DIRECT AND INDIRECT SPECIES INTERACTIONS IN AN EARLY OLD-FIELD PLANT COMMUNITY

The success of species in a community is determined not only by the direct interactions between species but also by indirect interactions. Indirect effects will occur if an associate species changes the abundance of intermediate species in a chain of interactions that ultimately affect the growth of some focal species. A model of species interactions in a community is presented here and used to quantify direct and indirect effects in a five-species weedy plant community. This method quantifies indirect effects by measuring changes in the abundance of the intermediate species and then estimating how such changes will affect the focal species. The experiments demonstrated that the direct effects in this community were generally negative (competitive) and could be quite large. Indirect effects were positive (facilitative) and negatively correlated with the direct effects, which thus acted to mitigate the usually larger competitive effects. The magnitudes of most species interactions in this community were primarily determined by large direct effects of Ambrosia artemisiifolia and, to a lesser extent, Agropyron repens. The species that exhibited the least competitive release following the removal of a competitor were those that were the most suppressed, because of the nonlinear nature of their response to increasing abundance of competitors.