Population declines and generation lengths can bias estimates of vulnerability

Abstract

Wildlife Society Bulletin 2004, 32(3):979–982 Peer edited A recent analysis of avian population trends (Dunn 2002) has engendered debate over the use of trend data in attempts to identify species of conservation and management concern (Butchart 2003,Dunn 2003). Dunn (2002) used breeding-bird survey data (Sauer et al. 2003) and methods proposed by the International Union for Conservation of Nature and Natural Resources ([IUCN] 2001) and Gibbons et al. (1996) to identify vulnerable species. Dunn (2002) found that many bird species triggering conservation alerts (i.e., perceived to be imperiled) due to population declines were not actually suitable candidates for management attention because the species were widespread and still common. As Dunn (2002) stressed, alerts based exclusively on population declines can be misleading because extinction risk is a multidimensional problem requiring simultaneous consideration of several variables. Conservation and management alerts based on population trends are influenced by 1) the level of decline producing an alert, and 2) the time period over which trends are evaluated (Dunn 2002). Managers must carefully weigh these parameters and the costs (Millsap et al. 1990) associated with species considered vulnerable. However, the parameters commonly used vary considerably (Millsap et al. 1990, Gibbons et al. 1996, IUCN 2001). A fixed time period was used by Millsap et al. (1990) and Gibbons et al. (1996), while IUCN (2001) used flexible time periods determined by an organism’s generation length. The latter approach may seem more appropriate because it includes species-specific parameters, but the approach can produce distorted estimates of relative extinction risks. This note provides an example of this problem and serves to bolster recommendations provided by Dunn (2002) to consider population trends in combination with other variables associated with extinction risk. In 2002 the Florida Fish and Wildlife Conservation Commission (FFWCC) adopted criteria developed by IUCN (2001) for estimating extinction risk and incorporated the criteria in the state’s legal definitions of protected species (Florida Administrative Code Rule 68A-27.0012). The FFWCC’s goal was similar to IUCN (2001), namely to gauge the global extinction risk of hundreds of species falling under the agency’s jurisdiction (FFWCC 2001). The new criteria considered population declines as well total population size, area occupied, and geographic distribution. The history and development of FFWCC’s new laws were described in Cox et al. (2002). Proposed reclassifications of the red-cockaded woodpecker (Picoides borealis) and the gopher tortoise (Gopherus polyphemus) occurred soon after the new criteria were adopted (Cox et al. 2002). The petition for the red-cockaded woodpecker (FFWCC 2002a) concluded that the species Commentary: Population declines and generation lengths can bias estimates of vulnerability