IUCN Red List Guidelines - The IUCN Red List of Threatened Species

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Red List Guidelines 20 Although subpopulations typically have little demographic or genetic exchange, this may or may not amount to their complete isolation in this regard. 4.3 Mature individuals (criteria A, B, C and D) “The number of mature individuals is the number of individuals known, estimated or inferred to be capable of reproduction. When estimating this quantity the following points should be borne in mind: • Mature individuals that will never produce new recruits should not be counted (e.g., densities are too low for fertilization). • In the case of populations with biased adult or breeding sex ratios, it is appropriate to use lower estimates for the number of mature individuals, which take this into account. • Where the population size fluctuates, use a lower estimate. In most cases this will be much less than the mean. • Reproducing units within a clone should be counted as individuals, except where such units are unable to survive alone (e.g., corals). • In the case of taxa that naturally lose all or a subset of mature breeding individuals at some point in their life cycle, the estimate should be made at the appropriate time, when mature individuals are available for breeding. • Re-introduced individuals must have produced viable offspring before they are counted as mature individuals.” (IUCN 2001) 4.3.1 Notes on defining mature individuals This definition of mature individuals differs slightly from that given in version 2.3 of the Red List Categories and Criteria (IUCN 1994). Some groups have found the more recent definition of mature individuals to be less conservative and less precise, leading to a potential down-listing of some taxa (e.g., obligate co-operative breeders) even though their extinction risk has not changed. It must be stressed that the intention of the definition of mature individuals is to allow the estimate of the number of mature individuals to take account of all the factors that may make a taxon more vulnerable than might otherwise be expected. The list of points given with the definition is not exhaustive and should not restrict an assessor’s interpretation of mature individuals, provided they are estimating the number of individuals known, estimated or inferred to be capable of reproduction. The ability of an assessor to estimate or infer which individuals are capable of reproduction is paramount and highly contingent on the particular features of the taxon or group. Juveniles, senescent individuals, suppressed individuals and individuals in subpopulations whose densities are too low for fertilization to occur will never produce new recruits, and therefore should not be counted as mature individuals. On the other hand, in many taxa there is a pool of non-reproductive (e.g., suppressed) individuals that will quickly become reproductive if a mature individual dies. These individuals can be considered to be capable of reproduction. In general, the judgement will be best made by assessors with insight into the species’ biology. In the case of taxa obligately dependent on other taxa for all or part of their life cycles, biologically appropriate values of mature individuals for the host taxon might be used. This
Red List Guidelines 21 number may be much less than the total number of mature individuals of the host taxon, because generally other factors restrict the dependant taxon from utilizing all host individuals. The number of mature individuals can be estimated using the equation d * A * p, where d is an estimate of population density, A is an estimate of area, and p is an estimate of the proportion of individuals that are mature. However, this approach often leads to gross overestimates of number of mature individuals. Therefore, great care should be taken when using this formula to ensure that: (a) the area is appropriately selected and d is an estimate of the average over the entire A (for example, the estimate will be positively biased if A is set to EOO and d is based on samples from areas of highest density), and (b) p should be selected based on knowledge of the taxon (or related taxa) rather than being set to a default value (such as 0.5) because the proportion of mature individuals in a population differs markedly among taxa. Bounds on the estimate of number of mature individuals can be obtained by placing bounds on each of d, A, and p. The value from this approach will be an estimate if the values for d, A and p are all estimates, but should be considered to be an inference if one or more of these values are based on inference. 4.3.2 Colonial or modular organisms, such as corals, algae, bryophytes, fungi and many vascular plants As opposed to a unitary organism, such as a vertebrate, an insect and many vascular plants, the growth and development of a colonial (modular) organism is an iterative process in which “modules” are added step by step to the existing structure. In principle, the growth of a modular organism never ends and it has no final shape, size or age. A modular organism (the genet) can sometimes exist in a form of many parts (ramets), which can become more or less isolated from each other. Consequently, what constitutes a ‘mature individual’ in a colonial or modular organism is not always clear. Still, it is important to define ‘mature individual’ for such organisms, since ‘mature individual’ is used under criteria C and D to capture the effects of threats and demographic stochasticity to a small population. In defining ‘mature individual’ for colonial organisms, it is important to identify entities that are comparable in extinction proneness to discrete mature individuals of animals and other organisms. As a general rule, the ramet, i.e., the smallest entity capable of both independent survival and (sexual or asexual) reproduction should be considered a ‘mature individual’. For instance, in those cases where the organism appears in well-distinguishable units, each such unit would be counted as one mature individual. Examples may be a bryophyte tuft (e.g., of Ulota) or a discrete cushion (e.g., Brachythecium), a lichen thallus (e.g., Alectoria) or foliose patch (e.g., Parmelia), or a coral discrete entity (e.g., a brain coral Diploria or sun coral Tubastrea). If the delimitation of ramets is not obvious, but the species lives in or on a discrete and relatively small substrate unit limited by a certain resource, e.g. a piece of cow dung, a leaf or a dead tree branch, each unit colonized by the species should be counted as a single mature individual. In many other cases, like reef-forming corals, cliff-growing lichens and groundgrowing fungi, the organism grows in large, more or less continuous entities that could be divided into smaller pieces without obviously harming the organism. In principle, the smallest such entity (ramet) that an organism could be divided into without causing its death
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IUCN Red List Guidelines - The IUCN Red List of Threatened Species

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