Ranking Taxa For Conservation Priority

The World Conservation Union (IUCN) has initiated schemes whereby species can be signaled as of conservation concern through being included on a global Red List of Threatened Animals and progressively allocated to a category of threat severity based on quantitative estimates of risk of extinction. The year 2000 Red List includes 747 insect species, including representatives of 15 orders, but is dominated by Lepidoptera (284), Hymenoptera (152), and Odonata (137 species); daunting though this number may seem, it is no more than the detected tip of the iceberg of needy insect taxa. Many of the species included have not been evaluated critically in relation to their close relatives, for example; some are listed as the result of the zeal of individual nominators; and many insect groups have no such champions to promote their welfare.

A number of regional red data books dealing with insects have established more local priorities, as have a greater number of Action Plans and similar documents arising from country- or state-based conservation legislations. In common with other taxa, the "listing" of an insect on a schedule of protected taxa often confers legal obligation to define and pursue the necessary conservation measures needed to ensure its well-being. It is important to recognize that simply being "rare" does not necessarily indicate conservation need. Many insects are known from single localities or otherwise very small areas. "Rarity" has connotations of one or more of small numbers, limited distribution, and ecological specialization, with the rarest species being ecological specialists occupying very small areas and occurring in very low numbers. However, rarity can be a stable condition. Conservation concern arises more properly from threats caused by human intervention increasing the level of rarity, such that a risk of extinction is imposed on a formerly stable balance or a trajectory of decline is accelerated. Conditions of rarity may predispose the species to stochastic effects and increase its vulnerability if the external threat spectrum increases.

Unlike many conservation assessments for vertebrates, quantitative population data on insects are rarely available, and even large numerical fluctuations between successive generations may be entirely normal. Detection of numerical decline is thereby difficult, and the quantitative thresholds for allocating a species to the IUCN categories of "critically endangered," "endangered," and "vulnerable" generally cannot be met. For most insects for which any biological information is available, which is a small minority, even the basic pattern of population structure is generally unclear. Many butterfly species previously assumed to have closed populations, for example, are now known to manifest a metapopulation structure, wherein discrete demographic units (nominally subpopulations) occur disjunctly in patches of habitat across a wider area, and the whole population is maintained through rolling series of extirpations and recolonizations of the suitable habitat patches in the wider environment. Thus, even loss of whole apparent populations may be entirely normal, and the practical conservation dilemma is to distinguish these from declines and loss caused by imposition of external threats.

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