Mammalian responses to changed forest conditions resulting from bark beetle outbreaks in the southern Rocky Mountains

Abstract

AbstractSpruce beetle (Dendroctonus rufipennis) and mountain pine beetle (Dendroctonus ponderosae) outbreaks have impacted millions of acres of conifer forest from Alaska to northern Mexico. These species are native to North America, and periodic outbreaks have shaped the structure and composition of conifer forests for millennia. However, the extent and severity of current outbreaks, fueled by favorable climatic conditions and increased susceptibility of forests, are unmatched in recorded history. To characterize the response of a suite of mammalian species to beetle‐induced changes in vegetation in the southern Rocky Mountains, we deployed cameras at 300 randomly selected sites during summer 2013–2014. Selected sites spanned gradients of years elapsed since bark beetle outbreaks (YSO) and severity. We fit single‐season occupancy models to detection/non‐detection data collected for each species to examine a variety of plausible relationships between use of a given stand and YSO, severity, or both. Ungulates exhibited a positive association with bark beetle activity, although the nature of these associations varied by species. Elk (Cervus canadensis) were positively associated with severity, but not YSO; mule deer (Odocoileus hemionus) exhibited the opposite relationship. Moose (Alces alces) responded in a quadratic fashion; use of forest stands adjacent to preferred willow habitat peaked 3–7 yr after an outbreak commenced, but only at high severity. Similarly, yellow‐bellied marmot use of impacted stands adjacent to rock outcroppings followed a quadratic trend, but only at high severity. Red squirrel (Tamiasciurus hudsonicus) use declined in severely impacted stands, likely as a response to diminished cone crops. Golden‐mantled ground squirrels (Callospermophilus lateralis) and chipmunks (Neotamias spp.) exhibited a shallow negative relationship with YSO, as did coyotes (Canis latrans). Contrary to our hypotheses, black bears (Ursus americanus), American marten (Martes americana), snowshoe hares (Lepus americanus), and porcupines (Erethizon dorsatum) did not appear to be substantially influenced by beetle activity. Red fox (Vulpes vulpes) use was positively associated with YSO, but overall use declined as severity increased. Note that changes in probability of use described here could reflect changes in abundance, home range size, habitat use, or some combination, and in several cases, there was considerable uncertainty across competing models.