This post is based on an interesting paper that I've had in my blogging folder for a while now. The researchers (Berger & Gese, 2007) looked at the impact of interference competition between wolves and coyotes on the coyotes. The study was based in the Greater Yellowstone Ecological Area, & was possible because wolves were reintroduced into Yellowstone National Park in the 1990s, following a long period when they'd been locally extinct due to hunting.
And what is interference competition? It can take the form of harassment of one species by another, stealing the other species' prey (also called kleptoparasitism), and in some cases killing the competitor. Berger & Gese's working hypothesis was that both distribution and abundance of coyotes was limited by interference competition with wolves.
To test it, they compared data on coyote populations where wolves were present with data from wolf-free areas. Coyotes at both sites were radio-tagged, which meant the researchers could follow their movements & could also determine whether each animal was part of a resident pack or was a 'transient'. They could also find dead coyotes and, where possible, determine the cause of death. Disturbingly, the largest single cause of mortality was poisoning & hunting by humans, despite the fact that hunting coyotes was illegal throughout the study area. However, a breakdown of the mortality data showed that while most resident coyotes were killed by humans, wolves were the primary dause of death for transient coyotes. Berger & Gese suggested that this could be because the transients weren't familiar with the area and so were more likely to unknowingly encounter wolves - or that, because they travelled alone, they weren't able to benefit from warnings from other group members.
As far as the wolf-coyote relationship was concerned, Berger & Gese found that in terms of population size, coyotes were the dominant species throughout their study area. However, when wolves were reintroduced to the area, historical data showed that numbers of coyotes fell, & the researchers also found that coyote population densities were higher in the wolf-free part of the study site. This suggests that interspecific competition had some effect on the coyote population size.
How did this work? While the sample sizes in this study were fairly low, it seems possible that wolves may affect coyote populations by killing transient coyotes. In other words, the wolves appear to affect coyote abundance in the study area. But there doesn't seem to be an effect on coyote distribution, because coyote home ranges were within the home ranges of the resident wolf packs. However, there was also some suggestion that the overlapping areas were used in different ways, or at different times, by the two species. The researchers point out that wolves represent not only a cost (risk of lethal attacks) to the coyotes, "but also a potential energetic benefit, as scavenging from wolf kills represents an important food source for coyotes." The need to balance the costs and benefits may well explain the overlaps between the two species' home ranges.
So - a nice 'natural' experiment, made possible by an attempt at ecosystem restoration through the reintroduction of wolves to part of their original range :-)
K.M. Berger & E.M. Gese (2007) Does interference competition with wolves limit the distribution and abundance of coyotes? Journal of Animal Ecology 76: 1075 - 1085