Does Daphnia density determine demographic destiny?
The measurement and assessment of life history traits, such as fecundity and survival, typically rely on life tables generated from individual-based assays, where growth, maturation and reproductive rates are measured in isolated individuals. In reality, individuals often live in populations, which together form communities. For many species, it is possible to distinguish single individuals and measure their individual life history traits acting in populations, in wolves for example. However, for other species individuals are indistinguishable, for example in zooplankton, so it is impossible to mark and track a single animal. In these indistinguishable and non-markable animals, the problem with traditional life history assessment is that it is based on traits measured in individuals and these trait values are then extrapolated to population level without measuring life history traits of individuals in the populations. To overcome the limitations of individual life history trait assessments, other parameters are necessary to describe populations, for example the rate of change of population size using population growth rate or Edmondson egg ratio. Extrapolating from individuals to populations is suitable for estimating population level effects as long as individuals in the population do not interact. However, if individuals do interact, which is common, then effects of those interactions between individuals, must also be considered.
The main aim of my research was to determine life history trait parameters of individuals and simultaneously assess population growth rate variation directly in Daphnia populations grown at different population densities. By using several genotypes potentially adapted to different environmental conditions, I was also able to test whether life history traits and population growth rates differed between single and mixed genotype populations.
For this I conducted two life history assays using 10 resurrected Daphnia galeata clones ineither different initial densities or populations starting with 2 individuals of mixed or singlegenotypes. Both experiments were conducted without food limitation, to disentangle the effect of density from competition for food.
In all clones, population growth rate decreased with higher initial density, even though food availability was increased too. Of all life history traits I examined, only one, investment in reproduction, changed with increasing initial density, while neither age nor size at maturity, nor Edmondson egg ratio differed between initial densities. Instead Edmonson egg ratio decreased with rising ephippia production. In every life history trait, there were large differences between clones. Additionally, populations grown in single genotype populations showed no difference in population growth rate compared to mixed genotype populations.
It appears that initial population density depresses population growth rate most likely through fecundity reduction in populations, even when food is not limiting, and is independent of genotypic diversity. I could point that in D. galeata decreasing investment in current reproduction is linked to increasing investment in future reproduction. This study shows that in Daphnia experiments considering life history traits, it is important to regard density-dependence. Furthermore because genotypes differed greatly in the strength of the association with density, and in individual life history parameters, this study highlights that experiments using Daphnia, or any other clonal organism need to be based on multiple genotypes to allow more generalizable results.