The effect of past defaunation on ranges, niches, and future biodiversity forecasts

Humans have reshaped the distribution of biodiversity across the globe,
extirpating species from regions otherwise suitable and restricting
populations to a subset of their original ranges. Here, we ask if
anthropogenic range contractions since the Late Pleistocene led to an
under‐representation of the realized niches for megafauna, an emblematic
group of taxa often targeted for restoration actions. Using
reconstructions of past geographic distributions (i.e., natural
ranges) for 146 extant terrestrial large‐bodied (>44 kg)
mammals, we estimate their climatic niches as if they had retained their
original distributions and evaluate their observed niche dynamics. We
found that range contractions led to a sizeable under‐representation of
their realized niches (i.e., niche unfilling). For
29 species, more than 10% of the environmental space once seen in
their natural ranges has been lost due to anthropogenic activity, with at
least 12 species undergoing reductions of more than 50% of their
realized niches. Eighteen species mainly now be confined to
low‐suitability locations, where fitness and abundance are likely
diminished; we consider these taxa climatic refugees. For those
species, conservation strategies supported by current ranges risk being
misguided if current, suboptimal habitats are considered baseline for
future restoration actions. Because most climate‐based biodiversity
forecasts rely exclusively on current occurrence records, we went on to
test the effect of neglecting historical information on estimates of
species’ potential distribution – as a proxy of sensitivity to climate
change. We found that niche unfilling driven by past range contraction
leads to an overestimation of sensitivity to future climatic change,
resulting in 50% higher rates of global extinction, and underestimating
the potential for megafauna conservation and restoration under future
climate change. In conclusion, range contractions since the Late
Pleistocene have also left imprints on megafauna realized climatic niches.
Therefore, niche truncation driven by defaunation can directly affect
climate and habitat‐based conservation strategies.