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(Grabherr 1989). When the sedge disappeared, the lichen's productivity decreased, which could lead to a population decline for lichen-dependent species. Keystone species often function as predators that control the numbers of prey species. Their predatory effects can increase the biological diversity of subordinate prey species by reducing densities of a preferred dominant prey species. For instance, in central Chile a rocky intertidal community had a higher diversity index when the major gastropod predator was present than when the predator was absent (Duran 1989). In the absence of predation, the dominant competitor could itself be the keystone species if its presence determines the distribution and abundance of subordinate species. For ex- ample, the cotton rat (Sigmodon hispidus) was the largest and competitively dominant species that affected the smaller and subordinate species in the small mammal com- munity discussed in the case study below. We believe that continuous progress will most likely come from conservation efforts focused at the population level (Shaffer 1981). With well-understood organ- isms, such as small mammals, we can test some of the theories that bear on questions of population persistence for poorly-understood organisms, and supply generaliza- tions for those theories. As we mentioned, there are some problems associated with a reductionist approach, but there are few alternatives given the time and resources available. In this review, we will examine the role of population processes in determining biological diversity. We first briefly discuss how the basic concepts of minimum viable population and metapopulation relate to biological diversity. We then present data from our ongoing study of the effects of habitat fragmentation on population dynamics of small mammals and how this fragmented landscape affects biological diversity. We conclude with recommendations for future research on the effects of habitat fragmentation on population processes. Minimum Viable Population Concept Gilpin and Soule (1986) considered two kinds of population extinctions, deter- ministic and stochastic. Deterministic extinctions are due to forces that inexorably result in the disappearance of a population. For example, deforestation in the tropics would be a deterministic force for different species of trees. The outcome is pre- dictable if deforestation continues at its present rate. Stochastic extinctions are those due to random events. Shaffer (1981, 1987) distinguished four sources of variation that could result in the random extinction of a population: (1) demographic stochas- ticity due to random events in individual survival and reproduction; (2) environmental stochasticity due to unpredictable changes in abiotic factors such as weather, or biotic factors such as predators, competitors and parasites; (3) natural catastrophes such as fires and floods, which occur at random intervals; and (4) genetic stochasticity due to genetic drift and inbreeding, which may affect individual survival and reproduction. Several points are worth noting about the distinction between deterministic forces of extinction and stochastic forces. First, the relative effect of stochastic forces increases as populations become smaller. Second, many extinctions are caused by a deterministic event reducing population size to such an extent that stochastic forces will eventually lead to extinction. Third, different stochastic forces operating at low population densities may interact to cause extinctions. For example, an environmental perturbation could reduce population size to a level where a loss of variation in the Population Processes ? 253