Competition and coexistence in sympatric bobcats and pumas

1. What they did

Bobcats and pumas share much of western North America, and pumas are known to kill bobcats. Pumas are three to six times larger, so on body size alone the two cats should not compete much, yet both are known to eat prey ranging from small rodents to deer. The author asked whether pumas affect where bobcats are active, how their diets compare, and whether interference competition (one predator harming or killing another) shapes their relationship.

The study took place in and near the Huachuca Mountains of south-eastern Arizona, an area rich in prey and with 16 species of carnivores.

• Space use. Bobcat and puma tracks and scats were recorded on 27 transects along dirt roads and trails at Fort Huachuca and on loops along the San Pedro River, split by habitat: grassland, desert scrub, riparian forest and oak and pine woodland. Transects were 0.9 to 9.2 km long, and each was surveyed at least three times, at least 2 months apart, between October 1998 and January 2000. Sign was identified by comparison with published field guides and located with a Global Positioning System (GPS) receiver and a topographic map. Activity was measured as track sets per km.
• Diet. Felid scats were collected on these surveys and during other field studies from 1997 to 2002, identified by odour, size and shape, then washed through sieves. Hair, claws, teeth and bone fragments were identified against reference collections.
• Analysis. The author checked that enough scats had been collected, then compared the two cats’ diets: what they ate, how varied the diet was (niche breadth), the average weight of prey, and how much the diets overlapped.

2. What they found

Space use

• The 27 transects were surveyed 90 times. Sign of bobcats or pumas was found on all but one transect: bobcats only on 7, pumas only on 1, and both on 18 (67%). Both cats left sign on 30 of the 90 surveys (33%), in every habitat type.
• Bobcat activity was similar across all four habitats. Pumas were more active in riparian forest and woodland than in grassland or scrub.
• Bobcat activity was the same whether or not puma sign was found on the same survey (0.28 track sets per km).

Diet

• 61 bobcat scats held 96 prey items from 17 taxa; 64 puma scats held 73 items from 14 taxa. Both samples were large enough to describe the diet.
• Bobcats ate mainly rodents (33% of items) and lagomorphs such as cottontails and jackrabbits (32%), then ungulates (16%), birds and reptiles (10%) and carnivores (8%). Eight taxa each made up more than 5% of items: cottontails, deer, small rodents, small birds, rock squirrels, pocket gophers, jackrabbits and woodrats. Identifiable juvenile coatis, cottontails and deer made up 7% of the diet, a minimum because juveniles could not always be recognized.
• Pumas ate mainly ungulates (69%) and carnivores (21%). Deer alone made up 55% of the diet, and only deer, coatis and peccaries each made up more than 5%. Remains of a bobcat were found in one puma scat.
• Bobcats had a broader diet than pumas (standardized niche breadth 0.39 against 0.13). Their average vertebrate prey weighed 652 g, against 13,736 g for pumas.
• The bobcat diet overlapped the puma diet by 20%, but the puma diet overlapped the bobcat diet by 56%. The combined overlap was 36%.

3. What they concluded

• Bobcats showed little sign of avoiding pumas in space. They may avoid pumas at a finer scale or in time, which this study did not measure. With no data on prey availability, the author could not tell whether the habitat differences reflected preferred prey or avoidance.
• As expected, bobcats ate mostly rabbits and rodents and pumas mostly deer, consistent with earlier studies.
• Because pumas have a narrower diet that overlaps strongly with the bobcat diet, pumas may be more vulnerable than bobcats to changes in prey numbers, for example during drought. Bobcats eat many fast-breeding prey species.
• Bobcats can kill deer, and deer make up a large share of bobcat diets where pumas are absent or rare. Bobcats may kill fewer deer where pumas are present, to reduce the risk of being killed at a carcass.
• Pumas killed and ate bobcats, so interference competition may work through intraguild predation (one predator killing a competitor). The author hypothesizes that bobcats may adjust their foraging or diet to reduce this risk, but could not rule out that their diet and habitat use simply reflected the prey available.
• More studies in other habitats and conditions, including more competitors and prey availability, are needed.

4. How tracking was or was not used

Tracking was central to the study. Space use rested entirely on sign surveys: bobcat and puma tracks and scats were found along transects, identified with published field guides, and counted as track sets per km to compare activity between habitats and between surveys with and without puma sign. The diet data came from felid scats identified in the field by odour, size and shape. No trailing was reported. The acknowledgements refer to this work as “the tracking study.”

5. Tracker or observer certification

Not reported. The paper does not say who identified the tracks and scats or how skilled they were, beyond comparison with published field guides (Murie 1975; Aranda Sanchez 1981; Rezendes 1999). M. Tuegel is thanked for field assistance and H. Shaw for collecting some puma scats. This matters because both the activity index and the diet comparison depend on telling bobcat and puma sign apart; scat identification was not confirmed with DNA.

6. Other technologies, skills or innovations

• Sign located with a GPS receiver and a United States Geological Survey (USGS) topographic map.
• Scat contents identified with microscopes, reference collections and a hair-mounting technique (Carter and Dilworth 1971).
• Resampled diversity curves (Brillouin index) to test whether enough scats had been collected.

7. Implications for conservation (from the paper)

• Pumas, with their narrow, deer-based diet, may be more vulnerable than bobcats to changes in prey numbers such as those caused by drought.
• Larger predators can shape the diets and behaviour of smaller ones by killing or intimidating them, effects that may extend to other levels of the food chain.