restricted access VI.10 Support Services: A Focus on Genetic Diversity
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VI.10 Support Services: A Focus on Genetic Diversity Oliver R. W. Pergams and Peter Kareiva OUTLINE 1. Genetic diversity is the most fundamental of all ecosystem services 2. Rapid anthropogenic change and the role of evolution 3. Can one quantify the value of genetic diversity? 4. Practical outcomes from the valuation of genetic diversity 5. Summary Empirically and theoretically we know that genetic diversity is essential for rapid evolution. In the face of a rapidly changing world driven by unprecedented human impacts, the ability to evolve rapidly may be one of nature’s most precious commodities. Examples of the economic value of genetic diversity are numerous and compelling, but methods for formal economic valuation of this ecosystem service are not well formulated. Even without good methods for dollarizing genetic diversity as an ecosystem service, there are ways of quantifying its value that help inform sustainable and judicious resource management strategies. GLOSSARY ecosystem services. Goods (food, fuel, building materials ) and services (flood control, disease regulation, etc.) that benefit humans and are provided by natural ecosystems heterozygosity. The proportion of individuals in a population that have two different alleles for a particular gene microevolution. The occurrence of small-scale changes in allele frequencies in a population over a few generations resilience. The ability of a system to resist or recover from disturbances and perturbation so that the key components and processes of the system remain the same 1. GENETIC DIVERSITY IS THE MOST FUNDAMENTAL OF ALL ECOSYSTEM SERVICES The importance of genetic diversity is well known to agronomists, who for nearly a century have spoken of genetic variety as a resource to enhance crop vigor and productivity. Testimony to this value is the fact that half the yield gains in major U.S. cereal crops since the 1930s are attributed to genetic improvements (Rubenstein et al., 2005). We are able to breed and select for crops that meet different environmental challenges only because of the genetic variety in those crops, and the goal of plant breeders is typically to maintain as much genetic diversity as possible in case it is needed at some future date. More generally, a central theorem of evolution is that the rate of evolution is proportional to the amount of genetic variation. The quantitative connection between the rate of evolution and the amount of genetic variation provides the foundation for genetic diversity as perhaps the most fundamental of all supporting ecosystem services. It is clear that if there were zero genetic diversity within each species, even modest environmental change or human disturbance would imperil the species and the ecological services that species provide. In order for humans to get a return from nature (in the form of fisheries, timber, soil fertility, and so on) in a varying environment, species must harbor genetic diversity —how much we cannot say, but some for sure. A second related appreciation for genetic diversity can be traced to the origins of conservation biology, which sought to identify minimum viable population size on the basis of genetic principles, resulting in computer models of extinction probability. The importance of minimum viable population size applies to many of the world’s species, which have only small populations remaining. For example, 17% of the world’s bird species are confined to small populations on islands, and of these, 23% are classified as threatened (Johnson and Stattersfield, 1990). The International Union for Conservation of Nature (IUCN) estimates that 3032 of the 40,295 (7.5%) plant and animal species evaluated are critically endangered (category CR: IUCN, 2007), which means they are down to very small populations. It is probably fair to treat this percentage as a lower bound: some species with only small populations are considered stable enough not to be CR. It is well known theoretically that the smaller a population is, the more rapidly genetic variation is lost (figure 1), and the more likely the population is to lose rare alleles. These theoretical relationships are a matter of practical concern because we know from scores of empirical examples that lower heterozygosity (which will occur as rare alleles are lost) leads to lower rates of growth or reproduction. There is a substantial body of work on the relationship between heterozygosity and growth in annual and perennial crop plants, forest trees, birds, fish, molluscs, reptiles, and vertebrates including humans. It is more difficult to find real-life conservation applications that focus on loss of rare alleles, but the concept is clear. When...