contiguous areas of protected habitats are a critical tool for maintaining both species and ecosystem functions.

During the 1970s and 1980s, the debate over the ideal reserve size was largely based on species-area relationships and island biogeog-raphy theory. Species-area relationships are plots of the numbers of species found in discrete places (for example, islands of various sizes in an archipelago) against the estimated area of those places. Essentially, the larger the area sampled the more species will be found, although the relationship is not linear (hence species-area curves). As area increases, fewer and fewer species are added because the area gradually approaches saturation from the larger, regional pool of species. MacArthur and Wilson (1967) developed the theory of island biogeography around these and other empirical relationships. They argued that the number of species on an island represents a dynamic equilibrium between immigration and extinction of species. The rate at which species colonize an island is related to the island's distance from the nearest landmass; because the landmass serves as a source of species, the closer an island is to it, the higher the rate of immigration. Larger islands, however, support larger population sizes, and therefore have lower extinction rates. Consequently, for two islands the same distance from the nearest landmass, the larger one will have a greater number of species. Species-area relationships are important because they demonstrate that, all else being equal, any decrease in available habitat area results in a decrease in the number of species that can be supported by that area.

Based on the similarity between real islands and protected areas within human-dominated landscapes, island biogeography theory has been applied to protected areas in fragmented landscapes to estimate the number of species that protected areas or habitat patches can support. Although some researchers have also attempted to extrapolate predictions as to whether particular areas are adequate for preventing extinction of particular species or even ecosystems, the theory is based on general statistical patterns and assumes equilibrium conditions that may be ill-suited for most practical habitat management. For specific conservation applications, other theories that more directly address metapopulation, metacommunity, and landscape dynamics are likely to be more useful to decision makers.

Because funds for acquiring and managing protected areas are always limited, and many people were (and still remain) interested in devising general principles for the design of protected areas, a debate emerged in the 1980s about whether it was better to design systems made up of a few larger areas or systems of numerous smaller ones. This debate, labeled with the acronym SLOSS (Single Large Or Several Small), was derived in part from island biogeography theory but was more directly concerned with conservation issues. Researchers discussed, for example, whether for a given species, a single large population or multiple smaller populations might be more vulnerable to extinction from environmental catastrophes or disease. More recent thinking about reserves, however, de-emphasizes the pursuit of general, multipurpose designs. Rather, given all the site-specific variables and options about what to protect and what not to protect, the design of protected areas should be conducted around specific objectives. If a system is intended to protect a very rare species made up of three remaining populations, for example, a small reserve around each population may be better than a single, larger reserve around one population. If the goal is to provide the greatest possible protection to a relatively intact ecosystem, however, a single, large area may be more effective than other scenarios.

Larger habitat patches, or protected areas, tend to proportionally minimize the degradation of habitat that often occurs near their perimeters. Such edge effects, caused by dramatic changes in the ecological characteristics at certain edges (either habitat transition areas known as ecotones, or reserve boundaries), effectively decrease the size of core protected areas for sensitive species (Laurance and Bier-regaard, 1997). Alteration of physical variables (for example, light levels, wind disturbance, and humidity in forests) or biological variables (such as the invasion of harmful, invasive species or predators, including human poachers) may contribute to negative impacts on populations near these edges. Because small and narrow areas have larger perimeters relative to their areas, protected areas or habitat patches with this shape will suffer disproportionately from edge effects compared to larger and rounder patches.

Another way to increase the effective size of protected areas is to create buffer zones that surround core areas and help minimize some of the human-caused edge effects. Within these buffer areas, relatively benign human activities are typically allowed, while more harmful activities are prohibited. Buffer areas are one way that management of protected and surrounding areas can be integrated to achieve larger landscape and ecosystem objectives that are not possible in the core protected areas themselves.

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