Nutritional Ecology of Microtine Rodents: Linear Foraging-Rate Curves for Brown Lemmings

Abstract

Models of herbivore-plant interactions have often been based upon the assumption that the daily intake by herbivores approaches an asymptote at high levels of forage availability (the type II functional response curve of Holling). Daily intake can be viewed as the multiplicand of foraging rates (intake per unit time while feeding) and foraging time (time per day spent feeding), both of which are functions of forage availability (biomass per unit area). The only mammals for which both of these functions have been measured are sheep. The results generally produce an increasing exponential approach to an asymptote for foraging rate and an exponential decline for foraging time; their multiplicand produces a type II curve for daily intake. We measured foraging rates for brown lemmings in arctic tundra to see if results for sheep apply to herbivores in general. Apparently, they do not. Foraging rates for brown lemmings increased as a linear function of food availability, and the slope of the function differed between seasons and years. Although an upper limit to foraging rates must exist, it was not approached at the maximal levels of food availability found in arctic tundra. Changing slopes were interpreted as a response to maturity of the vegetation. These results contradict the usual assumptions made about foraging-rate curves for herbivores. Data for sheep indicate that the type II functional response curve may apply to ruminants, but it should not be applied to nonruminant herbivores without further testing.