Regulating Reproductive Effort in a Changing Environment

Abstract

Using the ratio of effective reproduction to survival rate, with time scaled in units of time till independence, as an index to age specific reproductive effort, we ask how optimal reproductive effort should vary when environmental conditions change. With an idealized portrayal of the effects of environmental change introduced to a fitness set analysis of the trade-off between reproduction and survival, we find that the optimal reproductive effort is indifferent to environmental change. Deviations from this null case are obtained in a more general model, where we find that a pattern of increasing reproductive effort with environmental amelioration and decreasing reproductive effort with environmental deterioration is predicted when older breeding age classes are less affected by environmental change that are younger age classes, or when the potential adult survival rates are less affected by environmental change than are the potential survival rates of dependent young or the potential natality rates. Where the converse of these conditions obtains, the reverse pattern of change of reproductive effort will be favored. The assumptions of the null-case model provide precise definitions of the threshold conditions at which the pattern of reproductive effort will switch from one type to the other. The circumstances under which regulation of reproductive effort is predicted to be expressed as an increase in reproductive effort in more favorable times and a decrease in less favorable times are commonly met, especially in the life histories of many long-lived vertebrates. Thus the analysis of optimal life histories offers a competing explanation for some patterns of reproductive behavior that are otherwise described as social adaptations that evolved in response to the selective advantage of population regulation.