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Study finds artificial light is drawing deer to urban areas — and cougars aren't far behind

By Graham Dudley, KSL.com | Posted - Nov. 13, 2020 at 12:44 p.m.



SALT LAKE CITY — A new study co-authored by Utah researchers found that artificial nighttime light in urban areas is drawing in mule deer that want to graze all night, but it's also bringing in the predators who hunt them.

Published last month in the journal Ecography, the study says cougars are able to successfully hunt within light-polluted areas by sticking to the shadows and dark spots to make their kills. It also found that deer in urban areas forage more often throughout the night than wildland deer, which usually stick to the dawn and dusk hours. This could create more traffic hazards for unsuspecting nearby drivers.

The study was headed by University of Michigan researchers and was co-authored by David Stoner and Terry Messmer of Utah State University, Randy Larson and Brock McMillan of BYU, Kent Hersey and Daniel Olson of the Utah Division of Wildlife Resources, and others.

According to a University of Michigan study summary, the data came from NASA-NOAA Suomi satellite data providing "detailed estimates of nighttime lighting sources," as well as from GPS location data on 486 mule deer and 117 cougars from four western states, including Utah. "In addition, wildlife agencies provided locations of 1,562 sites where cougars successfully killed mule deer," it says.

Stoner said it's important to remember that, for the animals involved, human lighting is "evolutionarily novel."

"Someone might say, 'Well, what's the problem for wildlife?'" Stoner said. "The thing is, throughout history the brightest night would be a full moon. ... That is the brightest nighttime light animals, or any life on earth, has ever experienced before 150 years ago when we started harnessing electricity."

He said there's been little research done about the effect of this light on wildlife. "It's still a relative unknown," Stoner said.

The Intermountain West, where the study was focused, is one of the fastest growing regions of the United States with increasing levels of artificial light. Stoner said the region will contend more frequently with wildlife in urban areas as it expands. "The attraction of the urban areas (to deer) is the food, there's food available, and it's predictable," he said.

But the study found light-polluted areas can create an "ecological trap" for deer — that is, it makes them easy to hunt by concentrating them in a particular area. Deer may perceive that the enhanced lighting makes them safe from predators, as most prefer to stay away from harsh light and human activity. In some bright areas, researchers found that to be the case. But in others, the cougars are simply lurking, and waiting, nearby.

"That's where the concerns are," Stoner said. "How do we deal with this problem that's one of our own making? We've created this lush, attractive environment, and the animals are drawn to it, and that has a cascading effect."

Both Stoner and Hersey of the DWR emphasized that cougar attacks on humans are exceedingly rare. Stoner said the "major problem" is deer and elk on highways, rather than cougar attacks. And while one might suspect that deer and cougar populations are increasing only in exurban areas, they said it's actually the Salt Lake City area that may see them the most.

"Urban deer has become a hotter topic," Hersey said. "As more and more development is occurring up further on the hillside, and expanding out, the number of calls we get dealing with nuisance animals ... have increased."

"The way cougars are hunted ... is really constrained by land use," Stoner added. "Several years ago, I mapped out the distribution of hunting pressure on cougars and looked at places where it was heavy, light and virtually nonexistent. And one of the places that showed up on my map was the Wasatch Front — primarily right behind Salt Lake City. ... The Wasatch Mountains behind Salt Lake have virtually no hunting pressure of cougars."

The DWR recommends that Utahns who come upon "aggressive" cougars:

  • Do not run
  • Make themselves look intimidating and make eye contact with the cougar
  • Pick up children, if present, so they don't panic and run
  • Fight back if attacked

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