Does fluctuating resource availability increase invasibility? Evidence from field experiments in New Zealand short tussock grassland

Abstract

The theory of fluctuating resource availability proposes that the susceptibility of a plant community to invasion by new species (i.e., invasibility) depends upon conditions of intermittent resource enrichment coinciding with the presence of invading propagules. We compared the response of a rapidly invading forb (Hieracium pilosella L.) between different experimental treatments in a short tussock grassland in New Zealand, over 6–12 years, to determine whether the theory explains differences in invasibility. The theory predicts that environments subject to periodic resource enrichment will be more invasible than those with more stable resource-supply rates. In our study, H. pilosella did not increase more rapidly in treatments subject to periodic resource pulses (fertiliser and water) than in those with more stable resource supplies. Also contrary to the predictions of the theory, the rate of invasion of H. pilosella did not increase following an increase in the rate of supply of water or nutrient resources, or following treatments that temporarily reduced resource uptake in the community, including grazing. H. pilosella did not increase immediately following abrupt increases in water and nutrient supply and removal of the dominant grass species with herbicide, as predicted by the theory, although temporary increases in resident exotic guilds indicated that the intensity of competition for resources was reduced. Neither H. pilosella nor resident exotic guilds showed increased cover growth rates following resumed grazing. The rate of invasion by H. pilosella was not correlated with species richness, a result consistent with one of the predictions of the theory. Therefore, short-lived events that temporarily reduced or suspended competition did not appear to determine the invasion success of this particular species in this region. In New Zealand’s perennial short tussock grasslands, the characteristics of the resident plant community may be more critical than resource fluctuations in determining invasion success of H. pilosella. Invasion of H. pilosella may be most successfully controlled here by promoting a successional physiognomic shift to a taller, shrub-and-tussock-dominated canopy that competitively excludes low-growing forbs.