Anthropogenic light pollution due to man-made artificial light sources such as streetlights, car headlights, businesses, and even homes is common throughout the US. Artificial light can cause disorientation and ineffective foraging and/or escape behavior for many animal species. Impact studies have been conducted on sea turtles, bats, and insects, but little research has been done to understand the effects of artificial light on nocturnal ground mammals. The nocturnal Stephen’s kangaroo rat has eyes adapted for sharp nighttime vision. Studies of other kangaroo rat species have suggested that bright moonlight may decrease foraging and activity rates due to increased visibility to predators such as owls or coyotes.
This summer, Alicia Bird, our Frabotta Summer Fellow in Applied Animal Ecology, examined the impact of anthropogenic light pollution on Stephen’s kangaroo rat foraging behavior. The kangaroo rats are endangered, and they only persist in a few remaining fragmented habitats throughout San Diego and Riverside counties. Their preferred habitat is open temperate grassland. As their habitats are encroached on by human development, it is important for us to learn how our activities and further development might affect this species’ survival.
Alicia set up multiple arrays on our research site at Lake Skinner, just outside of Temecula, California. This site has previously been home to several of our ‘roo rat studies, including translocations (see Roo Rats Released!). Each array contained a series of cardboard trays set in a row at increasing distances from an artificial light source. Trays were level with the ground and covered with dirt.
Over a period of about a week on a new moon night, Alicia placed 12.5 grams of seed on each tray and turned on the lights just before sunset. We used three light treatments: a low-watt bug light, a flood light, and a “no light” control. Alicia returned to the site just before sunrise to collect, sift, count, and/or weigh the remaining seeds. ‘Roo rats are granivores, or seed collectors, that use their cheek pouches to carry seeds to caches in their complex burrow systems.
Tracks and tail drags in the sand where used to assess their foraging presence. Cottontail rabbits were also found at this site; however, their presence did not affect the amount of seed foraged. The rabbits did have fun chewing on the wires of the lights, so Alicia had to bunny-proof them with wire mesh!
Significantly more seeds were collected from the control arrays compared to the bug and flood light arrays. There was little difference in the amount of seed foraged between the bug and flood light treatments. However, there was a difference in the distance at which each artificial light treatment impacted foraging activity. The bug light reduced foraging activity within 15 meters of the light source, whereas the floodlight affected foraging up to 35 meters away. Overall, this project suggests that kangaroo rats reduce their foraging levels under artificial night lighting.
Currently, there are no regulations on development near Stephen’s kangaroo rat habitats. They are often described as keystone species in grassland communities; therefore, the disturbance created by artificial night lighting may have far reaching influence over predator-prey relationships and species competition at the community level. Further study is needed to understand if these little mammals can habituate to artificial light and if it ultimately affects their survival in areas with ecological light pollution.
Christine Slocomb is a senior research technician at the San Diego Zoo Institute for Conservation Research. Read her previous post, Kangaroo Rats and Pocket Mice Burrow In