It is axiomatic that most species in a community are rare. Rarity is often associated with risk of extinction, but many rare species persist at low abundance. Negative density dependence has been proposed as a mechanism that could explain both low abundance and the ability to persist at low abundance because it would both limit population growth at greater abundance and promote increase from low density. We conducted field experiments to test the hypothesis that negative density dependence could contribute to stable low abundance of two perennial plant species in a high diversity community. We used long-term community survey data from a high-diversity plant community to identify pairs of congeneric perennial plant species in which one species persisted at low abundance and the other was consistently common. We planted these two pairs of species across a density gradient in the field and measured individual growth and herbivore damage. After one season of growth, we tested for evidence that the low abundance species had a stronger negative response to increasing density, either a greater decline in growth rate or an increased rate of herbivore damage, than its congeneric common species for each pair.
Results/Conclusions
For one species pair, the low abundance species had significantly greater growth and less herbivore damage than its common congener at all densities after one growing season. There was no significant interaction between species abundance class and density for either variable. For the second pair, the common species grew significantly more at all densities. There was no effect of density on herbivore damage for either species, and no significant interaction between density and species. These initial results do not support a role for negative density dependence in response to competition or herbivory in explaining how rare species persist. If differences in density dependence emerge over a longer time frame, plant traits associated with strong negative density dependence might be useful for distinguish species that are rare but stable from those at risk of extinction from a community.