Since the first Action Plan was published in 1986, a number of developments have made clear the need to change not just the threat ratings of species, as a result of new information about their status, but also the rating system itself. In the original plan, species were given a priority rating for conservation action based on three parameters:
The estimated Degree of Threat to their populations, on a scale of 1 (lowest threat) to 6;
The Taxonomic Uniqueness of the species, on a scale of 1 (several close relatives) to 3; and
The Association of the species with other threatened primates, with a rating of 1 (little overlap with other threatened primates) or 2 (major overlap with one or more threatened forms).
Using this system, the lowest-rated African primate species had 3 points, and the highest (six species) had 9 points.
Since the first plan was published, the Primate Specialist Group has produced two further plans, one for Asian Primates (Eudey 1987) which used the same system as the African plan, and one for Lemurs (Mittermeier et al. 1992). The lemur plan used a five rather than six-point scale for Degree of Threat, basing a taxon's score on its estimated total population size (e.g., 1 = total population probably greater than 100,000, 5 = total population probably less than 100), and it used a 4-point scale for taxonomic uniqueness (from 1 = a member of a large species group, to 4 = only member of a monotypic family). The lemur plan did not use a score for association with other threatened taxa, but instead gave any taxon an extra point if it is not known to occur in any protected area. On this system, individual lemur species scored between 2 and 7 points.
In 1991, Mace & Lande proposed a new set of threatened species criteria, based on probabilities of extinction calculated from estimates of population size, geographic range, degree of fragmentation, and rate of decline. This proposal provoked considerable debate and led to IUCN-sponsored workshops at which attempts were made to refine the proposed Red List threat categories and the criteria for their use (see Mace et al. 1993). The final version of this proposal was approved by the IUCN Council in 1994 (IUCN 1994).
This new categorization system has not been adopted in all its details by the Primate Specialist Group because (a) it relies heavily on numerical information about population sizes, distributions and/or rates of change that are generally not available, and (b) its strict application would result in the downgrading of some taxa to lower threat categories than are widely felt to be reasonable. On the other hand, the new system does have some merits; for instance, it focuses attention on the difficulty of categorizing extinction threats, and it draws attention to the range of factors that should be considered in allocating taxa to threat categories.
This plan gives African primate species a priority rating for conservation action based on:
The estimated Degree of Threat to the survival of their wild populations.
Their Taxonomic Distinctiveness.
We have eliminated a weighting based on Association with Other Threatened Primates in this revision. This was an ambiguous quantity, which tended to underrate the importance of taking action to protect some highly threatened species simply because they did not happen to coexist with other threatened primates.
In a draft of this plan we followed the lemur action plan and added one point to a score if a species is not known to be present in an effective protected area. Almost all African primate species occur in at least one area in which wildlife is legally protected, but in many of these areas actual protection on the ground is weak, and completely effective protection is rare. Because members of the PSG disagreed as to what level of failure in a protection system would justify adding an extra point to a priority rating, this evaluation has not been included in the final version of this plan.
We use five categories of threat that are comparable to the new IUCN categories, but which employ less precise numerical information and larger population thresholds. The categories are: Critically Endangered, Endangered, Vulnerable, At Risk and Low Risk; and the criteria by which species have been assigned to these categories are as follows:
1. Low Risk
The species faces a low risk of extinction at the present time. It has a wide geographical distribution, broad ecological tolerances, and the total population is probably >100,000.
2. At Risk
Not significantly threatened at present, but the species has a limited distribution and/or ecological tolerances and is, therefore, at risk of becoming vulnerable in the next 30–40 years.
3. Vulnerable
Vulnerable to extinction within approximately the next 100 years for one or more of the following reasons: the species has a very restricted geographical distribution (<100,000 km2); surviving populations are small (total number of individuals remaining probably <100,000) and/or highly fragmented; most populations are seriously threatened by habitat destruction and/or hunting; most populations are declining and the species has a low rate of reproduction.
4. Endangered
In danger of extinction within 30–40 years because the surviving population is very small (probably <10,000 individuals remain) and is declining rapidly as a result of habitat alteration and/or hunting; or, the surviving population probably numbers <25,000 individuals and is very highly fragmented and declining rapidly.
