Pumas as Ecosystem Engineers: Ungulate Carcasses Support Beetle Assemblages in the Greater Yellowstone Ecosystem

1. What they did

Ecosystem engineers are animals that create or change habitat for other species, as beavers do with their dams. Predators are rarely thought of this way. But pumas often leave large parts of their kills behind, and for many beetles a carcass is more than food: it is where they find mates, lay eggs and raise their young. Carrion beetles need fresh carcasses in spring and summer, just when natural deaths are few. The authors asked whether pumas, by leaving carcasses on the landscape, act as ecosystem engineers for beetles.

The work was part of a long-term puma study in the southern Greater Yellowstone Ecosystem, Wyoming, United States:

• Finding kills. Pumas were caught with trailing hounds in winter and fitted with Global Positioning System (GPS) collars that took a location every 2 hours. The team visited places where a puma’s locations clustered, once the puma had left. At remains, they looked at signs of struggle, blood patterns, bite marks on the carcass and which body parts had been eaten, to decide whether the puma had killed the animal or was scavenging.
• Sampling beetles (May to October 2016). At 18 fresh puma kills (elk, deer and one bighorn sheep), they set three barrier pitfall traps around the carcass and the same set-up at a control site 20 m away in similar vegetation. They emptied the traps weekly, and beetles were identified to species at Montana State University.
• Measuring what pumas provide. CyberTracker-certified researchers investigated the kill sites and estimated the age of the prey. From prey weights, time the pumas spent at each kill and feeding rates of captive pumas, the team estimated how much meat each puma ate and left behind, and calculated each puma’s kill rate and weekly carrion production.
• Comparing carcasses and controls. They compared beetle numbers, number of species and diversity between carcasses and controls over time, and tested whether caching by the puma or the prey species (elk or deer) made a difference.

2. What they found

Pumas and carrion

• The team investigated 501 GPS clusters of six pumas and found 190 kill sites, 261 bed sites, 21 carcasses the pumas had scavenged rather than killed, and 29 sites with no clear evidence of behaviour.
• On average each puma killed 1.4 animals and produced 37.9 kg of carrion a week, with individuals ranging from 24.7 kg to 57.5 kg a week.
• Pumas cached (covered with debris) 11 of the 18 carcasses sampled for beetles.
• Beetle sampling started on average 15 days after a kill and lasted an average of 12 weeks.

Beetles

• 24,209 beetles of 215 species from 33 families were caught; 83% were at carcasses.
• The northern carrion beetle made up about 60% of all beetles, and 98% of them were caught at carcasses.
• Eight families, with 113 species between them, were caught significantly more at carcasses than at controls. Most are known to use carcasses for courtship, mating, egg-laying, shelter or food for their young. Beetles of the family Curculionidae were a surprise, as they mostly eat plants; the authors suggest they came for the half-digested plants in the prey’s stomachs.

Carcasses versus controls (12 carcasses analysed)

• Six carcasses were left out of the models: five sampled late in the season, when cold sharply reduced beetles, and one taken over by coyotes after the second collection.
• In the first 8 weeks, carcasses had a large to very large effect on beetle numbers, number of species and diversity. Over the full 19 weeks the effect was medium.
• Beetle numbers and species were highest in the first collection week and then declined. The authors think they may have missed the first wave of beetles, because sampling began only after the puma left, and that scavenging mammals may have kept beetles away early on.
• Diversity was usually higher at controls, because the many northern carrion beetles at carcasses made the catch uneven.
• Caching made no difference to beetle numbers or species. Beetle numbers fell faster at deer carcasses than at the larger elk carcasses.

3. What they concluded

• Puma kills had a large to very large effect on beetle communities, which is strong evidence that pumas act as ecosystem engineers, creating habitat for beetles that depend on carrion.
• Carrion is an important but short-lived resource; the effects were strongest in the first 8 weeks and at the larger elk carcasses.
• By killing about 1.4 animals a week, pumas provide a steady supply of fresh carrion across the landscape in spring and summer, during key life stages for beetles and other species.
• Research on carrion left by predators has mostly looked at vertebrate scavengers. Understanding how predators shape the resources invertebrates depend on could help predict the knock-on effects of removing predators.

4. How tracking was or was not used

Tracks and signs were used to interpret what had happened at the puma GPS clusters. At each site with prey remains, the team read signs of struggle, blood patterns, bite marks on the carcass and which body parts had been eaten to tell a puma kill from scavenging, and recorded whether the puma had cached the carcass. They also told kill sites from bed sites. The prey’s age was judged from the remains. Pumas were caught for collaring with trailing hounds, which followed the pumas until they climbed a tree.

5. Tracker or observer certification

The authors used CyberTracker-certified researchers to investigate kill sites and estimate the approximate age of prey, citing Elbroch et al. (2011). The level of certification and which team members held it are not given. Co-author L. Mark Elbroch and Joshua M. Barry did the field work, with field help from others named in the acknowledgements.

6. Other technologies, skills or innovations

• GPS collars (Lotek Globalstar S or Iridium M; Vectronics Globalstar GPS Plus) uploading locations by satellite 1 to 6 times a day; locations shown in ArcGIS 10.0 and loaded onto handheld GPS units to find clusters in the field.
• Barrier pitfall traps: two plastic cups of odourless soapy water at each end of a 3-foot metal barrier, three per site, placed to leave corridors so some beetles could still reach the carcass.
• Beetle identification to species by an entomology laboratory (Marsh Laboratory, Montana State University).
• A study design that the authors say can be applied to predators anywhere.

7. Implications for conservation (from the paper)

• Where apex predators are few or have been wiped out, invertebrate communities that depend on carrion may suffer.
• Invertebrates are often overlooked but provide essential ecological services that people depend on.
• Predator-provided carrion may be more valuable to beetles than carrion from disease, starvation, road kill or hunters’ offal, because predators supply it all year and in many habitats, including in spring and summer when beetles need it.