Impact of Demographic Parameters upon a Population of Spruce Grouse in New Brunswick

Abstract

Densities, age ratios, survival rates, production, dispersal rates (in autumn), and recruitment rates are documented for spruce grouse (Dendragapus canadensis) at Sevogle, New Brunswick, during 1976-83. These data were used to predict the relative impact that each parameter had upon grouse density in the breeding period. This approach indicated that population size of spruce grouse was not controlled by the effects of a single parameter. Density of immigrants, the rate at which they were recruited, and the rate at which locally produced juveniles were recruited were predicted as the most important to population change. Their effects, however, did not in all cases exceed the impact from density of residents the previous year and their survival to the next year. J. WILDL. MANAGE. 51(4):771-777 Life history traits are strongly influenced by spatial and temporal variation in at least certain environmental factors, and variations in life history traits are equally important to their evolution, as are mean values (Stearns 1976:37). This should be true particularly for species occupying varied habitats over a broad geographical area. Spruce grouse are distributed across coniferous and mixed-wood boreal forests of North America, typically at low densities although Szuba and Bendell (1983) document exceptions. Population data have accumulated, but we do not yet understand the meaning of spatial differences that occur in demography and certain life history traits. Substantial differences in production and mortality of spruce grouse occur among different forest types at different latitudes across the range of the species (Ellison 1974; Keppie 1979, 1982); demography in New Brunswick appears closer to that in Alaska than for the Franklin's race (D. c. franklinii) in Alberta. A comparative approach will help form generalities about life history. The purpose of this paper is, 1st, to document demographic characteristics of spruce grouse at 1 locality in New Brunswick and to make selected comparisons with grouse at Gorge Creek, Alberta. Second, from these data I estimate the relative impact that each parameter had upon changes in population density. Many authors note variation in 1 or more demographic rates but do not balance them against each other and changes in population (Bendell 1972:81). That failure still is prevalent, especially if key factor analyses on mortality (e.g., Watson [1971], Weeden and Theberge [1972]) are inappropriate (Kitching 1976, Krebs 1978:2465). Present analyses document the potential influence that production, mortality, and dispersal processes have upon population size and predict that several act simultaneously to control grouse density. I especially thank the 10 persons that have worked in the field with me, 1976-83. I thank F. C. Zwickel and R. A. Lewis for review of preliminary analyses and their cautions about sample sizes and generality. I appreciate reviews by A. Boer and J. Towers. Field studies were supported by the Nat. Sci. and Eng. Res. Counc. Can. and the Univ. New Brunswick. STUDY AREA AND METHODS Spruce grouse were studied at Sevogle in northeast New Brunswick (46N, 660W), Canada, from 1976 to 1983, excepting 1981. Fieldwork was conducted in 2 areas, SNW and WB, 1.5 km apart and separated by the Big Sevogle River. Logistic problems prevented both forest tracts from being worked each year with equal intensity. A mosaic of different stands occurs at Sevogle, principally dense black (Picea mariana) and red (P. rubra) spruce-balsam fir (Abies balsamea); lightly stocked black spruce-jack pine (Pinus banksiana) (Redmond 1982:fig la); dense, young jack pine (Redmond 1982:fig. ib); fully stocked mature jack pine, all principally dry upland sites; and white cedar (Thuja occidentalis)-spruce bogs. Intolerant hardwoods, such as white birch (Betula papyrifera) and red maple (Acer rubrum), occur on wet sites. The Sevogle area is surrounded by extensive softwood forest, principally spruce-fir, typical of that in the east, and which is occupied by grouse. Field methods were similar to those used earlier in southwest Alberta (Keppie 1979), with 2-