Darkness as Refuge

Reprinted from Western Tanager 88(6):22-24.

The challenges faced by the Western Snowy Plover (Charadrius nivosus nivosus) are many as it winters and breeds along the coast of southern California.  Beach grooming by heavy equipment perpetually remove valuable habitat and flatten out once-undulating dunes.  Beachgoers and their pets use the land where they once nested.  Predators come from all directions, from falcons to roaming cats.

Volunteers with Los Angeles Audubon and other chapters have monitored wintering populations of Western Snowy Plover extensively for twelve years.  It is usual to think about what plovers do during the day, but what about at night?  The lights of the greater Los Angeles area reflect in the sky and brighten the beaches, not to mention the array of parking lot lights, floodlights, and other illumination along the shores of the Santa Monica Bay.

Researchers have found that light at night can make it easier for predators to find prey, so I wondered whether Western Snowy Plovers would pick their roost sites to avoid the brightest areas of the beach.  That was back in 2016, and eventually I secured funding from the federally funded Sea Grant program to investigate.  Through this program I was able to work with graduate student Levi Simons, who completed this work while he was finishing his Ph.D. in biology at USC and as a post-doctoral scholar at UCLA. 

The first thing we needed was a map of the typical level of light pollution on beaches.  Light at night is measured by satellites, but the pixels are much larger than plover roosts, and we did not know whether the light experienced on the beach correlated with the light seen from the air.

To measure light on the beach, we used a Sky Quality Camera and its software, which consists of a DLSR camera with a fisheye lens that can image the whole sky at once.  Levi then visited 515 locations on sandy beaches that covered Ventura, Los Angeles, and Orange counties, with an array of field assistants and took full-sky images during the new moon.  These measurements document the amount of light pollution without the addition of the moon.

We needed to create a map of exposure with these samples, so we used the satellite-derived data for light pollution, along with a measurement of how much of the sky is visible from any given point along the beach.  The idea here is that most of the light comes from reflected and scattered sources in the sky, and it will be darker in places where the sky is less visible because it is blocked by a bluff, dune, or mountains.  With these two pieces, we created and published a map of beach-level exposure, measured in units that are understandable to lighting professionals and regulators (Simons et al. 2020).

Then we needed the locations of Western Snowy Plover roosts and were able to obtain them thanks to the many Audubon volunteers who have participated in plover surveys, and from the US Navy for Point Mugu.  To understand the role of light pollution, we used a computer model to account for the elements of suitable habitat already known from previous research (distance above sea level, distance to freshwater, distance to saltwater, slope, land use behind beach, and beach width) and then tested to see if light pollution contributed anything more to understanding where plover roosts were located.  Light at night turned out to be the second most important factor influencing roost location, right after distance to freshwater, and right before beach width (Simons et al. 2022). 

The models showed that the chances of finding a Western Snowy Plover roost went down dramatically when the light pollution level increased beyond 0.05 lux.  I have measured the light from a full moon in Los Angeles at 0.2 lux.  The light from a half moon is around 0.02–0.03 lux.  We concluded that those areas that start with more than a half moon’s worth of light pollution are less valuable as roost sites, which is consistent with my original hypothesis that for a small bird on an open beach, darkness is, in fact, a refuge.

Our results were consistent with an anecdotal report from the Ventura County Shorebird Conservancy at Ormond Beach.  They found Western Snowy Plover nests between 2000 and 2010 on either side of a power generating station there, but none along a 1-km stretch directly in front of the power plant.  The Conservancy contacted the power plant staff and requested that they reduce the lighting, which they did.  Then, in 2012 and 2013, ten nests were found in front of the generating station (California Sea Grant Extension Program 2022).

We have published our results, along with an evaluation of light pollution on grunion runs.  For grunion, the probability of a substantial spawning event on a beach increases up to 0.1 lux and then declines with more light.  These results have also been shared with the regulators who protect both California’s coastal resources and who are responsible for the Western Snowy Plover.  By providing thresholds at which the effects of light pollution can be observed, regulators should be able to provide guidance for beach managers and coastal development to reduce these impacts and better share the shore for all twenty-four hours of the day.

References

California Sea Grant Extension Program. 2022. Explore Beaches: Artificial Lighting. University of California, Santa Barbara, Santa Barbara, California.

Simons, A. L., K. L. Martin, and T. Longcore. 2022. Determining the effects of artificial light at night on the distributions of western snowy plovers (Charadrius nivosus nivosus) and California grunion (Leuresthes tenuis) in Southern California. Journal of Coastal Research 38:302–309.

Simons, A. L., X. Yin, and T. Longcore. 2020. High correlation but high scale-dependent variance between satellite measured night lights and terrestrial exposure. Environmental Research Communications 2:021006.