Nowhere to hide: pumas, black bears, and competition refuges

1. What they did

Pumas are top predators, but where they live alongside American black bears, bears are the dominant competitors and often take over puma kills. This is called kleptoparasitism (food stealing). One idea for how competing carnivores coexist is that the weaker species uses “competition refuges”: places or times where they are less likely to meet the stronger one. The researchers asked how black bears affect the way pumas hunt and eat.

They worked in two study areas in the western United States: western Colorado and the Mendocino National Forest in northern California. They captured 18 pumas (11 in Colorado, 7 in California) and fitted them with Global Positioning System (GPS) collars that took a location every 2 hours. Field teams visited clusters of locations to find and identify kills, and placed video cameras at kills where pumas were still feeding. Bear presence at kills was recorded from video footage, footprints, scats and other signs.

Each study area was split into a “bear season” (when bears were active and found at puma kills) and a “no-bear season”. The researchers then tested three questions:

• Season and site. Did prey size, handling time (time spent at a kill), search time (time between kills) and kill rates differ between seasons and study areas?
• Why pumas leave meat behind. Was the amount of time pumas spent at large kills (40 kg or more) in the bear season better explained by optimal foraging (leaving when prey are easy to catch) or by bears taking the kill?
• Hiding places. Did pumas make kills in places where bears were slower to find them? They compared kills found by a bear within 48 hours with kills found after 96 hours or more, or never.

2. What they found

Kills and bears

• In Colorado, the teams investigated 1001 GPS clusters and documented 437 kills and 8 scavenging events. In California they investigated 598 of 609 clusters and identified 352 prey and 4 scavenging events.
• In the bear season, black bears were detected at 47.8% of puma kills of 40 kg or more in Colorado, and 77.2% in California.
• Cameras showed bears displacing pumas at 29 kills in each area. Pumas came back only once to 3 of those kills in Colorado (10%) and 8 in California (28%).
• Once in California, an adult female puma returned and killed a 3-year-old female bear that had pushed her off her kill.

Seasons

• In the bear season, pumas killed smaller prey, spent less time at each kill (43.2 hours on average, against 89.1 hours in the no-bear season) and searched for less time between kills (69.4 hours against 98.7 hours).
• Kill rates were higher in the bear season: 1.48 against 0.84 animals per week, and 1.14 against 0.77 ungulates (hoofed animals) per week. This is 77% and 48% more. These rates did not differ between the two study areas.
• Measured in kilograms per day, kill rates were the same in both seasons, but higher in Colorado (9.0 kg per day) than in California (6.0 kg per day).

Why pumas leave meat behind

• The best model explained time at a kill by prey size, the search time before the kill, and whether a bear was present. Pumas stayed longer at bigger prey and after longer searches, and left sooner when a bear came.
• After kills where a bear was present, pumas made their next kill sooner.
• This pattern does not fit optimal foraging. It fits bears driving pumas off their kills.

No hiding places

• None of the landscape features tested (elevation, slope, aspect, ruggedness, distance to the nearest river or habitat type) predicted how quickly a bear found a puma kill. Kills found quickly and slowly were often close together. The chance of a bear finding a kill was the same across the landscape.

3. What they concluded

• Black bears strongly affect how pumas feed. Pumas did not use spatial competition refuges to avoid bears. Instead, they appear to make up for lost meat by killing more often.
• The only relief pumas had from bears in both areas was a time refuge: winter, when bears hibernate.
• Higher summer kill rates are not caused by bears alone; prey such as newborn ungulates are more available then, and carcasses spoil faster in the heat. Pumas may also choose smaller prey in summer, which they can eat quickly before bears arrive.
• Because other scavengers also feed at puma kills, the effects of food stealing on pumas are likely underestimated.
• A dominant competitor may indirectly affect prey populations by making a top predator kill more. Predation can only be understood within a community framework, and future studies should separate the effects of predators and competitors on prey.

4. How tracking was or was not used

No trailing was reported. Tracks and signs identification was central to the field work at GPS clusters:

• Prey identification: teams searched each cluster for prey remains, including hair, skin, rumen and bone fragments.
• Kill or scavenge: the state of the remains, the presence and position of bite marks (throat, nose or back of the head indicating a kill) and which body parts had been eaten were used to decide whether the puma had killed the animal or was scavenging.
• Bear presence: bears at puma kills were detected from video footage, footprints, scats and other signs. These detections underpin all the main results.

5. Tracker or observer certification

93% of all field investigations of clusters were carried out by CyberTracker certified observers (citing Elbroch et al. 2011), which the authors say ensured consistent effort and expertise across both study areas. The level of certification is not given. Several people are thanked for their help with kill site investigations.

6. Other technologies, skills or innovations

• GPS collars (Lotek 7000SAW with Argos uplink, or Lotek Iridium 2D) taking locations every 2 hours.
• GPS cluster analysis to find kill sites, with locations loaded onto handheld GPS units to find clusters in the field.
• Motion-triggered video cameras (Bushnell Trophy Cam) at active kills, which recorded bears displacing pumas.
• Bear and no-bear seasons defined from the field data (the earliest and latest dates bears were found at kills), rather than calendar seasons.

7. Implications for conservation (from the paper)

• Kleptoparasites may indirectly affect prey populations by raising the kill rates of top predators. This may matter especially for rare prey in systems with many prey species.
• Puma kills stolen by bears may in turn feed bear populations, and bears also prey on deer fawns. Whether this increases or buffers predation on ungulates is still an open question.
• Predation studies should look at the whole community: predators, prey, carrion, scavengers and competitors together.