Predators Under Nightlights

This is a guest post by Dr. Mark Ditmer.

Streetlights like these have meant that for some animals, hiding in the dark is now increasingly difficult (Image Credit: ME Stoner, CC BY 2.0, Image Cropped)

Artificial nightlight alters the predator–prey dynamics of an apex carnivore (2020) Ditmer et al., Ecography,

The Crux

The earth is no longer dark at night – artificial lighting has degraded the dark nighttime conditions that many species have evolved with throughout their evolutionary history. This change is only accelerating, with human expansion and intensity of radiance continuing to increase annually. We already know that elevated light levels can disrupt ecological processes like pollination or migration, as well as have a litany of negative effects on individual species, from physiological stress to predation risk. But it’s hard to get an idea of how the increase in ‘light pollution’ affects free-roaming wildlife, especially large mammals, and especially at scales relevant for making conservation policy.

In areas like the American west, the rapid growth of urban areas and the accompanying spread of light pollution create a rapidly changing ecosystem, one that sees many conflicts between humans and wildlife. One particularly species of particular interest is the mule deer (Odocoileus hemionus), which seeks out sources of forage on the edges of and within towns and cities (e.g. parks, farms), especially in arid regions. The primary predator of mule deer – the cougar (Puma concolor) – also navigates and hunts near human development where their prey congregate, but tend to avoid human presence more so than deer.

Today’s authors wanted to assess how artificial lighting, both where it occurs and its intensity, can shape the behaviors and predator-prey interactions of these species across the American West ranging from the edges of bright urban regions, such as Salt Lake City (Utah) and Reno (Nevada), to areas receiving minimal light pollution like Grand Canyon National Park.

What They Did

The authors used a massive dataset that included GPS-locations from 263 mule deer, 56 cougars, and 1,562 locations where cougars successfully killed mule deer. The resulting location data were combined with estimates of anthropogenic light pollution (more on this in Did You Know?).

Several different analyses were performed on the combined light and GPS-location data, along with other variables representing environmental (e.g., snow cover, land cover, terrain) and human factors (e.g., distance to roads, housing density). The aim was to figure out whether A) light has any influence on the behavior of each species, B) cougars avoid areas with high light pollution, allowing deer to forage freely wherever and whenever they want (the ‘predator shield hypothesis’), or C) cougars exploit the higher densities of deer seeking forage around areas with elevated light pollution (e.g., parks, golf courses, agriculture; the ‘ecological trap hypothesis’).

Did You Know: A Space Agency’s Ecological Impact

In this study we used remote sensing data to determine the amount of light pollution in a given environment. Yet the sensors only pick up the total amount of light, and can’t tell us what is a product of our activity and what is a natural source of light. To separate the two, we used light data which was recently developed by the U.S. National Aeronautics and Space Administration (NASA). This dataset removes the contributions of natural sources of light (e.g., moonlight, fire, atmospheric spray) from our data and results in values of just the human-created nighttime light emissions.

What They Found

The behaviors of both species changed greatly with levels of light pollution, as did the predation risk for deer. The behaviour changed across different scales as well. Cougars killed deer in study sites with the high amounts of light pollution, but within those sites (e.g., edge of Salt Lake City, Utah) cougars selected to hunt and kill in the relatively darkest locations. In contrast, in the darker study areas, cougars killed deer in areas with the relatively more light pollution than the surrounding area. However, even though cougars killed deer in the darkest spots within the bright urban interface, those locations generally had much higher levels of light pollution than the brightest kill sites in the low light pollution study areas.

The study found that cougars made kills in bright regions, but generally only within the darker parts of those regions (Image Credit: ME Stoner, CC BY 2.0)

Deer living in brighter urban areas tended to forage at night, potentially to avoid direct human interactions. This shift might have benefited deer by avoiding humans, but as they sought out more natural and dark locations in these areas, cougars would wait in ambush.

In the end, the authors concluded that their findings fell in a gray zone between the predator shield and ecological trap hypotheses dependent on scale. Areas with high levels of light and subsequent human activities provide excellent foraging opportunities for ungulates (as this study measured as well), but adaptable predators can follow and take advantage – at least in environments that they feel are safe enough.


This is an observational study, so it’s hard to fully tease apart what effects are driven by light and what are driven by other human factors. We did our best to account for the other more traditional sources of the human footprint, reporting effect sizes for each, but there’s always a chance we’re attributing some effects to light pollution that could be caused by some other aspect of our presence.

So What?

Work like this shines a light on (pun intended) how different species will respond to the ongoing urbanization trends humans are driving in much of the planet.

Although many wildlife ecology studies consider various human alterations to habitats and the consequent changes in animal behavior, most studies fail to consider the sensory environment and the pollutants (e.g., noise, light) that can impact wildlife populations in their analyses. How wildlife use an ecosystem can impact everything from human-wildlife interactions to pulses of nutrients to the soil based on shifting areas of kill sites/carcasses.

Dr. Mark Ditmer is a post-doctoral researcher at the Centre for Human-Carnivore Co-Existence and Wittemyer Lab at Colorado State University. You can read more about him at his profile here or follow him on Twitter @MDitmer.

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