Recolonizing wolves influence the realized niche of resident cougars

1. What they did

Wolves were reintroduced to Yellowstone National Park and central Idaho from 1995, after being absent since 1926. In the Southern Yellowstone Ecosystem, wolf numbers then rose from 15 to as many as 91 between 2002 and 2012, while a resident cougar population was already there. The researchers used this natural experiment to ask whether wolves, the dominant competitor, were changing how cougars live: what they hunt, where they hunt, how often they kill, and how they die.

• Capturing and collaring cougars. From 2001, cougars were caught in winter, when snow made it easier to find them, using hounds to tree or corner them. They were fitted with Very High Frequency (VHF) radio collars or Global Positioning System (GPS) collars with mortality sensors.
• Finding kills. The team searched sites where VHF-collared cougars had stayed put for 24 hours or more, and GPS clusters (groups of locations) where a cougar had stayed for 8 hours or more. They identified prey from hair, skin, rumen and bone fragments, used bite marks and feeding patterns to tell a kill from scavenging, aged prey by their teeth, and recorded the habitat.
• Investigating deaths. When a collar stopped moving, they visited the site as soon as possible and used signs of struggle and of other animals to work out the cause of death. Necropsies, usually led by a veterinarian, and laboratory disease tests added to this.
• Testing for change. They tested whether the share of elk, deer and elk calves in cougar diets, and the share of kills in forest, changed from 2002 to 2012, in summer and in winter. They then compared models explaining the changes with elk availability (elk per wolf), deer availability (deer per elk), snow depth and, for calves, elk cow-calf ratios.
• Kill rates. For GPS-collared cougars (2005 to 2012), they built models to predict which GPS clusters held kills, checked them against clusters visited in the field, and used them to estimate kills per week over time.

2. What they found

Prey and diet

• Between January 2002 and October 2012 they recorded 411 winter and 239 summer prey killed by 28 female and 10 male cougars, plus 37 prey of unmarked cougars. Only 29 prey were not ungulates.
• In summer, deer made up 42.4% of the diet and elk 38.3%. In winter, when deer migrate almost entirely out of the area, deer were only 7.2% and elk 74.4%.
• In summer, the share of deer in cougar diets rose and the share of elk fell over the study. In winter, there was no change.
• The best model for both changes was elk availability (elk per wolf). Both elk and deer were declining, deer faster than elk, while wolves increased steeply. By 2012, elk per wolf was about 14% of the 2002 value.

Elk calves and habitat

• The share of calves among elk killed rose in summer but did not change in winter. Cow-calf ratio was the strongest predictor, followed by snow.
• The share of summer kills found in forest rose over the study; in winter it did not change. On average 76% of summer kills and 58% of winter kills were in forest.

Kill rates

• From 2005 to 2012, 17 GPS-collared cougars formed 2729 clusters. The team visited 309 clusters from 14 cougars, an average of 8 days after the cluster formed, and found kills at 269.
• The best statistical model classified only 49.7% of known kill and non-kill clusters correctly. The simple rule “the cougar stayed more than one night” found kills at 86% of kill sites but also assigned kills to 40% of sites where none were found. The authors used this rule, as visits were often late and some kills were likely missed.
• Kill rates did not change over time in either season. Mean rates were 0.90 animals per week in summer and 0.99 in winter.

Cougar deaths (2002 to 2012)

• Kittens: 10 starved, 5 were killed by wolves, 5 died of unknown causes, 2 were killed by an unidentified predator, 2 by other cougars, 1 died of disease and 1 was killed by wildlife managers.
• Subadults and adults: 16 were legally hunted, 9 starved, 5 were killed by other cougars, 5 died of disease, 5 of unknown causes, 3 were killed by wildlife managers, 1 was killed illegally and 1 died of old age.
• All starvations, kittens and adults, happened after 2005. No cougars were hunted from 2007 to 2012.

3. What they concluded

• Cougars switched from elk toward deer in summer as wolves increased. Because both elk and deer were declining, changing prey numbers alone did not explain the switch; competition with wolves at least partly did.
• Elk numbers fell only from 14,956 to 11,982, far more than the low-density cougar population needs, so the authors did not see this as cougars running short of elk (exploitation competition). They favoured another explanation: cougars moved into more complex, forested habitat to avoid wolves, where they met more forest-dwelling mule deer. The rise in summer kills in forest supports this.
• Cougars took more elk calves in summer, not fewer as predicted. Cow-calf ratios stayed steady, probably helped by supplemental feeding of elk, so calves may not yet be scarce enough to limit cougars.
• No change in kill rates was found, but the authors think this reflects their methods (late site visits, the more-than-one-night rule missing small prey and kills lost to competitors, and mixed GPS fix rates) rather than what was really happening.
• Wolves killing cougar kittens is strong evidence of direct (interference) competition. The starvations may also partly reflect competition with wolves.
• Together with earlier evidence that cougars chose home ranges further from wolves, the results show new and ongoing competition. Resident cougars may have been “naïve intermediaries” relearning to live with a dominant competitor, and their realized niche may now resemble the one they had when wolves last shared Yellowstone.

4. How tracking was or was not used

The study rests on reading tracks and signs at sites found by telemetry. At kill sites, the team identified prey from remains (hair, skin, rumen and bone fragments) and used bite marks and feeding patterns to decide whether the cougar had killed the animal or was scavenging. They aged prey from tooth eruption and wear. At death sites, they used signs of struggle and signs of other animals to judge the cause of death and whether animals present had acted as predators or scavengers, citing a tracks and signs field guide (Elbroch 2003). Site searches where snowfall may have hidden evidence were left out of the kill-site model. Cougars were captured in winter, when snow made it easier to tell whether a cougar was present, and hounds were used to trail them; the paper does not describe people following tracks themselves.

5. Tracker or observer certification

Not reported. No qualification or skill test is given for the people who investigated kill and death sites, although the prey, kill-or-scavenge and cause-of-death findings all depend on their sign interpretation. The authors note that field crews often reached clusters many days late and likely missed evidence of some kills.

6. Other technologies, skills or innovations

• GPS collars (Telonics, Televilt, Vectronics) taking locations every 3 to 10 hours, mostly six to eight a day, and VHF collars (Telonics), with mortality sensors that signalled when a collar had not moved for 8 hours or more.
• A Python script (developed by Knopff and colleagues) to find GPS clusters: two or more locations within 100 m and 48 hours.
• Logistic regression models of cluster attributes (more than one night, hours at night, fidelity, distance to the next cluster) to predict kill sites, compared with the simple more-than-one-night rule.
• Necropsies and laboratory disease testing at the Wyoming State Veterinary Laboratory.

7. Implications for conservation (from the paper)

• Recolonizing wolves are reshaping how resident cougars hunt and survive, through both prey switching and direct killing of kittens.
• Species reintroductions offer rare chances to test how a dominant competitor affects a subordinate one, since carnivore numbers are hard to manipulate in the wild.
• More intensive fieldwork may be needed to detect changes in kill rates, since late site visits and the methods used can miss small prey and kills taken over by competitors.