Fine-scale Spatial and Temporal Variation in the Demography of a Perennial Bunchgrass'

Most life history studies assume that demographic transition rates within a population are temporally and spatially invariant. A violation of these assumptions may have important consequences in the analysis of long-term processes such as population growth and stability. To examine the potential problems, I studied the demography of a size-classified population of Danthonia sericea, a long-lived grass species, using standard matrix analysis techniques. The study was conducted for two June-to-June transition periods at five locations along a well defined soil/vegetation gradient in a field located in Durham, North Carolina. Demographic transition rates, and predictions of long-term population growth, differed markedly between transition periods and among locations, with the biggest differences being attributable to location. Elasticity analysis indicates that, in all cases studied, a large proportion of long-term population growth can be ascribed to demographic processes associated with individuals in the largest size classes. However, this aspect of the demography of Danthonia sericea does not differ appreciably among locations or between years and, consequently, does not explain the observed demographic variability. I argue that some of the variability arises from differences in the rate of recruitment of smaller individuals into the larger size classes. Although elasticity analysis indicates that the raw rate of recruitment through reproduction is of little importance to population growth, germination and early establishment success may play a significant role in determining long-term population growth rates, particularly if good recruitment years are infrequent. This can be shown if more than one transition period is considered explicitly in the demographic analysis. It is suggested that future demographic studies account for year-to-year variation in transition rates by incorporating stochastic matrix theory in the analysis of long-term population processes.