Consequences of individual growth rate heterogeneity on population dynamics in plants

Background/Question/Methods

Ecologists have long recognized that individuals within stage or age classes phenotypically vary, but this variation has only recently been added to population models. In plant populations, individuals may vary in growth rate. A simple model shows that when all individuals of an annual plant start at the same size and grow exponentially until the end of the year (with individual growth rates drawn from a normal distribution), the distribution of final sizes is log-normal, with mean final size greater than the individual with the mean growth rate. Increasing the variance of growth heterogeneity increases the mean final plant size and, if reproduction is related to plant size, increases the mean per-capita seed production (if reproduction is a nonlinear function of size, then the variance as well as the mean of the size distribution needs to be considered). Consequently, this increases the population growth rate. Here we examine how perennation and size-dependent mortality affect this relationship between individual growth rate variation and population growth rate. We built a model assuming density independence with two different relationships between size and mortality: increasing survival with size to an asymptote and maximum survival at intermediate size.

Results/Conclusions

Growth rate heterogeneity introduces variability and skew into size-at-age distributions among individuals in a population of perennial plants; this can increase the mean size-at-age. Size-dependent mortality further skews the size distributions and changes the relationship between mean growth rate and final size, with the shape of the distribution depending on the size-mortality relationship. This introduces a distribution of age-specific fertility and consequently a distribution of generation times (the latter effect can be particularly pronounced in monocarpic perennials and other species with a minimum flowering size). Regardless of the effects on mean generation time or lifetime expected reproduction, we found that the concave relationship between generation time and population growth rate guarantees that generation time variability will increase the population growth rate. Therefore, population models need to explicitly consider the variability in as well as the mean of size at age in order to correctly predict the population growth rate. These results may be particularly applicable to understanding the speed of invasion front advancement. An open question remains the extent to which these outcomes are tempered by physiological and architectural constraints on the plasticity of the individual growth rate.