Rapid morphological change in a small mammal species after habitat fragmentation over the past half-century

Study Aim: To compare the rapid shifts in body size of mainland
and island populations of a native rodent and examine the mechanisms
underlying these changes. Location: Thousand Island Lake, China,
which was created in 1959 when the Xin’anjiang Dam was constructed for
generating hydroelectricity. Taxon: The Chinese white-bellied
rat, Niviventer confucianus. Methods: Field surveys were
conducted from 2015 to 2018 to collect data on body size of the rodents
from a set of islands and nearby mainland sites. We constructed multiple
linear models to examine the relationships between body size (length and
mass) of rodents and biological variables (predators, interspecific and
intraspecific competitors, and food availability). We also conducted
structural equation modeling (SEM) by constructing models via confirmatory
path analysis. Results: All island populations of N.
confucianus had significantly larger body size (both body mass
and body length) than their mainland counterparts. Moreover, populations
on small and more isolated islands had larger body size than their
relatives on big islands. The relative absence of predators (large-bodied
mammals, snakes, and raptors) on islands was most strongly associated with
shifts in the body size of rodents. The documented changes occurred after
only a half-century of fragmentation. Main conclusions: The
observed rapid body enlargement of rodents after habitat fragmentation is
consistent with a release from predation pressure. SEM indicated that
island area, rather than island isolation, had positive effects on the
richness of predators, interspecific competitors and food resources, which
then had an indirect impact on body size of the rodents. In this study, we
report a remarkably rapid case of mammal morphological shifts in a small
mammal in response to habitat fragmentation. Given the omnipresence of
dams and other anthropogenic disturbances, our findings suggest that a
wave of rapid phenotypic shifts in terrestrial vertebrates is taking place
in the Anthropocene.