A Great Escape: resource availability and density-dependence shape population dynamics along trailing range edges

Populations along geographical range limits are often exposed to
unsuitable climate and low resource availability relative to core
populations. As such, there has been a renewed focus on understanding the
factors that determine range limits to better predict how species will
respond to global change. Using recent theory on range limits and
classical understanding of density dependence, we evaluated the influence
of resource availability on the snowshoe hare Lepus americanus along its
trailing range edge. We estimated variation in population density, habitat
use, survival, and parasite loads to test the Great Escape Hypothesis
(GEH), i.e. that density dependence determines, in part, a species’
persistence along trailing edges. We found that variability in resource
availability affected density and population fluctuations and led to
trade-offs in survival for snowshoe hare populations in the northeastern
USA. Hares living in resource-limited environments had lower and less
variable population density, yet higher survival and lower parasitism
compared to populations living in resource-rich environments. We suggest
that density-dependent dynamics, elicited by resource availability,
provide hares a unique survival advantage and partly explain persistence
along their trailing edge. We hypothesize that this low-density escape
from predation and parasitism occurs for other prey species along trailing
edges, but the extent to which it occurs is likely conditional on the
quality of matrix habitat. Our work indicates that biotic factors play an
important role in shaping species’ trailing edges and more detailed
examination of non-climatic factors is warranted to better inform
conservation and management decisions.