Type 3 functional response of mice to gypsy moth pupae: is it stabilizing?

We conducted field experiments in 2002 and 2003 to determine whether the functional response of white-footed mice (Peromyscus leucopus ) to gypsy moth (Lymantria dispar ) pupae is decelerating (e.g. type 2) or accelerating (e.g. type 3) at low pupal density. In both experiments, live gypsy moth pupae were deployed in June (prior to the appearance of natural pupae) at densities of approximately 1, 8, and 35 pupae per mouse home range in oak-forest grids in upstate New York and monitored over 10 days for signs of predation. Pupae were deployed 1.5-m high on tree boles in 2002, whereas in 2003 the three density treatments were crossed with a height treatment: ground level vs 1.5-m high. The relationship between daily predation rate (proportion of pupae eaten/day) and pupal density was significantly positive in both years, indicating an accelerating functional response. Daily predation rates on ground-level pupae were substantial in the lowest density treatment, suggesting that dense mouse populations could drive gypsy moths to extinction despite an accelerating functional response. Daily predation rates on elevated pupae increased over several days in the medium and high density treatments, suggesting a lagged shift from groundto tree-level foraging by mice. Within the high-density treatments, predation rates on pupae showed no apparent relationship with the number of pupae on a tree. Our results disagree qualitatively with simple models of type 3 functional response, in which predation rate of prey approaches zero as prey approach extinction, and support the contention that an accelerating functional response alone may be insufficient to prevent prey extinction.