Assessing Threats and Setting Priorities for Conservation

The purpose of this comment is to discuss and to compare briefly some systems for prioritizing species for conservation attention. This comment is written in response to a proposed new International Union for the Conservation of Nature and Natural Resources (IUCN) hierarchical categorization, described in Conservation Biologjy by Mace and Lande (1991), and based on a species' probability of extinction. Resources (e.g., time and money) available for the conservation of species and ecosystems are invariably in short supply relative to the needs for those resources. Accordingly, setting priorities for conservation actions is a necessary and major preoccupation of governmental and nongovernmental organizations concerned with the conservation of species and ecosystems. Priorities for the allocation of resources to individual species are often based primarily on an assessment of the threats to those species, and secondarily on other factors. For example, the U.S. Fish and Wildlife Service has two priority species ranking systems-one for listing actions and one for recovery actions. For listing as "endangered" or "threatened," species are ranked on a scale of 1 through 12 based on magnitude (high or moderate to low) and immediacy (imminent or nonimminent) of threat, and taxonomic distinctness (monotypic genus, species, or subspecies). For recovery actions, species are ranked on a 1 through 18 scale on the basis of (again in descending order of importance) the degree of threat, recovery potential, taxonomic distinctness, and conflict with economic alternatives. These priority ranks, even though they are based on qualitative criteria, are considered to be guides to action and not "inflexible frameworks for determining resource allocations" (Fay & Thomas 1983). Many states in the U.S. have developed their own systems in order to prioritize conservation actions (e.g., Millsap et al. 1990) and/or in order to designate species into priority categories including "official" state listing categories such as "endangered," "threatened," and "special concern." Assignment of species to these categories is usually based on an analysis of factors such as the number of subpopulations or occurrences and the number of individuals of the species in the state, threats to the species and/or its habitat, and population and/or habitat trends. Quantitative systems in which points are assigned to various subcriteria and then totaled have been used as guides to assigning species to categories in several states (e.g., New York; Nye & Riexinger 1981). The utility of such quantitative rating systems is debatable. Individual species are not equally comparable in terms of all considerations, and adequate data are often lacking, causing species assessments to be variably subjective, despite the apparent but misleading precision of the ranks. Reflecting these concerns, several states (including New York) have recently adopted semiquantitative listing criteria similar to those first developed in Michigan in 1981 (Michigan Department of Natural Resources, unpublished). The International Union for the Conservation of Nature and Natural Resources (IUCN) publishes Red Data Books and Red Lists that provide lists and data on animal species known to IUCN to be threatened with extinction. Species identified as threatened by IUCN are assigned a category purportedly indicating the degree of threat. These categories are extinct, endangered, vulnerable, rare, indeterminate, insufficiently known, threatened, and commercially threatened (IUCN 1988). Providing little guidance regarding the assignment of species to categories, the current IUCN system is considered by some to be overly subjective. Mace and Lande (1991) propose a new system of IUCN categories of threat and a set of specific quantitative criteria which they suggest are applicable at least to most vertebrates. In their proposed system, species are