Читать книгу Fundamentals of Conservation Biology - Malcolm L. Hunter Jr. - Страница 54

The IUCN Red List of threatened species now requires that a species meet at least one of five quantitative criteria. Shown below are the five criteria for Critically Endangered. The primary differences for Endangered and Vulnerable are certain key numbers; these are shown in parentheses as values for EN and VU. The specificity of these criteria may seem rather naive given the uncertainty that often surrounds these kinds of data; see Akçakaya et al. (2000) for a system for dealing with this uncertainty. See Mace et al. (2008) for further discussion of the assessment process.

A taxon is Critically Endangered when the best available evidence indicates that it meets any of the following criteria (A to E), and it is therefore considered to be facing an extremely high risk of extinction in the wild:

1 Reduction in population size based on any of the following:An observed, estimated, inferred, or suspected population size reduction of ≥90% (EN 70%; VU 50%) over the last 10 years or three generations, whichever is the longer, where the causes of the reduction are clearly reversible AND understood AND ceased, based on any of the following:Direct observation.An index of abundance appropriate for the taxon.A decline in area of occupancy, extent of occurrence, and/or quality of habitat.Actual or potential levels of exploitation.The effects of introduced taxa, hybridization, pathogens, pollutants, competitors, or parasites.An observed, estimated, inferred, or suspected population size reduction of ≥80% (EN 50%; VU 30%) over the last 10 years or three generations, whichever is the longer, where the reduction or its causes may not have ceased OR be understood OR be reversible, based on any of (a) to (e) under A1.A population size reduction of at least 80% (EN 50%; VU 30%), projected or suspected to be met within the next 10 years or three generations, whichever is the longer (up to a maximum of 100 years), based on any of (b) to (e) under A1.An observed, estimated, inferred, projected, or suspected population size reduction of ≥80% (EN 50%; VU 30%) over any 10‐year or three‐generation period, whichever is longer (up to a maximum of 100 years), where the time period includes both the past and the future, and where the reduction or its causes have not ceased, based on any of (a) to (e) under A1.

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.