2 Geographic range in the form of either B1 (extent of occurrence) OR B2 (area of occupancy) OR both:Extent of occurrence estimated to be less than 100 km2 (EN 5000; VU 20,000) and estimates indicating at least two of a–c:Severely fragmented or known to exist at only a single (EN 5; VU 10) location.Continuing decline, observed, inferred, or projected, in any of the following:Extent of occurrence.Area of occupancy.Area, extent, and/or quality of habitat.Number of locations or subpopulations.Number of mature individuals.Extreme fluctuations in any of the following:Extent of occurrence.Area of occupancy.Number of locations or subpopulations.Number of mature individuals.Area of occupancy estimated to be less than 10 km2 (EN 500; VU 2000), and estimates indicating at least two of a–c:Severely fragmented or known to exist at only a single (EN 5; VU 10) location.Continuing decline, observed, inferred, or projected, in any of the following:Extent of occurrence.Area of occupancy.Area, extent, and/or quality of habitat.Number of locations or subpopulations.Number of mature individuals.Extreme fluctuations in any of the following:Extent of occurrence.Area of occupancy.Number of locations or subpopulations.Number of mature individuals.
3 Population size estimated to number less than 250 (EN 2500; VU 10,000) mature individuals and either:An estimated continuing decline of at least 25% (EN 20%; VU 10%) within 3 years (EN 5; VU 10) or one generation (EN 2; VU 3), whichever is longer, ORA continuing decline, observed, projected, or inferred, in numbers of mature individuals AND at least one of the following (a–b):Population structure in the form of one of the following:No subpopulation estimated to contain more than 50 (EN 250; VU 1000) mature individuals, ORAt least 90% (EN 95%; VU 100%) of mature individuals are in one subpopulation.Extreme fluctuations in number of mature individuals.
4 Population size estimated to number less than 50 (EN 250; VU 1000) mature individuals. (See www.iucnredlist.org for an alternative criterion for VU.)
5 Quantitative analysis showing the probability of extinction in the wild is at least 50% (EN 20%; VU 10%) within 10 years (EN 20; VU 100) or three generations (EN 5), whichever is the longer (up to a maximum of 100 years).
For all of these organizations, the decisions about listing species were historically based on the best judgment of biologists rather than specific, quantifiable criteria. With a better understanding of the process of extinction and better data about species (e.g. population size, rate of decline), these decisions are now made systematically using criteria like those illustrated in Box 3.2 (Mace et al. 2008).
Unfortunately, the phrase “rare and endangered” has become a bit like “assault and battery”; most people use it without really understanding what it means. You might be surprised to know that many species are quite rare but not endangered with extinction and, conversely, that some endangered species are not particularly rare. For example, the African elephant probably has a total population over 500,000, but is listed by the IUCN as Vulnerable because it is considered to be in jeopardy. On the other hand, in the fynbos and succulent karoo ecosystems of southwestern South Africa there are hundreds of plant species with very small population sizes that live in fairly pristine environments and show no evidence of population decline (Cowling 1992). In other words, rarity can be a species’ natural state. Consequently the IUCN uses tighter standards for species that are rare yet not currently in decline. For example, a population that is in decline would be listed as “endangered” if it had fewer than 2500 individuals, but a population that is stable would only be listed as “endangered” if it had fewer than 250 individuals. Lastly, some species restricted to single areas, like small oceanic islands, are nearly always considered vulnerable, even if their populations number in the many thousands, because any disturbance may threaten the entire range of the species.
The idea that rarity can be a natural state is easier to understand if we go beyond simply equating rarity with having a small total population. Deborah Rabinowitz (1981) described rarity on the basis of three separate characteristics: (1) having a low population density; (2) being restricted to an uncommon type of habitat (e.g. caves or desert springs); or (3) being limited to a small geographic range (e.g. a single island or lake). We will return to the issue of rarity in Chapter 7, “Extinction Processes.” Suffice it to say here that rare species need to be monitored carefully because their status can quickly shift from secure to endangered.
The Instrumental Values of Species
When we think about the instrumental value of a species, we take a very human‐centric approach: Can I eat it? Can I make it into clothing or shelter? Can I burn it to keep me warm? Or, in the market‐based economies in which most of us live: Can I sell it? Materialistic uses of a species may be the core of instrumental values, but this is not the whole story. People also value species for purely aesthetic or spiritual reasons; species have instrumental value as members of ecosystems and as models for science and education; and conservation biologists use certain species to expedite their larger goal of maintaining biodiversity. Some instrumental values are conceptualized as functions of whole ecosystems, not individual species; we explore ecosystem values, especially ecosystem services, in the next chapter. Note that the term “ecosystem services” has become a popular catchphrase for all the instrumental values associated with biodiversity, both goods and services, whether tied most closely to ecosystems, species, or genes.
Economic Values
Food
Except for salt and a few other additives, everything we eat started out as an organism, an element of biodiversity. Often, we do not even recognize all the organisms involved, for example, the enormous array of microorganisms that are essential to the processes by which we produce cheese, yogurt, bread, chocolate, coffee, vanilla, and the various pickled foods and alcoholic beverages. Despite their fundamental role, the instrumental value of species as food is usually considered the domain of agricultural and food scientists rather than conservation biologists, because the vast bulk of our food comes from a relatively small number of domesticated species (Prescott‐Allen and Prescott‐Allen 1990; Khoury et al. 2014). Maintaining the genetic diversity of domestic species is a component of conservation biology as we will see in future chapters, but it is not in the mainstream of conventional conservation biology, which usually focuses on wild species. Nevertheless, there are at least three ways in which conservation biologists who work with wild species are involved with the instrumental value of species as food for people.
First, most domesticated species are closely related to species that are still wild, and these wild relatives are a critical source of genetic material, germplasm, for agricultural breeders trying to improve domesticated species (Khoury et al. 2013). Indeed, in many cases (e.g. pigs, coconuts, and carrots), there are both wild and domesticated populations of the same species. Maintaining viable populations of the wild relatives of crop plants and livestock falls squarely within the purview of mainstream conservation biology, especially if the wild relatives are threatened with extinction. For example, yaks and water buffalos are important livestock in parts of Asia, and the wild populations of both species are in danger of extinction. We lost the wild version of the domestic cow, the auroch, back in 1627 (Szafer 1968). A well‐known example of the potential role of wild relatives is found in the teosintes, wild relatives of corn (or maize) that were thought to be extinct until rediscovered in southern Jalisco, Mexico, in 1978 (Iltis et al. 1979; Hufford et al. 2012). Because teosintes are perennial (regrowing each year from a root system, not new seeds) some of their genetic material, if transferred to corn, could increase its resistance to some diseases and, perhaps, could even enable