An Experimental Study of Competition among Fugitive Prairie Plants

We assessed competition among five wind—dispersed perennial fugitive plant species. The species constitute a guild on small open sites formed by foraging badgers on the Cayler Prairie Preserve in northwestern Iowa, USA. The outcome of interactions among individuals at a site depended on the intervals between immigrations. When immigrations were separated by more than one growing season, preemptive competition occurred: initial colonists were not affected, but most later arrivals died during the first growing season, and none survived to maturity. When multiple immigrations occurred within a single year, exploitative competition depressed growth and propagule production of co—occurring species in inverse proportion to their propagule size. By manipulating the density of colonization sites and the production of propagules, we demonstrated a direct nonlinear relationship between immigration and intra— and interspecific competition. With infrequent immigration, neither preemptive nor exploitative competition was likely among offspring; reproductive success of parents was proportional to the likelihood of immigration. As immigration increased, preemptive and exploitative competition among offspring was accelerated, and per—propagule gains in parental reproductive success declined. Losses of reproductive success were inversely related to species—specific exploitative capabilities. For individuals of each species, the competition threshold was that immigration rate above which reproductive success was lowered due to the combined effects of preemptive and exploitative competition are directly related (as with competition for sites or resources within sites), competition increases with the density of the limiting resource and will have greatest effect on plants at high densities of the limiting resource. Our results further indicate that intra— and interspecific competition are likely to influence reproductive success within local populations of these fugitive species. Mean fecundities among adjacent populations of fugitive plants were more similar than expected for noninteracting populations distributed along a gradient of site densities, suggesting that interspecific competition has influenced life history attributes in this guild. Differences in life history attributes influencing immigration and exploitative capabilities may result in there being regions along the gradient within which each species is competitively superior. Persistence in the guild may be maintained by: (1) an inverse relationship between immigration and exploitative ability, (2) variation in the density of sites, and (3) the occurrence of both preemptive and exploitative competition on sites.