National Wildlife Research Center (NWRC)

PROJECT GOAL: Improve current knowledge of predator ecology, physiology, and behavior relative to depredations on species of human concern, and assess predator responses to management practices, and develop control aproaches that effectively target alpha coyotes.

Project Accomplishments 2007

Interactions Among Wolves, Coyotes, and Pronghorn Antelope:  Does Wolf Recovery Help Pronghorns?—High coyote predation rates on pronghorn fawns are common throughout the western United States.  Scientists at the NWRC Logan, UT, field station used a natural experiment created by wolf recolonization in the southern Greater Yellowstone Ecosystem to evaluate whether wolf recovery would decrease the abundance of coyotes and subsequently increase pronghorn fawn survival due to reduced coyote predation.

Results from a 3-year study involving spatial contrasts of wolf densities, coyote densities, and pronghorn fawn survival at control (wolf-free) and experimental (wolf-abundant) sites provided strong evidence of wolves’ decreasing coyote numbers and increasing fawn survival.  The scientists documented a more than fivefold increase in pronghorn fawn survival at sites used by wolves during summer and a nearly sixfold increase in fawn survival at sites used by wolves year ’round.  Results indicated a negative relationship between coyote and wolf densities, suggesting that interspecific competition facilitated the increase in observed fawn survival.  Whereas densities of resident coyotes were similar between control and experimental sites, the abundance of transient coyotes was markedly lower in areas used by wolves.  Thus, differential effects of wolves on solitary coyotes may be an important mechanism by which wolves limit coyote populations.

These results suggest that widespread extirpation of wolves may contribute to high rates of coyote predation on pronghorn fawns.  The results also support a growing body of evidence demonstrating the importance of top carnivores’ influencing smaller predators and their prey in structuring the dynamics of terrestrial systems.

Effects of Coyote Population Reduction on Swift-Fox Demographics in Southeastern Colorado—The distribution and abundance of swift foxes have declined from historic levels.  Causes for the decline include habitat loss and fragmentation, incidental poisoning, changing land-use practices, trapping, and predation by other carnivores.  Coyotes overlap the geographic distribution of swift foxes, compete for similar resources, and are a significant source of mortality for swift-fox populations.  Current swift-fox conservation and management plans to bolster declining or recovering fox populations may include coyote population reduction to decrease predation.  However, the role of coyote predation in swift-fox population dynamics is not well understood.

To better understand the interactions of swift foxes and coyotes, scientists at the NWRC Logan, UT, field station compared swift-fox population demographics (survival rates, dispersal rates, reproduction, density) between areas with and without coyote population reduction.  On the Piñon Canyon Maneuver Site in Colorado, NWRC scientists monitored 141 swift foxes for 65,226 radio-days from 1998 to 2000 with 18,035 total telemetry locations collected.  In areas where coyotes were removed, the juvenile swift-fox survival rate increased and survival was temporarily prolonged.  Survival patterns for adult foxes also were altered by coyote removal, but only following late-summer coyote removals, and again only temporarily.  Coyote predation remained the main cause of juvenile and adult swift-fox mortality in both areas.

The increase in juvenile fox survival in the coyote removal area resulted in a compensatory increase in the juvenile dispersal rate and an earlier pulse in dispersal movements.  The adult-fox  dispersal rate was more consistent throughout the year in the coyote removal area.  Coyote removal did not influence the reproductive parameters of the swift foxes.  Even though juvenile survival increased, swift-fox density remained similar between the areas due to the compensatory dispersal rate among juvenile foxes.  NWRC scientists concluded that the swift-fox population in the area was saturated because all suitable habitat was being used by foxes.

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Logan, UT, Field Station