Quantifying the cost of reproduction and exploring its effects using an integral projection model
Demographic costs of reproduction, such as diminished survival or fecundity following a year of high fecundity, are especially important to understanding the evolution of plant life histories and the effects of temporal variation in reproduction. We asked whether, and how, demographic costs of reproduction affect population dynamics. In plant populations, both survival and fecundity are functions of plant size. We therefore measured the effects of reproduction on future size as well as on survival and fecundity, using demographic data from a population of Bouteloua rigidiseta, a perennial grass. To determine effects on population dynamics, we incorporated cost of reproduction into an integral projection model (IPM). Because IPMs allow choices among a wide variety of possible functions to predict survival and fecundity from size and previous reproduction, they are well-suited to this task.
The cost of reproduction in this grass population was most apparent in the negative effects of reproduction on future plant size. Both survival and future fecundity were strongly affected by plant size, so prior reproduction indirectly reduced both future survival and future fecundity. The most sensitive metric of the cost of reproduction was relative growth rate (size[yr t+1]) / size[yr t]), which was on average approximately 15% lower in plants that had reproduced the previous year than in plants that had not. The potential population growth rate and other properties of the population's dynamics predicted by our IPM were quite sensitive to the cost of reproduction. However, a cost of reproduction of the magnitude we measured had little effect on population dynamics in most versions of the model (a few % change in population growth rate, for example). The results of the IPM analysis depended strongly upon the precise function used to predict individual plant size one year from its size the previous year, especially this function's predictions of the growth rates of the largest plants. The greater the cost of reproduction to the growth rate of the largest plants, the greater the reduction in the rate of population increase.