Conserving Earth

Conserving Earth's Biodiversity E.O. Wilson and Dan L. Perlman Island Press, 2000

• • • •

Although the original equilibrium theory of island biogeography suggested that a single large reserve is better than several small reserves, the idea was almost immediately attacked when Jared Diamond stated it explicitly in a 1975 paper. 1 Depending on the biology of a region, either a single large reserve or several small ones may support a greater number of species; however, the two strategies will often protect different sorts of species. Several small reserves can protect a wide variety of habitats and many different ecological communities, and it is unlikely that all the reserves will be devastated by single fire, disease, exotic species invasion or other catastrophe. Large reserves are necessary for species such as top carnivores that might not survive in the absence of such reserves, and they can accommodate the needs of many species with smaller ranges.

One of the longest running (and, at times, most divisive) debates in conservation biology has been the SLOSS question: if total area protected is roughly equivalent, should we protect a Single Large Or Several Small reserves? Although early papers argued that the equilibrium theory of island biogeography supported one or the other position, 1 ecologists now generally agree that the theory is equivocal on this question. 2 Either a single large reserve or several small reserves could protect more species, depending on the slope of the species-area curve and the proportion of species held in common among the small reserves. 3 What Gets Protected It is important to keep in mind that different sizes of reserves will protect different sorts of species, even if a large reserve and a system of several small reserves protect the same number of species. The large reserve will protect larger species with greater habitat and range requirements, while the set of smaller reserves can protect more locally endemic species with smaller ranges. Indeed, arguments in the SLOSS debate have been heavily influenced by the research bias of the ecologist in question: those who study insects and other small organisms tend to favor several smaller reserves, while those who study vertebrates tend to favor a few larger reserves. Clearly, reserve design depends in part on the intended target species or groups. Reasons for Large Reserves Those who favor large reserves argue that some species would not be able to survive without a system of such reserves. 4 Large vertebrates — especially mammalian predators, such as bears or cougars — tend to persist only in expansive areas of relatively undisturbed habitat. Many smaller animals, on the other hand, seem able to survive within the fragments of habitat left in an unprotected landscape and even in developed or cleared areas. Advocates of large reserves also point out that as habitat destruction and fragmentation proceed, large areas of habitat will not survive unless protected, while smaller parcels may persist without protection. 5 Further, large reserves may be easier to maintain and manage than several scattered parcels. Reasons for Small Reserves Multiple small reserves have their own advantages, however. They may cover a wider variety of habitats than one or a few large reserves, and may thus help to preserve more ecosystem diversity. In a landscape that harbors many locally endemic species with small ranges, multiple small reserves may be necessary to protect as many species as possible. Multiple reserves are also less likely to be simultaneously devastated by a single random event — such as fire, flood, disease, or invasion by an exotic species — whereas a single large reserve might be destroyed by one such disaster, natural or human-produced. 6

No Single Answer When all of these factors are taken into consideration, no overall advantage to either one large reserve or many small reserves emerges. Comprehensive systems of reserves — such as that designed by conservation biologist Jared Diamond for the Indonesian province of Irian Jaya — will probably contain both large and small reserves, in order to suit the needs of different species and communities and to protect against different kinds of threats. 7 To better understand how science is performed, and to observe passionate scientists at work, it is worth reading some of the papers from the SLOSS debate. 8-11