You would never guess it from their soft eyes and timid demeanour, but the swift-​footed deer is North America’s most dan­ger­ous mam­mal to humans.

Each year deer cause 1.2 mil­lion vehi­cle col­li­sions in the U.S., trig­ger­ing more than 200 deaths, some 29,000 injuries and $1.66 bil­lion in costs asso­ci­ated with vehi­cle dam­age, med­ical bills and road cleanup. These stag­ger­ing fig­ures are in part because deer’s nat­ural preda­tors — large car­ni­vores such as wolves and cougars — have declined in pop­u­la­tion, leav­ing large ungu­lates like deer to repro­duce mostly unchecked.

A team includ­ing Uni­ver­sity of Washington’s Laura Prugh has for the first time begun to quan­tify the eco­nomic and social impact of bring­ing back large car­ni­vores. Using cougars (Puma con­color) and their value in reduc­ing deer-​vehicle col­li­sions as a case study, the researchers found that within 30 years of cougars recol­o­niz­ing the East­ern U.S., large cats could thin deer pop­u­la­tions and reduce vehi­cle col­li­sions by 22 per­cent — each year pre­vent­ing five human fatal­i­ties, 680 injuries and avoid­ing costs of $50 million.

The study is pub­lished online on 13 July (arti­cle accepted though final edit­ing to be done) in the jour­nal Con­ser­va­tion Let­ters. The student-​led project was ini­ti­ated dur­ing a com­mu­nity ecol­ogy class Prugh taught in 2014 at the Uni­ver­sity of Alaska Fairbanks.

“Car­ni­vores are so con­tro­ver­sial and there’s a lot of fear, anx­i­ety and resis­tance when they are rein­tro­duced or recol­o­nize an area. We are hop­ing that show­ing peo­ple how their lives could really ben­e­fit in a tan­gi­ble way from hav­ing large car­ni­vores around could help peo­ple become more accept­ing of liv­ing with them.”

Cougars con­ser­va­tion sta­tus
Cougars, also called moun­tain lions, pan­thers and pumas, used to live through­out most of the U.S. and Canada. State-​sponsored bounty hunts to pro­tect live­stock and humans from the cats led to their com­plete removal from the Mid­west and east­ern states by the early 20^th century.

Breed­ing pop­u­la­tions have since recol­o­nized their for­mer habi­tats in South Dakota, North Dakota and Nebraska, and indi­vid­ual males have made it to Con­necti­cut and, most recently, Ten­nessee. It is likely just a mat­ter of time before new breed­ing pop­u­la­tions pop up far­ther east, Prugh said.

In the mean­time, with­out as many preda­tors, the deer pop­u­la­tion has grown across the U.S., par­tic­u­larly in the east­ern states. While these ungu­lates pro­vide ample hunt­ing oppor­tu­ni­ties, an over­abun­dance of deer has led to more col­li­sions with vehi­cles as well as impacts on land­scapes and veg­e­ta­tion from deer’s her­bi­vore diet.

“I think every­one on the East Coast has either hit a deer or knows some­body who’s hit a deer, so it’s a very real prob­lem for peo­ple,” Prugh said. “An over­abun­dance of ungu­lates might be a hunter’s par­adise, but it comes with prob­lems as well.”

The researchers cal­cu­lated the cougars’ impact by com­par­ing white-​tailed deer pop­u­la­tion den­si­ties and the num­bers of deer killed by vehi­cles with and with­out cougar pre­da­tion. Their mod­els showed that cumu­la­tively over 30 years, 155 human deaths and more than 21,000 injuries could be pre­vented by the pres­ence of cougars in 19 east­ern states. A sin­gle cougar would kill 259 deer over its aver­age six-​year lifes­pan, pre­vent­ing eight col­li­sions and sav­ing nearly $40,000 in asso­ci­ated costs.

The researchers were con­ser­v­a­tive in their esti­mates of the ben­e­fits cougars could bring. They assumed that three out of four deer killed by cougars would have died from other causes with­out cougars present, thereby reduc­ing the impact of cougars on deer mor­tal­ity by 75 per­cent. They also lim­ited the pos­si­ble range for cougars to large forested areas, although cougars could prob­a­bly live in rural and sub­ur­ban areas as they do in west­ern states. Finally, the researchers assumed cougars would prey on deer at the same rate in west­ern and east­ern states. In real­ity, cougars would likely hunt deer at a higher rate in east­ern states, because out west they have more prey options such as elk.

A cougar stands over its prey.
Image credit Brian Kertson/​Washington Depart­ment of Fish and Wildlife.

“The mod­el­ling frame­work and assump­tions we made stacked the cards against the cougars being able to reduce the deer pop­u­la­tion. We didn’t have any expec­ta­tions that cougar pre­da­tion would be enough to drive the deer down, and yet it did,” Prugh said. “Cougars are deer spe­cial­ists and they tar­get adults. With a long-​lived species like deer, remov­ing adults in prime breed­ing age can really have an impact on pop­u­la­tion growth.”

The researchers were able to com­pare their mod­elled results with an actual exam­ple in South Dakota, where a viable cougar pop­u­la­tion lives in the Black Hills. The data clearly showed that after cougars repop­u­lated the region in the 1990s, deer-​vehicle col­li­sion rates markedly dropped. This real-​life test case was strong evi­dence of a trend that could hap­pen else­where, Prugh said.

The authors acknowl­edge that re-​establishing cougars across the U.S. has its costs. Attacks on humans, pets and live­stock could become more com­mon, though their esti­mates show that cougars would actu­ally save five times the num­ber of peo­ple they would kill by way of pre­vent­ing deer-​vehicle col­li­sions. The researchers next hope to com­plete full cost-​benefit analy­ses in smaller sub­sec­tions of the coun­try where cougars are present.

(Source: Uni­ver­sity of Wash­ing­ton news release, 14.07.2016)