Are Small Populations of Plants Worth Preserving?

Abstract

Maintenance of genetic variation is considered essential for the long-term survival of a species (Frankel & Soule 1981). Consequently, the rate at which genetic variation is lost in isolated populations is of great concern to conservation biologists who must decide how to apportion limited resources to protect rare or endangered species. Spending money and time to protect a population in which genetic variation will erode rapidly may be a poor investment. It is thought that small populations lose a large amount of genetic variability because of genetic drift and thus have reduced chance of longterm viability (Franklin 1980; Soule 1980; Lande & Barrowclough 1987). Most models of the rate of loss of genetic variation in small populations assume that all genotypes have equal fitness (i.e., selective neutrality). Neutral models of genetic drift assume that the amount of heterozygosity remaining after t generations is (1-V/2Ne)t, where Ne is the effective population size (Wright 1969). Based on these models, it has been estimated that effective population sizes of at least 50 are needed to avoid harmful loss of genetic variation in the short term (Franklin 1980; Soule 1980; Frankel & Soule 1981; but see Simberloff 1988). Some empirical studies have shown that selection against homozygotes occurs during early stages of growth in natural populations of plants. Consequently, models that assume selective neutrality may be mislead-