Cohort Variation in Reproduction and Survival: Implications for Population Demography

Abstract

In two different species of cervid, red deer (Cervus elaphus) and moose (Alces alces) pronounced cohort differences in fitness have important demographic consequences. In particular, where the environmental conditions into which a cohort is born permanently affect reproduction and survival, even if conditions ameliorate in later life, populations may fluctuate. Although cohort variation in survival in both moose on Isle Royale, USA, and red deer on Rhum, Scotland, has been attributed to differences in nutritional status during the period of early development, there are marked contrasts in the underlying causes of the differences in the nutritional status at birth. In the moose, cohort variation in survival was related to density-dependent changes in the food availability, while in red deer density-independent, weather, factors determining the onset of spring grass growth influenced both survival and reproduction. However, where cohort differences in early development do not persist they are unlikely to have significant affects on lifetime reproductive success and population fluctuation. For example, although cohort differences in early growth and the onset of breeding in a Norwegian red deer population varied in relation to spring weather, animals were able to compensate for retarded early development, and acheive large adult size and maximum reproductive potential. The available evidence suggests that, where food supply is limited by random stochastic processes, population fluctuation is likely to be small compared with long-term changes in climate like the predicted global warming.