Parsimonious interpretation of the impact of vegetation, food, and predation on snowshoe hare

Krebs et al. (1995) summarized an 8-year experiment that measured the response of hare density to feeding, predator reduction, and plant fertilization, and concluded that the system has three interacting trophic levels. Food addition and predator reduction treatments each had positive effects, and together were multiplicative. These results were consistent with working assumptions of the importance of food and predation to the hare cycle, but the authors were puzzled by results of fertilization. They reported that all elements of the flora in the boreal forest responded dramatically to fertilization, but in spite of this, no response in hares was measured. Fertilization, it was concluded, was an ineffective method of food addition. This paradox could be resolved if it were understood how the three elements of this natural system interact, and whether or not they are sufficient to account for experimental observations. Therefore we constructed a parsimonious three-trophic-level model to account for the experimental results of Krebs et al. (1995), and to gain further insight into the interaction among vegetation, hares, and predators.