A flexible approach for prioritizing rare taxa for ex situ collection

Given their sheer numbers, it is necessary to prioritize rare taxa for ex situ collection. Prioritization is inherently a values-driven exercise, and, just as value judgements can reasonably differ between people, the perspective of who is doing the prioritizing matters as well.

Ed Guerrant, the director of Rae Selling Berry Seed Bank & Plant Conservation Program at Portland State University, has attempted to create a way that taxa can be prioritized using multiple criteria. With this process, the weight attributed to different criteria can be modified by the user.

The approach described here was specifically developed to assist the BLM strategically prioritize for ex situ seed collection the 636 taxa on the Oregon and Washington BLM Sensitive and Strategic Species (SSS) list. It builds on Farnsworth et al (2006. Biological Conservation 128:1-12) for the six New England States, which assigns numerical values to each of many variables, with the resulting order of ranking determined by the sum of all ranks. Those having the lowest total have the highest priority. It yields a single rank order list of priorities. The approach created by Guerrant relies on many of the same factors, but how these factors are treated differs in several significant ways.

First, in the New England approach, the relative weights of the different variables, while not explicit, vary greatly due to the differing range of values with which different variables are assigned. Those differences have a dramatic influence on the final scores and thus rank order.

In addition to whether a taxon can be stored ex situ, the basic variables considered here concern the geographic range, number of element occurrences in Oregon and Washington, the proportion of element occurrences for which ex situ samples exist, how many and what proportion of element occurrences occur on BLM lands, as well as legal status and other priority ranks (e.g. G-ranks.)

To ameliorate potential effects on relative weighting inherent in differing scales among variables, the priority value assigned to each variable were adjusted to a standard range of 0 to 10 for all variables in order to facilitate the explicit assignment of differing relative weights to different variables, should that be desired. This approach allows the effects of particular variables on the overall priority rank order to be determined. Ultimately, this approach represents a significant advance and has the potential to be a first step in the development of what could become an ongoing, updatable and flexible (database based) tool for prioritizing species for ex situ collection.