Cohort effects and deer population dynamics

Among-individual variation of life history traits in a given population of deer has most often been accounted for by differences among sex and age classes and by current environmental variation (mostly changes in density and climatic conditions). However. among-individual differences in fitness can also be generated by differences in environmental conditions during the year of birth. Such cohort effects can be divided into two different components. First, among-year differences in environmental conditions at birth may produce large yearly variation in recruitment that generates a direct numerical effect (i.e., a high proportion of newborns are recruited in good years, whereas very low proportions are recruited when environmental conditions encountered by newborns during their first weeks of life are poor). Second, when recruited into the population, individuals born in a good year may reproduce earlier, reach a larger body mass, and have a higher reproductive success than individuals born in a poor year. We call the long-lasting influence on individual fitness of environmental conditions during the year of birth a delayed quality effect. Here, we first review briefly evidence of numerical and quality effects recently accumulated in deer populations. Then, by using long-term monitoring (> 20 y) of two contrasted roe deer (Capreolus capreolus) populations, we assess the influence of the among-cohort differences in reproductive traits, age- and sex-specific survival rates, and population growth rate. Results show that cohort effects (1) are widespread in deer populations and (2) may have a major influence on population dynamics, especially in low-performance populations. Cohort effects should therefore be accounted for in management plans of deer populations.