Density regulation in annual plant communities under variable resource levels

Density regulation is assumed to be common, but is very rarely tested experimentally. Using annual plant communities, we tested the hypotheses that 1) regulation of abundance in plants occurs at the level of entire communities, not just within species, and 2) such regulation is strongest when resources are most limiting. We transplanted different amounts of seeds from two diverse source communities in Israel to an experimental garden and monitored plant densities and cover for two years under different irrigation regimes. Both total density and total plant cover showed strong evidence of community-level regulation; plots sown at higher than average natural density declined or stayed the same in total abundance over time, while plots sown at lower than average natural density increased in abundance over time. This convergence of community abundance was strongest with the lowest irrigation in both source communities, consistent with the hypothesis of stronger regulation when all resources are more limiting (light levels were high, regardless of irrigation level). The main mechanism of regulation was strong density dependent recruitment, while survival was either density independent or inversely density dependent. Thus, the results also emphasize the need for direct experimental studies of the population dynamic consequences of interactions, as well as of individual level consequences.