Determinants of the abundance of invasive annual weeds: community structure and non-equilibrium dynamics

The dynamics of an annual pasture community are described from a five-year experimental and monitoring study The community was dominated by two grasse s (Lolium rigidum and Vulpia bromoides) and a legume (Trifolium subterraneu m). Fits of population dynamic models to per capita rates of population cha nge indicate that interactions between the grasses were generally strong, w hile interactions between the grasses and legumes were weaker. Most, but no t all, of the net effects of competition on population growth could be attr ibuted to interactions occurring during plant growth. Phase-plane analysis indicated that, for a constant environment, a joint equilibrium of the two grasses is unstable since interspecific competition between Lolium and Vulp ia is stronger than intraspecific competition. Consequently: the community will tend to a mixture of only one or other of the grass species and T. sub terraneum, depending on the founding composition of the pasture. Analysis o f data taken from a year in which a drought occurred (1993-1994) demonstrat ed profound effects on all three species. Modelling of the long-term impact s of the effects of repeated droughts showed that disturbance of this form overrides the founder effect observed under constant conditions. Consequent ly, Vulpia is ultimately able to invade any mixture of the other species in Environments where stochastic disturbances occur.