Density‐Dependent Demography in Two Grasses: A Five‐Year Study

To determine the strength, frequency, and nature of density—dependent processes in natural plant populations, the densities of two perennial grass species, Bouteloua rigidiseta and Aristida longiseta, were perturbed in an otherwise undisturbed community. Both seed additions and adult removals, of each species separately, were used. Individual plant survival, growth, and reproduction were monitored, as well as recruitment to the population and the numbers in each size class of each species, for 5 yr. The responses to the experimental perturbations were small relative to the magnitude of the perturbations, indicating that density dependence in demographic parameters in both populations was weak throughout the period. The results therefore do not support the suggestion of Fowler (1986) that the weak density dependence in the Bouteloua rigidiseta population in the 1st yr of the experiment was a temporary result of drought. Significant responses to the density manipulations were scattered among annual intervals, characters, and replicates. This may be simply a result of testing responses at the limits of detectability, or it may reflect intermittent and scattered density dependence. Density effects were generally much smaller than other differences among quadrats and differences among plants within quadrats, indicating that other factors had much more effect on a plant's fate.