Impact of alternative metrics on estimates of extent of occurrence for extinction risk assessment

Abstract

In International Union for Conservation of Nature (IUCN) Red List assessments, extent of occurrence (EOO) is a key measure of extinction risk. However, the way assessors estimate EOO from maps of species' distributions is inconsistent among assessments of different species and among major taxonomic groups. Assessors often estimate EOO from the area of mapped distribution, but these maps often exclude areas that are not habitat in idiosyncratic ways and are not created at the same spatial resolutions. We assessed the impact on extinction risk categories of applying different methods (minimum convex polygon, alpha hull) for estimating EOO for 21,763 species of mammals, birds, and amphibians. Overall, the percentage of threatened species requiring down listing to a lower category of threat (taking into account other Red List criteria under which they qualified) spanned 11-13% for all species combined (14-15% for mammals, 7-8% for birds, and 12-15% for amphibians). These down listings resulted from larger estimates of EOO and depended on the EOO calculation method. Using birds as an example, we found that 14% of threatened and near threatened species could require down listing based on the minimum convex polygon (MCP) approach, an approach that is now recommended by IUCN. Other metrics (such as alpha hull) had marginally smaller impacts. Our results suggest that uniformly applying the MCP approach may lead to a one-time down listing of hundreds of species but ultimately ensure consistency across assessments and realign the calculation of EOO with the theoretical basis on which the metric was founded.

Hot and bothered: Using trait-based approaches to assess climate change vulnerability in reptiles

“…Environmental tolerances and exposure to climate change were calculated from distribution maps published on the IUCN Red List, using those parts of the distribution where species are recorded as 'extant', 'probably extant', 'native' or 'reintroduced' (Joppa et al, 2015). Distribution maps on the IUCN Red List usually exist in the form of generalised range polygons, thus potentially including large areas of unoccupied, potentially unsuitable areas; this can make calculations of climatic requirements and tolerance from these distribution maps unrepresentative .…”

Including environmental niche information to improve IUCN Red List assessments

“…AOO provides information about the effective spatial distribution within EOO and therefore differentiates between species with a restricted spatial distribution, which are often habitat specialists, and common species (SPSC, 2016), which might have the same EOO as specialists but a very different extinction risk. These two measures are therefore complementary (Gaston & Fuller, 2009;Joppa et al, 2016).…”

The use of range size to assess risks to biodiversity from stochastic threats

“…For example, our approach could elucidate patterns of extinction risk from stochastic threats, providing support to Red List status assessments while exploring the limitations in the application of this criterion (Gaston & Fuller, 2009;Joppa et al, 2015). In particular, the same spatial metrics that we have investigated are employed for assessing risks to species, and we expect that all of our findings are equally informative for species listing protocols such as the IUCN Red List of Threatened Species.…”