Evolution of increased competitive ability in invasive nonindigenous plants: a hypothesis.

There are very few valid generalizations about invasive species, so that it is only possible to make weak, probabilistic predictions about which species will invade (Gilpin 1990; Daehler & Strong 1993). A phenomenon that has not received much attention, however, is the observation that, in alien environments, plants tend to be more vigorous and taller, producing more seeds than in their native distribution (Crawley 1987). The seed production of Chrysanthemoides monilifera (native to Sout'l Africa) in Australia and of Acacia longifolia (native to Australia) in South Africa is an order of magnitude higher where the plants are aliens (Noble 1989). The vigour and success of aliens in areas where they have been introduced has been attributed to both, more favourable environments, and to the release from natural phytophagous enemies (Crawley 1987). These alternatives can be evaluated according to two theories relating to resource allocation. The optimal defence hypothesis predicts that plants with limited resources will show trade-offs in biomass allocation among maintenance, growth, storage, reproduction, and defence (Coley et al. 1985; Bazzaz et al. 1987; Fagerstrom 1989; Herms & Mattson 1994; Lerdau et al. 1995). The environmental constraint hypothesis (Bryant et al. 1988), however, predicts that the evolution of defence mechanisms against herbivory will result in only minor reductions in growth. Resource availability is then the primary force determining whether there is surplus photosynthate available for allocation to secondary metabolism.