Effects of Size‐Selective Predation and Food Competition on High Altitude Zooplankton Communities

Two distinct communities exist in a series of small subalpine ponds on Galena Mountain, Colorado, at 3,400 m. In the deep ponds, two predators, the culicid larva Chaoborus and the axolotl Ambystoma tigrinum, coexist with two small herbivores, the copepod Diaptomus coloradensis and the cladoceran Daphnia rosea. In the shallow ponds the predatory salamander (Ambystoma tigirinum) coexists with three large herbivores, Daphnia pulex, Diaptomus shoshone (which may be predatory as an adult), and the fairy shrimp, Branchinecta shantzi, as well as the small Diaptomus coloradensis. Despite the proximity of deep and shallow ponds, each community has remained resistant to invasion from the other for at least 5 years. To determine why, I recorded the survival of organisms placed in cages in foreign ponds alone and in combination with the native species. Results revealed that chemical or physical conditions were limiting only for Daphnia pulex, which did not survive well in deep ponds. The large Diaptomus shoshone and Daphnia pulex could not invade deep ponds probably because they were heavily preyed upon by Chaoborus and the axolotl, respectively. This occurred in the presence of small prey, since laboratory experiments showed that predators selected large food items. Branchinecta shantzi was restricted by axolotl predation as well. Light predation pressures in shallow ponds were unimportant. Results suggest that Daphnia rosea could not invade shallow ponds because the larger native cladoceran D. pulex outcompeted it for food. Diaptomus coloradensis survives in shallow ponds because it avoids competition with its larger congenitor D. shoshone. Only the deep ponds are suitable for the aquatic Chaoborus larvae and axolotl since shallow ponds freeze completely each winter and periodically dry up during the summer.