DISEASE AS A CONTROL OF A POPULATION OF BLUE GROUSE, DENDRAGAPUS OBSCURUS FULIGINOSUS (RIDGWAY)

Abstract

A population of blue grouse was studied on its summer range at Quinsam Lake, Vancouver Island, to determine the factors of importance in population control. The population is stable with a density of 0.40 adult males and 0.78 yearling and adult females to the acre. A life table is constructed on the basis of a stable population, the death of 80% of the chicks in the first three months after hatch, and the death of 31% of the adults each year, as calculated from banding returns. The survivorship curve is negatively "J"-shaped. Mortality rates appear constant and independent of age after the first year of life. Space, weather, food, predators, and disease are considered as factors capable of population control. Six parasites are new records in this host. Two of the six, Plagiorhynchus formosus and Dispharynx nasuta occur commonly and almost exclusively in the chicks, where they cause extreme damage to the tissues of the gut. It is concluded that parasitism by these helminths is an important mortality factor in the chicks and a major cause of population stability—an equilibrium between death in the older age classes and replacement by surviving young.