Reproductive effort and reproductive values in periodic environments

Life-history theory concerns the optimal spread of reproduction over an org anism's life span. In variable environments, there may be extrinsic differe nces between breeding periods within an organism's life, affecting both off spring and parent and giving rise to intergenerational trade-offs. Such tra de-offs are often discussed in terms of reproductive value for parent and o ffspring. Here, we consider parental life-history optimization in response to varying offspring values of a population regulated by territoriality, wh ere the quality of the environment varies periodically. Periods are interpr eted as either within-year (seasonality) or between-years variation (cyclic ity). The evolutionarily stable strategy in a general model with two-phased periodicity in the environment ran generate either higher or lower effort in the more favorable of the two phases; hence knowing survival prospects o f offspring does not suffice for predicting reproductive effort-the future of all descendants and the parent must be tracked. We also apply our method to data on the Ural owl Strix uralensis, a species preying on cyclically f luctuating voles. The observed dynamics are best predicted by assuming dela yed reproductive costs and Type II functional response. Accounting for vary ing offspring values can lead to cases where both reproductive effort and r ecruitment of offspring are higher in the phase when voles are not maximall y abundant, a pattern supported by our data.