@ 1980, by the American Society of Limnology and Oceanography, Inc. Disjunct distributions of two aquatic predator9

Two planktonic predators found in high altitude ponds of the Rockies are distributed locally in a disjunct fashion. Larvae of Chaoborus americanus are well known planktivores and can eliminate the semipredacious copepod Diaptomus shoshone from a pond; but in spite of the close proximity of many of these ponds, early instars of Chaoborus are rarely found in the presence of D. shoshone, and the densities are extremely low in the few cases of coexistence. Feeding experiments show that D. shoshone adults prey on early instar Chaoborus, but the later the midge instar, the greater its survivorship in the presence of D. shoshone. The co-pepods are in turn preyed upon by 4th instar midge larvae. The life cycles of the two species are offset so that midge eggs hatch in the presence of D. shoshone adults. It is suggested that reciprocal predatory interactions such as this one are unstable. Following the pioneering work of Hrbacek et al. (1961) and Brooks and Dodson (1965), many investigators have helped establish the importance of ver-tebrate predators in aquatic communi-ties. More recently,.nvertebrate preda-tors have become recognized as having equally important influences on coexist-ing species (Dodson 1974; Kerfoot 1977, 1978; Li and Li 1979) in terms of distri-bution and adaptation. One invertebrate predator often studied is the larva of Chaoborus sp.-the phantom midge-a well known and common planktivore. In the past few years copepod preda-tion has also been recognized as impor-tant (Kerfoot 1977). Diaptomus shoshone Forbes is a large copepod found in the Rocky and Sierra Nevada Mountains (Wilson I959), first reported as predatory by Anderson (1967). It preys on coexist-ing herbivorous zooplankton (Sprules 1972; Dodson 1974), but Sprules showed that it is not an obligate predator. These copepods have other influences as well, as Sprules first recognized, since D. sho-shone adults can prey on Chaoborus lar-vae. The faunal communities in subalpine ponds of northwestern Wyoming typical-ly contain, among other species, D. shoshone and larvae of Chaoborus l This research was supported by a Wellesle