Effects of Detectability on Estimates of Geographic Range Size in Bignonieae

Abstract

Extinction risk has not been evaluated for 96% of all described plant species. Given that the Global Strategy for Plant Conservation proposes preliminary conservation assessments of all described plant species by 2010, herbarium specimens (i.e., primary occurrence data) are increasingly being used to infer threat components from estimates of geographic range size. Nevertheless, estimates of range size based on herbarium data may be inaccurate due to collection bias associated with interspecific variation in detectability. We used data on 377 species of Bignonieae to test the hypothesis that there is a positive relationship between detectability and estimates of geographic range size derived from herbarium specimens. This relationship is expected if the proportion of the true geographic range size of a species that is documented by herbarium specimens is given by the product of the true geographic range size and the detectability of the species, assuming no relationship between true geographic range size and detectability. We developed 4 measures of detectability that can be estimated from herbarium data and examined the relationship between detectability and 2 types of estimates of geographic range size: area of occupancy and extent of occurrence. Our results from regressing estimates of extent of occurrence and area of occupancy on detectability across genera provided no support for this hypothesis. The same was true for regressions of estimated extent of occurrence on detectability across species within genera. Nevertheless, regressions of estimated area of occupancy on detectability across species within genera provided partial support for our hypothesis. We considered 3 possible explanations for this mixed outcome: violation of the assumption of no relationship between true geographic range size and detectability; the relationships between estimated geographic range size and detectability may be an artifact of a negative relationship between estimated area of occupancy and the sampling variance of detectability; detectability may have had 2 opposite effects on estimated species range sizes: one determines the proportion of the true range of a species documented by herbarium specimens and the other determines the distribution of true range size for the species actually observed with herbarium data. Our findings should help improve understanding of the potential biases incurred with the use of herbarium data.

The sizes of species’ geographic ranges

“…However, more formal methods have also been applied, including calculation of the area that lies within (i) a minimum convex polygon or convex hull embracing the occurrences of a species (Shaw, Musina & Gichuki 2003; Standards and Petitions Working Group 2006; Miller, Enright & Lamont 2007; Sérgio et al . 2007; Sheth et al . 2008); (ii) an α–hull constructed by building a Delauney triangulation of the occurrence points, and removing connecting lines longer than a certain multiple of the average line length, thus deleting areas spanning largely unoccupied regions (Burgman & Fox 2003; Farnsworth & Ogurcak 2006; Sérgio et al .…”

Reassessing Neotropical angiosperm distribution patterns based on monographic data: a geometric interpolation approach

“…However, there also exist some inherent drawbacks, such as heterogeneous sampling of space and taxa because of varying accessibility of areas and attractiveness of taxa to collectors (Nelson et al 1990;Graham et al 2004;Schulman et al 2007;Sheth et al 2008) and systematic inaccuracy (Meier and Dikow 2004;Hopkins 2007;Tobler et al 2007). This problem can in part be avoided by using revised specimen data, which were reviewed by expert taxonomists and published in form of monographs, socalled monographic data (Thomas 1999;Knapp 2002;Hopkins 2007).…”

“…Examples of techniques for the estimation of species ranges are the convex hull (Willis et al 2003;Sheth et al 2008), the minimum spanning tree (Hernández and Navarro 2007) or the minimum bounding box (Graham and Hijmans 2006). Generating species ranges by means of a convex hull often results in overestimation of species ranges (Burgman and Fox 2003) and ignores disjunct distribution patterns, particularly for widespread species.…”

Conservation status evaluation of Berberis species across the Karakoram Mountain Ranges, Pakistan using IUCN Red List Categories and Criteria

“…Two approaches are most commonly used to calculate the area of distribution of any species (Sheth et al 2008;Harris et al 2007;Litvaitis et al 2006;Randrianasolo et al 2002;Riba et al 2002). Extent of occurrence (EOO) is a key measure of extinction risk (Joppa et al 2015), and the area of occupancy (AOO) provides a general measure of how robust the distribution will be to stochastic and directional threatening processes (Gaston and Fuller 2009).…”