5. Critically Endangered
In danger of extinction in the very near future as a result of extremely small population size (probably <1,000 individuals remain) and rapid decline through habitat destruction (including fragmentation) and/or hunting. No African primate species yet falls into this category.
Table 2 compares the criteria for the allocation of species to these categories with a simplified version of the criteria given for the comparable new IUCN categories. The population sizes used in our categories have been chosen to maintain some consistency among the original African primate plan, the Asian primate and lemur plans which followed it, and discussions held between members of the Primate Specialist Group and the Captive (now Conservation) Breeding Specialist Group in 1991 that led to the CBSG's Conservation Assessment and Management Plan for Primates (Stevenson et al. 1992). Our ratings are above all indices, indicating the relative degree of extinction threat faced by one species compared to others. Population size has not been given special weight in assigning species to different categories, because information on total population size is unavailable for the vast majority of African primate species.
Monkey meat is an important source of animal protein for many people in the African rain forest, but excessive hunting may carry some species to extinction. Here a hunter holds a female white-throated guenon he has just shot in Nigeria's Okomu Forest Reserve.
John F. Oates
Table 2 A Comparison of the Latest IUCN Threatened Species Categories with those used in this Plan
Table 3 Conservation Priority Ratings for African Primate Species*
The measure of Taxonomic Distinctiveness is reduced in this plan from a 3-point to a 2-point scale. With the elimination of one point from the Degree of Threat scale (compared with the original plan), an unreasonably high total priority score can result if a 3-point taxonomic scale is retained. For instance, since this plan regards the patas monkey as a member of a monotypic genus, giving 3 taxonomic points to this relatively very abundant primate (regarded as a crop pest in many places) would result in an unduly high priority rating for conservation action.
The criteria used to give a species a score of 1 or 2 for taxonomic distinctiveness are:
A species belonging to a genus, subgenus or species-group with 3 or more members, and/or its status as a full species is sometimes questioned.
A species with no more than 1 close relative (a member of the same species-group, subgenus or genus).
The results of applying this rating system are presented in Table 3. The highest-ranked species for conservation action is the drill (Mandrillus leucophaeus), with a total score of 6. Five species have a score of 5; these are the Diana monkey (Cercopithecus diana), Sclater's guenon (Cercopithecus sclateri), the chimpanzee (Pan troglodytes), bonobo (Pan paniscus), and the gorilla (Gorilla gorilla).
In the original plan, subspecies were given rather little attention. In part this was due to the uncertain taxonomic status of very many subspecies—a large number of the primate subspecies that have been named are of doubtful validity. However, certain kinds of intraspecific variability are both clear and discrete, producing distinctive local populations. Even though such populations may not have been given formal species status, they must surely be recognized in conservation planning, especially when a distinct local form is under threat but the species to which it belongs is relatively secure.
Table 4 Subspecies and Populations of Particular Conservation Concern
The white-crowned mangabey (Cercocebus atys lunulatus) is known only from western Ghana to the Sassandra-Nzo River system of the Ivory Coast and appears to be endangered.
S. Gippoliti
In the first action plan, 15 subspecies were listed as deserving special attention on the basis of being clearly distinct from other forms, having a very limited geographical distribution and/or a small population size, and being threatened or potentially threatened. It is now clear that more than 15 subspecies should be recognized with these characteristics, and it seems worthwhile to attempt a rating of the degree to which they are threatened.
The following rating system used for subspecies and other distinct local populations is different from that used for species (only threatened forms are considered; taxonomic uniqueness is not considered; and separate ratings are given for population size and distribution, and for degree of threat, since these are somewhat easier to estimate for localized populations): Population size and distribution are rated on a scale of 1–3: A score of 3 is given if approximately 1,000 or fewer individuals probably remain, or populations are limited to a total area of less than 500 km2; a score of 2 if 25,000 or fewer individuals remain, or the total area inhabited is less than 5,000 km2; and a score of 1 if less than 100,000 individuals remain, or the area inhabited is less than 10,000 km2. Degree of threat from hunting and/or habitat loss is also rated on a 1–3 scale: 3, severe; 2, moderate; 1, low.