Apparent stability masks underlying change in a mule deer herd with unmanaged chronic wasting disease

The contagious prion disease “chronic wasting disease” (CWD) infects mule deer (Odocoileus hemionus) and related species. Unchecked epidemics raise ecological, socioeconomic, and public health concerns. Prion infection shortens a deer’s lifespan, and when prevalence (proportion of adults infected) becomes sufficiently high CWD can affect herd dynamics. Understanding population responses over time is key to forecasting long-term impacts. Here we describe unexpected stability in prevalence and abundance in a mule deer herd where CWD has been left unmanaged. High apparent prevalence (~30%) since at least 2005 likely drove observed changes in the proportion and age distribution of wild-type native prion protein (PRNP) gene homozygotes among deer sampled. Predation by mountain lions (Puma concolor) may be helping keep CWD in check. Despite stable appearances, prion disease nonetheless impairs adult survival and likely resilience in this deer herd, limiting its potential for growth despite refuge from hunter harvest and favorable habitat and winter conditions.

A seasonal pulse of ungulate neonates influences space use by carnivores in a multi-predator, multi-prey system

The behavioral mechanisms by which predators encounter prey are poorly resolved. In particular, the extent to which predators engage in active search for prey versus incidentally encountering them is unknown. The distinction between search and incidental encounter influences prey population dynamics with active search exerting a stabilizing force on prey populations by alleviating predation pressure on low-density prey and increasing it for high26 density prey. Parturition of many large herbivores occurs during a short and predictable temporal window in which young are highly vulnerable to predation. Our study aims to determine how a suite of carnivores responds to the seasonal pulse of newborn ungulates using contemporaneous GPS locations of four species of predators and two species of prey. We used step-selection functions to assess whether coyotes, cougars, black bears, and bobcats actively searched for parturient females in a low-density population of mule deer and a high-density population of elk. We then assessed whether searching carnivores shifted their habitat use toward areas exhibiting a high probability of encountering neonates. None of the four carnivore species encountered parturient mule deer more often than expected by chance suggesting that predation of young resulted from incidental encounters. By contrast, we determined that cougar and male bear movements positioned them in proximity of parturient elk more often than expected by chance which is evidence of searching behavior. Although both male bears and cougars searched for neonates, only male bears used elk parturition habitat in a way that dynamically tracked the phenology of the elk birth pulse suggesting that maximizing encounters with juvenile elk was a motivation when selecting resources. Our results support the existence of a stabilizing mechanism to prey populations through active search behavior by predators because carnivores in our study searched for the high45 density prey species (elk) but ignored the low-density species (mule deer). We conclude that prey density must be high enough to warrant active search, and that there is high interspecific and intersexual variability in foraging strategies among large mammalian predators and their prey.

Conflict, coexistence, or both? Cougar habitat selection, prey composition, and mortality in a multiple-use landscape

Western North America is experiencing remarkable human population growth and land-use change. Irrigation and associated cultivation have led to colonization of urban-wildland interface (UWI) environments by mule deer (Odocoileus hemionus), and consequently, cougars (Puma concolor). In the wake of these changes, human-wildlife conflicts have increased in tandem with questions about long-term species conservation. To address these concerns, we fit 79 cougars with radio-telemetry collars in the Oquirrh Mountains near Salt Lake City, Utah (2002–2010). Our goal was to evaluate variation in cougar habitat selection, diet, and cause-specific mortality in a landscape dominated by urban, military, and industrial activities. We used radio-telemetry data in concert with Resource Selection Functions to address three hypotheses: (1) that cougars would select wildland over UWI land-uses; (2) prey composition would reflect differences in land-use; and (3) mortality would be predominantly human-caused. Cougars largely selected wildland habitats associated with seasonal mule deer presence, but contrary to expectation, they also selected habitats closer to urban and mined areas. Prey composition in the UWI did not differ from wildland habitats. Domestic ungulates represented only 2% of 540 recovered prey items and were found primarily in wildlands. Native ungulates comprised > 90% of the total kill, irrespective of season or land-use, suggesting that use of UWI habitats was linked to mule deer presence. Cougar mortality was disproportionately due to natural causes in wildlands, but individuals that died of human causes in UWI habitats were more likely to be inexperienced hunters, supporting young kittens, or compromised by physical handicaps. In general, presence of mule deer was the key predictor of cougar habitat use, even in this highly disturbed, anthropogenically altered landscape. As such, management designed to reduce conflict and ensure conservation will need to focus on urban deer, land-use planning, and targeted education campaigns to reduce food subsidies.

An innovative temporary escape ramp for deer and other wildlife

Concrete-lined water conveyance canals can be a significant source of mortality for ungulates and other wildlife, which can drown or become entrapped. Various types of wildlife escape structures have been deployed in canals with limited success. From 2011 to 2018, we used camera traps to monitor mule deer (Odocoileus hemionus) use of three different temporary wildlife escape structure designs with the goal of developing an effective escape structure for fawns. We monitored three to five locations at a hydroelectric water conveyance canal, operated by Pacific Gas and Electric Company in the foothills of Central California on the Sierra National Forest, in which trapped fawns had been detected previously during the maintenance period when the canal was dry. Mule deer activity and ramp use varied by year. During the monitoring period, deer were detected in the canal in all years except 2016 and 2017. Fawns and adults used the temporary escape structures to exit the canal in four of these years and 50% of mule deer detections showed ramp use overall. No deer were detected using the escape structures until jute netting and debris were added to the surface of the escape ramps in the third monitoring year. Prior to this modification, fawns were detected trapped in the canal investigating the ramps, but not using them to exit the canal. Deer may be more likely to utilize ramps covered with materials that mimic native ground cover. Seven other wildlife species were detected entering and exiting using the ramps with a general increase in use over years. Our temporary escape ramp design for small canals, when dry, appears to be novel and may be applicable in other areas. When compared to fencing or covering the canal, it is a relatively low-cost solution to reduce animal entrapment.