Trophic facilitation or limitation? Comparative effects of pumas and black bears on the scavenger community

Abstract

Scavenging is a widespread behaviour and an important process influencing food webs and ecological communities. Large carnivores facilitate the movement of energy across trophic levels through the scavenging and decomposition of their killed prey, but competition with large carnivores is also likely to constrain acquisition of carrion by scavengers. We used an experimental approach based on motion-triggered video cameras at black-tailed deer (Odocoileus hemionus columbianus) carcasses to measure the comparative influences of two large carnivores in the facilitation and limitation of carrion acquisition by scavengers. We found that pumas (Puma concolor) and black bears (Ursus americanus) had different effects on their ecological communities. Pumas, as a top-level predator, facilitated the consumption of carrion by scavengers, despite significantly reducing their observed sum feeding times (165.7 min ± 21.2 SE at puma kills 264.3 min ± 30.1 SE at control carcasses). In contrast, black bears, as the dominant scavenger in the system, limited consumption of carrion by scavengers as evidenced by the observed reduction of scavenger species richness recorded at carcasses where they were present (mean = 2.33 ± 0.28 SE), compared to where they were absent (mean = 3.28 ± 0.23 SE). Black bears also had large negative effects on scavenger sum feeding times (88.5 min ± 19.8 SE at carcasses where bears were present, 372.3 min ± 50.0 SE at carcasses where bears were absent). In addition, we found that pumas and black bears both increased the nestedness (a higher level of order among species present) of the scavenger community. Our results suggest that scavengers have species-specific adaptions to exploit carrion despite large carnivores, and that large carnivores influence the structure and composition of scavenger communities. The interactions between large carnivores and scavengers should be considered in future studies of food webs and ecological communities.

Figure 1. A map of the study area, in Mendocino National Forest.

The study area is outlined by the thick black line, within the greater context of the North Coast Range and California.

Figure 2. The detection rate of black bears among seasons at different carcass types.

Detection varied among seasons, and the detection for deer carcasses used for the black bear experiments, those killed by pumas, and the matching puma control carcasses are shown.

Figure 3. The seasonal distribution of scavenger species richness (number of scavengers present) and sum feeding time in minutes (total time scavengers spent feeding).

Each season is represented as a mean with the error bars representing standard error. We did not include samples on the graphs where there were <3 samples for a season.