Patterns of niche contraction identify vital refuge areas for declining mammals

Aim Investigation of realised niche contraction in declining species can
help us understand how and where threats are being either mediated or
tolerated across landscapes. It also provides insights into species’
sensitivity to environmental change that are unable to be identified
through analysis of declines in range size or abundance alone. Here, we
apply the recently proposed ‘niche reduction hypothesis’ to investigate
relationships between trends in niche breadth and geographic distribution
of declining species. Location Northern Australia Methods We compare and
contrast contemporary and historical datasets to examine the relationship
between extent of occurrence (EOO) and realised niche hypervolume, and
investigate changes in species’ utilisation of environmental space through
time via generalised linear modelling and bootstrapping of historical
values. We also use the ‘Maxent’ algorithm to create and stack
contemporary and historical ecological niche models (ENMs) and identify
regions where resilience to threatening processes is maximised. Results We
found larger mean reductions in niche hypervolume (39%) than EOO (30.5%),
with little correlation (r = 0.07) between the two measures, suggesting
that contraction of realised niche breadth can be largely independent of
reduction in EOO. We also identified a general set of environmental
conditions towards which species’ realised niches contracted. Comparison
of stacked ENMs allowed us to identify regions of natural refuge where
environmental conditions are associated with increased species resilience
to threats, and conversely, regions where habitat suitability has
declined. Main conclusions Examining species declines from an ecological
niche perspective provides a powerful tool for understanding how
environmental conditions, biotic interactions, and species traits shape
responses to local and global environmental changes. Quantifying
reductions in niche breadth is crucial as contraction to a narrower subset
of environmental space can reduce a species’ ability to tolerate other
threats and potentially lower adaptive capacity and genetic diversity,
increasing extinction risk.