Separating the effects of number of individuals sampled and competition on species diversity] an experimental and analytic approach

0 Species richness typically increases with the number of individuals sampled\ although many ecological processes that in~uence species richness are also well known to depend on density of individuals[ We separated the e}ects of density on species richness that are due to sampling\ from those due to density!dependent ecological processes such as competition or predation\ by manipulating the density of an entire community[ 1 A seed bank from a community of desert annual plants that occur on semi!stabilized sand dunes in Israel was collected from the _eld and sown in an experimental garden at a range of densities from 0:05 to eight times the natural density[ The species pool observed in the lowest density plots was used as the null community\ which was repeatedly sampled to calculate the species richness "and other diversity indices# in higher density plots that would be expected from sampling considerations alone[ The signi_cance of deviations of observed diversity from this expected diversity was then evaluated[ 2 Both observed and expected number of species increased substantially with the experimental increase in density[ However\ observed species richness\ the ShannonÐ Wiener diversity index and Simpson|s diversity index were often signi_cantly lower than that expected based on sampling considerations[ The magnitude of the deviation from expected increased signi_cantly with increasing density for richness and the ShannonÐWiener index[ This provides some of the _rst direct experimental evidence from diverse natural assemblages that increasing competition among all the indi! viduals in a community can lead to competitive exclusion[