When does increased population genetic diversity enhance colonization success?
Colonization is a critical filter for the establishment of most introduced populations, setting the stage for both short-term and long-term success. Increased propagule pressure (i.e., more founding individuals) usually enhances colonization success; however, this pattern may be driven by purely numeric effects, by population genetic diversity effects, or both. To determine the independent and interactive effects of propagule size and genetic diversity, we are conducting a factorial seed addition experiment in the field in Texas, using the ruderal mustard Arabidopsis thaliana. Our propagule size treatments were designed to characterize nonlinear recruitment curves, spanning five levels from 32 to 960 seeds per 0.25 m2 plot. These propagules were composed of one, four or eight genotypes, randomly selected from a pool of 25. After the first year, a reduced-competition treatment was initiated to increase variability in the plant communities where A. thaliana has colonized. We have now been monitoring colonization success and population dynamics in these plots for three generations post-introduction.
In the first generation, the abundance of reproductive individuals and total population fecundity both increased rapidly with increasing propagule size, quickly reaching their saturation points. Genetic diversity effects were most commonly observed where intraspecific densities were the greatest and A. thaliana individuals were thus more likely to interact. Under these conditions the proportion of flowering individuals and per-capita fecundity both increased on average with the availability of limiting resources (soil nutrients), although this positive relationship weakened with increasing genetic diversity and no significant increases occurred in our highest diversity treatments. These findings suggest that increasing genetic diversity in our experiment may be releasing populations from resource limitation, perhaps due to increased niche partitioning in more diverse populations. Preliminary analyses from year two data suggest these patterns also contribute to variation in population growth rates; year three data collection is currently underway. We will report on data from the first three generations post-introduction, incorporating functional trait data from a separate experiment as well so that genetic diversity and trait diversity data can be integrated for a deeper understanding of colonization success in this system.