The consequences of earlier reproduction in declining populations

Abstract

Conflicting claims have been published concerning the consequences of earlier reproduction. In particular, it has been claimed that earlier reproduction will increase the rate of decline in a declining population, have no effect in a stable population, and enhance the rate of growth in an increasing population. However, if fecundity and mortality are conceptually separated so that earlier reproduction is defined as moving the fecundity column of the life table forward in time but not changing the mortality column, then earlier reproduction will nearly always cause the rate of change of a population to become more positive. This definition is believed to be more consistent with what biologists think of as earlier reproduction. Also, this concept requires that the cost of reproducing earlier be explicitly addressed rather than hiding it in assumptions implicit in a definition of earlier reproduction. This paper reconciles published mathematical demography models with published biological claims concerning earlier reproduction.