When we teach our students predator-prey relationships, we tend to tell them a very basic story that have come from a few very good studies. It's not that all we have are simple models, but but we like to build up the basics before we dive into the complexities. Because when you get down to the complexities, a lot of counter-intuitive things happen which don't follow "common-sense" relationships.
At the last Alaska chapter of the Wildlife Society meeting, Steve Arthur presented one such study, which I had previously mentioned here by flinging out the abstract. Now the full paper by him and Laura Prugh is out in the Journal of Wildlife Management, and so I want to spend a little more time on the study.
They studied snowshoe hares (Lepus americanus) and Dall's sheep (Ovis dalli) in the central portion of the Alaska Range, illustrated by the figure to the right. It's worth noting that though both species are in the Alaska range, they have differing habitat requirements. Whereas hares are happy in the lower foresty areas, the sheep decidedly prefer the rocky hills where they can can try to avoid predators. However, while adult survival among sheep is fairly high, lamb survival is generally lower, as predators (lynx, coyotes, eagles, wolverines) can still prey on them.
They used road-side counts to index hare abundance, which they double checked against pellet count data - it's little things like that which make a good paper. For the sheep end, they collared a variety of ewes and fitted them with collars, as well as lambs. They re-located them about 3-5 times a week, and the collars were fitted with mortality sensors. On death, they went and determined cause of death of the individuals. One nit-pick here - scavenging is probably represented somewhat in the mortality data, but the two authors admit that. Still, you wouldn't expect scavanging and predation to vary independently of mortality, and survival is what they wanted to get at. Finally, they used surveys to index the total population of Dall's sheep through the period.
When they analysed the adult data, they found that the year really didn't have a major effect on their survival, and that survival was either modeled best as age-specific, or just constant. However, for lambs, the best explanations for the changes in survival were either a three year time lag in hare abundance, positively, or a negative relationship between survival and hare abundance with a one year time lag. The mechanism for changing survival appears to be changes in predation - primarily coyote and golden eagle predation. Accidents and disease seem to remain at a constant low-level throughout all years, whereas predation seems to vary among years. Eagles are best predators early on, whereas coyotes appeared to kill several months after ewes dropped their lambs. As I mentioned before, snowshoe hares and sheep share those predators.
This seems to suggest that although there's a positive relationship between abundance of hares and sheep, it is not, in fact, a mutualistic relationship. If they had just limited their study to abundance, they would have never seen the change in lamb survival. At that point, we would be led to incorrectly believe that snowshoe hares and sheep benefit from each-other's presence. It doesn't appear to be mediated by a functional response in predation - first, a previous study found that coyotes don't switch to sheep when they're low, but secondly, it would mean there would be a positive relationship between hare abundance and sheep survival, not negative.
It's a messy story, one that depends on the sort of data you gather, and, this simplified version I'm presenting here is leaving out further complications itself. The effects of predation can be complex, leading to strange results when all is said and done. Here, the population dynamics of Dall's sheep seem at first blush to be driven by a species that isn't even in the same habitat as them. It's only though diligent data collection the true picture comes out.
Arthur, S., & Prugh, L. (2010). Predator-Mediated Indirect Effects of Snowshoe Hares on Dall's Sheep in Alaska Journal of Wildlife Management, 74 (8), 1709-1721 DOI: 10.2193/2009-322
All figures from Arthur & Prugh 2010, used under fair-use rationale.