Nature reserves
1. The potential for epidemics should be considered in the design of nature reserves, not only because of the possibility of total annihilation, but also because of the genetic consequences of a severe knockdown in numbers following a major disease outbreak. Both kinds of events are likely in nature reserves, and their probabilities increase as natural populations become fragmented due to human habitat disturbance and development, and as the major reservoir of contagious disease - domestic plants and animals - encircles, ever more snugly, the remaining fragments of undisturbed wildlife. Species redundancy, therefore, must be a design criterion in any reserve system based on rational biological principles, assuming that simulataneous epidemics are an uncommon event in two or more reserves harbouring the same species. 2. Several important generalizations relevant to the design of nature reserves have emerged from the studies of island biogeographers. First, extinction rates in habitat patches are area dependent, the smaller the patch (or island), the higher the rates of extinction. Second, even the largest nature reserves, if left alone, will probably suffer major die-offs of species, accounting for a majority of birds and large mammals in a few hundred or a few thousand years. Third, the arguments for proximity of reserves and for dispersal corridors between reserves may have been overstated. The advantages and disadvantages of such aids to dispersal need to be examined on a species by species basis. The existence of corridors should never be used to justify small reserves since (a) few species will disperse along corridors, (b) corridors can transmit diseases as well as genes, and (c) corridors are vulnerable to changes in land use values and practices. 3. The maturing field of community ecology permits the following generalizations regarding design: (a) the stability of ecosystems (particularly in the tropics) may be very sensitive to the presence of a relatively small fraction of the species in an ecosystem, particularly large mammals, certain insects and birds, and certain key plants. The extinction of any of these species (itself a function of the area of the reserve) could precipitate a cascade of extinctions in ecologically linked forms; (b) designers of reserves must consider the size, distribution and longevity of disturbances from tree falls to cyclones and volcanic eruptions, since the disappearance, even if temporary, of a successional habitat from a reserve, will usually cause a sudden and dramatic devease in species diversity. In most cases, such events will be irreversible due to the limited amount of migration betuween reserves; (c) the creation of artificial disturbances will be necessary in small reserves, but the kind and amount of such imposed succession needs to be carefully planned and monitored, especially in the tropics, in light of the sensitivity of tropical species and interactions. 4. The basic genetic criterion for nature reserve design should be the long-term rule of genetic conservation. This rule states that the minimum effective population size for any species is of the order of 500. The choice of which species are to be considered for the application of this rule requires ecological sophistication, but it is evident that few reserves in the world are large enough to protect many of their mammals and birds from immediate decline in fitness (in the worst cases) or a long-term erosion of genetic variation and evolutionary potential. 5. The historical phase of reserve establishment is drawing to an end, it being mainly restricted to the latter half of the twentieth century Conservationists must now begin to attend more to the problem of maintaining what is already protected - otherwise it will leak away in very short order. 6. We call for basic research in genetic management. In addition, it is imperative that genetic management be applied immediately to many species in many reserves in order to stem the loss of genetic variation and fitness. The following approaches are suggested: (a) intensive genetic management of small populations where manipulations of matings and other demographic variables are possible; (b) culling to minimize loss of genetic variation; (c) the integration of artificial disturbance with rational genetic management; (d) artificial migration between populations in different r eserves or between reserves and captive populations.
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| Format: | biblioteca |
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Cambridge (EUA) Cambridge University Press
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| Subjects: | PLANIFICACION, RESERVA NATURAL MANEJADA, |
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