Regional priority setting for rare species based on a method combining three criteria

Abstract

We propose a method to establish regional-level priorities for plant species that is based on three criteria. These involve two main criteria associated with species rarity, (a) small range size and thus regional responsibility and (b) low population numbers and thus local rarity, and a third criteria associated with habitat vulnerability to distinguish species currently threatened by human activities. Application of the method shows how the three criteria can be objectively and readily quantified (using a system of equal numbers of ranks) and combined into a hierarchy of priorities. We show how this can be done by using regional responsibility as a first order priority followed successively by local rarity and habitat vulnerability. Since the criteria are closely associated with rarity, this method is easily understandable for decision makers. We illustrate the application of the method to plants (a species rich group) at two regional scales where the objectives are different. If adapted to the availability of different types of data, the three criteria could be used to produce comparable lists in different countries and regions.

Introduction

The conservation of biodiversity occurs via the implementation of policy with only limited resources (Balmford et al., 2005). This means that effective conservation requires the establishment of priorities. The development of criteria to establish extinction risks and red lists for rare species of global conservation concern by the World Conservation Union (IUCN, 2001, IUCN, 2003) has provided comprehensive information about the conservation status of species in a data-driven protocol that allows informed decisions for conservation (Rodrigues et al., 2006). However, priority setting for rare species does not equate to the elaboration of red lists based on extinction risk; the “assessment of extinction risk and setting conservation priorities are two related but different processes” (IUCN, 2003, p. 5).

Priority setting for rare species includes the assessment of extinction risk and also integrates ecological, phylogenetic and historical criteria, cultural preferences, the probability of success of conservation actions, availability of funds, and legal frameworks for species conservation. Methods of priority setting based on such criteria will thus produce different classifications from those based solely on extinction risk (Dunn et al., 1999, Pärtel et al., 2005, Selvi, 2007, Schmeller et al., 2008b). It is thus necessary to distinguish threat of extinction from priority setting (Gärdenfors, 2001, Gärdenfors et al., 2001, Eaton et al., 2005, de Grammont and Cuarón, 2006, Fitzpatrick et al., 2007, Keller and Bollmann, 2004, Miller et al., 2007, Schnittler and Günther, 1999). As several of these studies indicate, information on extinction risk may be lacking, especially for species rich groups of organisms.

There thus remains a real need to develop such methods to achieve priority setting based on easily quantifiable biological information. It is also necessary that such a method be (a) applicable at different spatial scales and (b) based on a small number of criteria which can be combined into a simple scoring scheme without having to apply complex weighting systems. Otherwise the utility of the method, in terms of a tool for management and decision makers, will be questionable. Finally, priority setting for rare species should integrate criteria which reflect how a species is rare.

Rarity has two main components (Rabinowitz, 1981). First, species may have a limited distribution and be endemic to a small area, where regional responsibility for their conservation is high. The concept of national or regional responsibility as a criterion for assessing conservation priorities is receiving increasing attention (Schmeller et al., 2008a, Schmeller et al., 2008b). The notion of regional or national responsibility has its parallel in the literature on priority setting for reserve selection, where the relative contribution of sites to the conservation of a particular type of habitat, species or other biodiversity indicator can be estimated in terms of their irreplaceability: unique sites having a maximum value (Noss et al., 2002, Pressey et al., 1994, Pressey and Taffs, 2001). Second, small populations and low numbers of populations may cause species to be rare within a region, although they may have a widespread geographic distribution. Low numbers of populations may have diverse natural causes, e.g. occurrence in specialised or naturally rare habitats, poor dispersal ability, or occurrence in the peripheral part of a species distribution.

In addition, species may be undergoing decline and loss of populations due to human activities. Species for which the habitat is vulnerable to human activities can have a high local extinction risk (in terms of the likelihood of habitat loss) compared to those which are naturally rare – as long as the habitat of the latter is not affected by human activities (Keller and Bollmann, 2004, Pärtel et al., 2005). Natural rarity and habitat vulnerability may thus need to be distinguished in the assessment of regional conservation priorities (Domínguez Lozano et al., 2003, Pärtel et al., 2005).

The purpose of this paper is to develop a method of priority setting for species rich assemblages based on a small number of criteria which reflect the different components of rarity and habitat vulnerability of rare species. We apply the method to two areas in the Mediterranean region of southern France to illustrate how the criteria can be adapted and scored at different spatial scales. The use of well defined levels for each criterion allows for quantitative assessment and objective ranking to be performed. We discuss the use of a hierarchical method of combining the criteria based on their relative importance at a regional scale.