The Comparative Effects of Large Carnivores on the Acquisition of Carrion by Scavengers

1. What they did

Large carnivores can shape which other animals get to eat carrion. Pumas regularly kill deer and so provide carrion, while black bears rarely kill adult deer but are strong scavengers that often take over puma kills. The researchers asked how each of these two large carnivores affects other scavengers’ access to deer carcasses in Mendocino National Forest, California.

• Puma kills. They fitted seven pumas with Global Positioning System (GPS) satellite collars, visited clusters of locations where a puma had spent at least 2 hours within 150 m, confirmed the puma was still nearby with very high frequency (VHF) radio telemetry, then wired the carcass in place and set a motion-triggered video camera. They filmed 58 puma kills of black-tailed deer.
• Matched control carcasses. For each kill, they placed a road-killed deer with the same amount of meat and stage of decay at a nearby site matched for habitat, with its own camera (58 controls).
• Black bear experiment. They placed 100 road-killed deer in a variety of habitats with cameras, and compared carcasses where black bears were “present” (first or second scavenger to arrive, 33 carcasses) with those where they were “absent” (never came, or came late and stayed 10 minutes or less, 46 carcasses). Twenty-one carcasses were dropped from the analysis.
• Measuring access. For each scavenger species and size class (large carnivores, mesocarnivores, small carnivores and birds), they measured presence (share of carcasses visited), total feeding time, and mean feeding-bout length. They used shorter bouts as a sign that a scavenger felt more at risk.

2. What they found

Puma kills

• Pumas stayed within 150 m of their kills for an average of 75.1 hours. Cameras recorded 10,775 videos at puma kills and 9,663 at controls.
• Pumas did not limit large carnivores as a group, but domestic dogs were at fewer puma kills (1.7% versus 13.8%), and coyotes marginally so.
• Mesocarnivores fed far less at puma kills: total feeding time 58.8 minutes versus 135.1 at controls, and shorter feeding bouts. Bobcats, fishers and gray foxes all fed for less time, and gray foxes had shorter bouts.
• Birds also fed for less time at puma kills (50.4 versus 152.9 minutes) and in shorter bouts.
• Small carnivores did better at puma kills: their total feeding time was 75% longer and their bouts longer. Spotted skunks fed significantly longer, and ringtails were marginally more often present. A model comparison suggested that fewer mesocarnivores and birds, rather than the puma itself, best explained small carnivores being present more often.

Black bear carcasses

• Black bears found carcasses an average of 48.5 hours after they were placed, and fed for an average total of 105.9 minutes.
• Black bears limited other scavengers more strongly than pumas did. Mesocarnivores fed 44 times longer where bears were absent (269.1 versus 6.1 minutes) and were present at far fewer bear carcasses (27.3% versus 84.8%).
• Large carnivores were present at 18.2% of bear carcasses versus 63.0% without bears. Small carnivores fed for less time, and ringtails and spotted skunks never came to carcasses where bears were present.
• Birds, common ravens and turkey vultures all fed for less time where bears were present. The one exception was that turkey vultures were present at more carcasses where bears were present (51.5% versus 21.7%).

Black bears and pumas

• Pumas scavenged at 15.2% of carcasses without bears, and at none where bears were present.
• After a black bear arrived at a puma kill, the puma did not return to 72.4% of its kills and returned only once to another 13.8%. Puma feeding time fell from 106.5 minutes before the bear arrived to 16.8 after.
• Taking over kills carried risk: in one case a puma killed a subadult black bear that had been feeding on its kill.

3. What they concluded

• Pumas provide carrion to scavengers across many levels of the food web, and are effective at keeping out smaller, subordinate competitors but not dominant ones. Most feeding by those subordinate scavengers probably happens when the puma is away or has left the kill.
• By holding back mesocarnivores and birds, pumas may set off a cascading pattern that gives small carnivores more access to carrion. This may be another explanation for cascades seen among carnivores.
• Black bears monopolize the carrion they find and are a major limit on the energy other scavengers can get from it. The authors suggest bears dominate because they can eat large amounts quickly, rather than by using their size to chase others away.
• Pumas, like cheetahs, abandon kills when a dominant scavenger arrives. This may force pumas to kill more often, so black bears may indirectly affect deer populations.
• The effects of large carnivores on scavengers depend on the traits, including body size, of both the carnivores and the scavengers, and competition at carcasses may lead to intraguild predation as well as mesocarnivore release.

6. Other technologies, skills or innovations

• Lotek 7000SAW combined ARGOS satellite GPS and radio telemetry collars, with locations downloaded every 3 days to find fresh kills.
• Motion-triggered video cameras with infrared flash: Bushnell ScoutCam at puma kills (60 seconds of video, 1 second pause) and Cuddeback IR at bear experiment carcasses (30 seconds of video, 60 second pause).
• Control carcasses matched to each kill site, in order, by primary habitat type, overhead tree species, canopy cover, secondary habitat type, distance to secondary habitat and elevation.
• Video clips available online, and the data in the Dryad Digital Repository.

7. Implications for conservation (from the paper)

• Because carrion is important in structuring ecological communities, anything that changes access to it is likely to affect scavenger populations and possibly how ecosystems function.
• The way pumas distribute the energy in carrion may act as a keystone effect that supports biodiversity, so changes in puma numbers may have unexpected and important effects on communities.
• Black bears may affect ungulate populations indirectly through their effects on top predators.