Comparability across and within groups
Amphibian, dragonfly and reptile maps have separate drivers of spatial
bias, and their biases differ both spatially and temporally as a
consequence of differing methodologies. Consequently, the main benefit
of using a single data source is entirely lost, as this inconsistency
precludes attributing differences between regions or taxa to genuine
differences rather than to methodological differences. For example,
reptile distribution maps frequently relied on either one-degree or
half-degree grids, with these boundaries clearly detectable on maps of
species range boundaries and turnover, with, for example,
>71% of range boundaries shared by >5 species
falling on either a political boundary or on a half-degree grid. The
fact that grids and political areas and other features are variably used
only further complicates these issues. Even odonata, where more
biologically-relevant river basin boundaries are used, the
near-universal use of such features (i.e. 92% of formerly mapped ranges
are on river basin boundaries) may still result in inaccurate maps with
both type one and type two errors. Thus, whilst trimming ERMs with
appropriate filters could be applied to most taxa, assessments of
gridding for reptiles before such an approach could be usefully applied.
For other taxa, once possible political boundaries have been assessed,
it may be possible to trim species ranges based on clear assessments of
habitat needs.
Regional biases are an especially notable issue. For example, only 31%
of US county boundary area (width=1km) has no amphibian boundary data,
whereas 98% of the land more than 500m from a boundary has no amphibian
range boundaries (Figure S2a). This is because of a specific initiative
within the US, and such inconsistent standards makes comparable analysis
between regions impossible (Blackburn et al., 2002). Once US Amphibians
which have all their borders on county borders are removed, only 40% of
species remain, and the remainder includes range-restricted and invasive
species. As county limits do not typically follow ecological boundaries,
they clearly do not represent species boundaries, thus, most forms of
trimming could not be used to accurately map ranges or diversity of such
a group.
In many cases, the development of published IUCN distributions
contradicts their general guidance (IUCN guidelines:
https://www.iucnredlist.org/resources/guidelines-for-appropriate-uses-of-red-list-data).
Despite the fact that former IUCN training material
(https://www.iucn.org/content/iucn-red-list-training-course-now-online)
explicitly said not to draw buffers around single locality points for
building distributions, this has clearly been done for hundreds of
species across taxa, extensively for some groups (Figure S2d). In
addition to other inconsistent biases both within and across groups,
these datasets lose the invaluable comparability that would normally be
expected from methods that are standardized temporally, spatially and
taxonomically. Consequently, further validation and refinement with
verified point data is necessary before these data are used for formal
conservation management.