EFFECTS OF POPULATION SIZE REDUCTION

Abstract

Threatened species have small, or declining populations. Once small, they lose genetic diversity, become inbred (with consequent reduction in reproductive fitness) and accumulate deleterious mutations. Consequently, Section II considers these factors in detail, as they contribute to extinction risk, and provide the essential background material for the genetic management of threatened species in Section III. Factors reducing population size Humans are reducing the size and distribution of wild populations through clearing and fragmentation of habitat, over-exploitation, pollution and the impact of introduced species. Of these, habitat loss is currently having the greatest impact, but threats from global climate change loom ever larger. Loss of genetic diversity Loss of genetic diversity in small populations reduces the ability to evolve in response to ever-present environmental change. There are four threats to genetic diversity: extinction of populations or species extinction of alleles due to sampling in small populations inbreeding reducing heterozygosity by redistributing genetic diversity among homozygous individuals and populations selection favouring one allele at the expense of others, leading to fixation. Overwhelmingly the major threat to genetic diversity is extinction of alleles in finite populations by genetic drift. All of the adverse genetic effects of population size reduction depend on the effective population size, rather than the census size. The effective population size is reduced by fluctuations in population sizes, high variation in family sizes, and by unequal sex-ratios. Chapter 11 deals with the effects of small population size on genetic diversity and the factors that influence effective population size.