Temporal trends in the spatial bias of species occurrence records

Large-scale biodiversity databases have great potential for quantifying
long-term trends of species, but they also bring many methodological
challenges. Spatial bias of species occurrence records is well recognized.
Yet, the dynamic nature of this spatial bias - how spatial bias has
changed over time - has been largely overlooked. We examined the spatial
sampling bias of species occurrence records within multiple biodiversity
databases in Germany and tested whether spatial bias in relation to land
cover or land use (urban and protected areas) has changed over time. We
focused our analyses on urban and protected areas as these represent two
well-known correlates of sampling bias in biodiversity datasets. We found
that the proportion of annual records from urban areas has increased over
time while the proportion of annual records within protected areas has not
consistently changed. Using simulations, we examined the implications of
this changing sampling bias for estimation of long-term trends of species’
distributions. When assessing biodiversity change, our findings suggest
that the effects of spatial bias depend on how it affects sampling of the
underlying land-use change drivers affecting species. Oversampling of
regions undergoing the greatest degree of change, for instance near human
settlements, might lead to overestimation of the trends of specialist
species. For robust estimation of the long-term trends in species’
distributions, analyses using species occurrence records may need to
consider not only spatial bias, but also changes in the strength of
spatial bias through time.