Shifting baselines increase the risk of misinterpreting biodiversity trends

Ecological studies quantifying the impact of land-use change on
biodiversity may be sensitive to the choice of reference points – or
baselines – particularly when sampling across human land-use
gradients and other space-for-time comparisons. Much depends on whether
the chosen baseline has already undergone shifts in species composition
because of hunting, habitat loss, and degradation. However, few studies
have assessed the influence of shifting baselines on estimates of
anthropogenic impacts. Using new survey data from five West African
land-use gradients, we examine how habitat patch size and structure
influence the estimated impact of land-use change on bird species richness
and functional diversity. We show that smaller forests have already lost
many forest-dependent birds, particularly those with large body size or
specialised ecological niches, leading to reduced estimates of
biodiversity loss after deforestation. The steepest biodiversity loss was
found in mid-sized forests, whereas relatively shallow declines were
estimated for the most extensive forests, despite their richer taxonomic
and functional diversity. In these larger forest blocks, accurate
estimates of biodiversity loss may require longer transects extending
beyond the biodiversity ‘shadow’ caused by the more extensive spillover of
forest species into the surrounding landscape, potentially linked to
source-sink dynamics. These findings suggest that biodiversity assessments
are highly sensitive to baseline selection and transect design,
highlighting the risk of underestimating land-use impacts unless shifting
baselines are carefully considered.