1. Conservation biology has two threads: the small-population paradigm which deals with the effect of smallness on the persistence of a population, and the declining-population paradigm which deals with the cause of smallness and its cure. 2. The processes relevant to the small-population paradigm are amenable to theoretical examination because they generalize across species and are subsumed by an inclusive higher category: stochasticity. 3. In contrast, the processes relevant to the declining-population paradigm are essentially humdrum, being not one but many. So far they have defied tight generalization and hence are of scant theoretical interest. 4. The small-population paradigm has not yet contributed significantly to conserving endangered species in the wild because it treats an effect (smallness) as if it were a cause. It provides an answer only to a trivial question: how long will the population persist if nothing unusual happens? Rather, its major contribution has been to captive breeding and to the design of reserve systems. 5. The declining-population paradigm, on the other hand, is that relevant to most problems of conservation. It summons an investigation to discover the cause of the decline and to prescribe its antidote. Hence, at least at our current level of understanding, it evokes only an ecological investigation which, although utilizing the rigour of tight hypotheses and careful experimentation, is essentially a one-off study of little theoretical interest. 6. The principal contribution of the small-population paradigm is the theoretical underpinning that it imparted to conservation biology, even though most of that theory presently bears tenuous relevance to the specific problems of aiding a species in trouble. It would contribute immeasurably more if some of the theoretical momentum so generated were channelled into providing a theory of driven population declines, thereby liberating the declining-population paradigm from the inefficiency of case-by-case ecological investigations and recovery operations. 7. The declining-population paradigm is urgently in need of more theory. The small-population paradigm needs more practice. Each has much to learn from the other. A cautious intermixing of the two might well lead to a reduction in the rate at which species are presently going extinct.
