The relationship between population means and variances in reproductive success: implications of life history and ecology

Abstract

Within populations, individuals vary in breeding performance even under identical environmental conditions. Here, we present a model of logistic improvement in reproductive success (proportion of fledglings raised per egg laid) with environmental quality for breeders with different capacities. The mean reproductive success in our model denotes environmental quality for breeding. Our model establishes a space of potential combinations of means and variances in reproductive success. In this space, positive, negative or absent trends are possible. The literature and our own long-term studies show that life-history strategies may markedly affect the sign and shape of the relationship between mean and variance in reproductive success. Populations of long-lived birds rarely achieve a high mean reproductive success, and associations between the mean and variance of breeding success in different years tend to be positive. Conversely, populations of short-lived birds present negative mean–variance relationships with high mean reproductive success and variance in good environmental conditions approaching zero. Other factors, such as resource distribution and mating system, may also affect the variance in reproductive success and mean–variance trends. Plotting means and variances in reproductive success for different years or populations may indicate important aspects of the life history and ecology of organisms.