Light pollution alters mountain lion behavior: But not in the way you might think
Review of Ditmer, M. A., Stoner, D. C., Francis, C. D., Barber, J. R., Forester, J. D., Choate, D. M., Ironside, K. E., Longshore, K. M., Hersey, K. R., Larsen, R. T., McMillan, B. R., Olson, D. D., Andreasen, A. M., Beckmann, J. P., Holton, P. B., Messmer, T. A., & Carter, N. H. (2020). Artificial nightlight alters the predator–prey dynamics of an apex carnivore. Ecography, First published on October 18, 2020. https://doi.org/10.1111/ecog.05251 Full text available at: https://onlinelibrary.wiley.com/doi/10.1111/ecog.05251
Back in 1986, after a few years living in the mountains bouncing from job to job as an environmental educator, river guide, and ski area photographer, I decided I needed to move Denver to get a real job and a real life. I started taking some classes at a local community college and answered an ad for a roommate at a nearby condominium complex. My new roommate, Steve, was an outdoorsy Colorado native who was active on a local search and rescue (SAR) team. When a hiker went missing in the high country, Steve would disappear for a few days to help track them down. A few years after we parted ways, my old roommate would gain a bit of notoriety after he made a gruesome discovery on one of his missions.
"in the neighbourhood this morning..." by marneejill is licensed under CC BY-SA 2.0
Colorado has seen a lot of change in the last 30 years. Cities and towns across the state have expanded and encroached on areas that until very recently had been open forest. With a lack of natural predators, populations of deer, elk, and other wildlife have flourished. As a result, deer, elk and other large animals can regularly be seen in city parks, gardens, and open spaces where suburbs merge into the front range mountains. It’s a story that has played out across the western U.S. for the past century and a half. When I was living in Boulder in the 1990s, it was common to see deer walking through town. I remember leaving my house for work one morning and seeing a big doe with two of my neighbor’s tulips hanging out of her mouth. That was part of the charm of living in Boulder back then. It was a thrill to feel like a part of nature even as we enjoyed a suburban lifestyle. However, what we were really experiencing was a manifestation of an ecosystem that was seriously out of balance.
On January 14, 1991, Scott Lancaster, 18, left for a trail run on a rocky hillside above his Idaho Springs high school. He was never seen alive again. A few days later, Steve was leading a search team about a half mile from the high school when someone shouted “We found him!” When Steve reached the scene, he saw the body, half covered with debris from the forest floor. Scott Lancaster had become the first confirmed mountain lion fatality in Colorado history. Mountain lions often stay in the area to protect recent kills and continue feeding on the remains. This cat was no exception. A few minutes after the discovery of Scott’s body, a team member yelled “Behind you!” Steve turned and saw a medium sized mountain lion carefully watching the group. A long chase ended with a bullet to the cat’s chest, and a subsequent examination found human remains in its stomach.
Photo by davedehetre is licensed under CC BY 2.0
The account of Scott’s death was chronicled in a riveting book by David Baron called Beasts in the Garden. As Baron illustrates, solving the puzzle of Lancaster’s death requires an understanding of a complex network of social and ecological factors that developed over many decades. He weaves a tale involving ranching practices, predator control, encroaching suburbs, our love of nature and affinity for seeing deer in our neighborhoods that culminated in the death of a teenager on a mountain slope in Idaho Springs, Colorado.
A recent article by Mark Ditmer and a host of contributors, has added another piece to this complicated puzzle: light pollution. The study shows that artificial light can disrupt the natural relationship between mountain lions and mule deer, their primary prey in the intermountain west. This disruption could lead to more encounters with humans. The study highlighted the following findings.
Deer show a preference for areas with relatively high light levels
Mule deer that live in naturally dark wildland locations are most active around dawn and dusk. However, those living around artificial lighting forage throughout the day and are more active at night than wildland deer -- especially during the summer. Some of the best areas for foraging are located near developed areas. This is particularly true in the relatively arid western US where vegetation begins to dry out by mid-summer. By comparison, the vegetation in our yards, gardens, parks, and agricultural areas are lush, green, and nutritious. The study reveals that deer are associating lighting with these prime feeding areas. In regions with high light levels, deer were feeding in areas that were 23% greener [based on enhanced vegetation index (EVI) values] relative to areas with lower levels of light. In addition, lighting encourages deer to feed throughout the night by mimicking their favorite times to browse: dusk and dawn. Other studies (Shannon et al. 2014) indicate that deer may select areas with high light levels because they perceive these areas as having less predation risk.
The study also examined other variables associated with human development such as distance to the nearest road, road density, and housing density. Controlling for these variables provides strong evidence that lighting was a key factor driving the results rather than some other aspect of human settlement.
Lighting did not deter cougars from hunting deer
NPS Photo. For more images of mountain lions see Santa Monica Mountains NRA Puma Profiles webpage
It has always been assumed that mountain lions are naturally wary of humans and would typically avoid developed areas whenever possible. However, this and other recent studies have cast doubt on this notion. The data show that mountain lions are not deterred from hunting in well lit areas and demonstrate that animals living in the urban-interface will kill prey in locations with light levels that far exceed their wildland counterparts. However, they preferred to hunt from the darkest locations within these areas. It should be noted that some areas and times of day did have too much light and human activity for cougars.
Almost all species on earth have evolved under a predictable light-dark regime - a constant, unchanging pattern guided by the sun, the moon, and the seasons. Interactions among animals are in many cases choreographed by these cycles. This study documents how artificial light is disrupting relationships that have evolved over thousands or millions of years. Ditmer and his colleagues provide important insights into this altered relationship by demonstrating how light impacts the predation process. As the authors point out, our lights are creating changes to the environment that ripple across entire ecosystems.
The fate that befell Scott Lancaster on that winter day in 1991 is exceedingly rare. Since 1890, there have been 25 documented fatal mountain lion attacks in North America. However, encounters between humans and these amazing animals are increasing. Predators will hunt where prey is abundant and as mule deer continue to take up residence in our neighborhoods, mountain lions will inevitably follow. As this study demonstrates, lighting our towns, our parks, and our gardens will not only fail to prevent this inconvenient truth but may actually encourage it.