Overlap between species and administrative boundaries
(%)
Analyses revealed pervasive biases throughout ERMs. For all groups high
proportions of species ranges overlapped with administrative borders
(Table 1; Figure S1). Non-coastal borders show that administrative
boundaries disproportionately impact range limit delineation, averaging
20-30% of non-coastal species range boundaries coinciding national
administrative boundaries alone. Provincial boundaries are also used,
increasing the average overlap by upto 10%.
To assess how species turnover relates to administrative areas, we
calculated the standard deviations of richness maps (Figure 1), then
identified what percentages of national administrative layers were
included in the higher richness classes, to see what proportion of
administrative boundaries fall within these hotspots. For reptiles, the
upper four classes were retained, covering 40% of national boundaries
(4% low turnover, 13% medium turnover 24% high turnover). This means
that the hotspots for reptiles are disproportionately delimited by
global administrative boundaries. In total, the overlap between
administrative boundaries and these upper classes equates to 37% of all
richness classes, but equals only 10% of the lowest quartile, 33% of
medium turnover and 82% of the highest turnover. However, as
low-diversity areas will necessarily have low turnover, this was then
extracted for areas with >3 reptiles species, and for these
regions 68% of national boundaries were on upper richness classes (43%
highest turnover).
For amphibians, 34% of national boundaries overlapped with richness
classes (two top richness classes, 27% high turnover), though equating
to only 10% of high-turnover areas (16% of the highest levels). For
birds, 52% of national boundaries coincide with turnover (3 upper
richness classes, 21% at highest level), this equates to 29% of the
highest bird turnover. For Mammals, 60% of national boundaries
coincided with high turnover areas (3 top richness classes, 29% at high
turnover), representing 26% of high turnover areas, and 35% of highest
levels. For Odonata, 40% of national boundaries were covered (three top
richness classes, with 25% at the highest turnover levels),
representing 9% of high turnover areas but 16% of the highest.
In terms of individual countries, countries with longer coastlines
obviously exhibited higher diversity changes, as coastlines typically
mark absolute distributional boundaries. Yet, despite this, some
countries with little coastline, and even landlocked countries, showed
comparable levels of turnover at political boundaries. For reptiles,
Nepal had levels of turnover at political boundaries roughly comparable
to that of coastlines, and China also shows exceptional turnover,
especially on its southwestern border (Figure 2); Bolivia showed some
similar patterns.
Without coastal boundaries included Northern Southeast Asian boundaries
are visible across taxa (excepting birds-Figure S1), with the Chinese
side of borders show much lower diversity than neighboring Thailand,
Vietnam and Myanmar. Iran also shows high turnover along administrative
boundaries, considerably below the areas it borders. What is striking is
for some countries, especially Southeast Asian countries, 100% of
non-coastal boundaries show overlap with peak areas for species
turnover, highlighting the disproportionately large role these borders
play in mapping species distribution (Figure S1).