How Do Any Species Persist? Lessons for Conservation Biology

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Abstract
Recent reformulation of the birth‐and‐death process model to include the effects of environmental variation on stochastic demography presents a pessimistic view of the prospects for persistence of small populations. The models indicate that modest increases in population size, or in the mean (positive) growth rate of the population, do not dramatically increase the mean persistence time. The key to long‐term persistence seems to lie in reducing the variance in the population growth rate.Natural populations, of those species that are not numerous, achieve their persistence through a variety of variance‐reduction mechanisms. These notably include: I) risk‐averaging by wide geographic distribution over areas that experience mutually independent environmental variation, and 2) reliance on restricted “hot spots” of especially favorable habitat where the local population growth rate is almost invariably strongly positive when the population is not crowded.These natural mechanisms depend on a spatial scale, and on species‐specific richness of habitat, which is not readily attained for many species on biological reserves with a “hamis‐off” management policy. However, these mechanisms can be mimicked by management intervention strategies‐particularly by reintroduction programs in an “archipelago” of geographically dispersed reserves that are otherwise mutually isolated and by resource augmentation, predator and disease control, and other survival‐and‐reproduction enhancement programs implemented specifcally at times when the population is at a low ebb.

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