Density effects in ecological systems are pervasive and varied, affecting resource availability and species interactions. Life table response experiments (LTREs) can be used to identify the life-history stages affected by density, as well as the consequences of density-induced trait changes for the population growth rate.  Natural densities vary over wide ranges and I propose that a regression-design LTRE can be used to better understand how life-history stages, individual traits and population growth rates are affected by continuous changes in density.
I am testing the effects of within-population changes in density on life history traits of the perennial composite Pityopsis aspera. Nine, 1-m² plots were erected at Tall Timbers Research Station, Florida in August 2009 across a natural range of densities, with all individuals in each plot mapped and followed. Branch number, flower head number, and life stage (seed bank, juvenile, adult rosette and adult stalk) were recorded for each individual to provide estimates of survival, growth and reproduction. These transition probabilities were used to create a matrix population model for each density. A regression-design life table response experiment (LTRE) is then used to identify which transitions are associated with density and how they contribute to an estimated population growth rates.

Results/Conclusions

Density does not significantly affect ramet and seedling survival, but does affect seedling and juvenile growth rates.  Density also has significant effects on reproductive strategies.  Average flower head number per plant peaks at intermediate densities, while vegetative reproduction is favored in high and low densities.  Reproductive tradeoffs resulting from density effects has implications for population growth and the ability of this species to invade new habitats.  These effects of density may also affect patterns of local adaptation due to gene swamping by individuals at intermediate densities.