Species replacement during early secondary succession: the abrupt decline of a winter annual

The factors that contribute to species establishment and decline determine the rate and pattern of successional change. We tested a commonly held assumption that competitive displacement is responsible for the loss of species during succession. Manipulative field experiments were used to examine the effects of interspecific competition on the population dynamics of Senecio sylvaticus, a winter annual that briefly dominates post-harvest sites in the western Cascade Range of Oregon. Senecio increased in density 400-fold from the first to the second growing season after disturbance but decreased precipitously in year 3 to 10% of the density and 0.5% of the biomass per plot of the previous year. Although interspecific competition reduced the cover and biomass of Senecio during its peak year, it had little or no effect on either the population increase or decline; the pattern of change was similar among all treatments. These counterintuitive results underscore the importance of testing, not simply assuming, that interspecific competition is responsible for the replacement of a species during succession.