RATE OF WEED SPREAD IN SPATIALLY HETEROGENEOUS ENVIRONMENTS

Communities subject to disturbance are apparently vulnerable to invasion in that they contain a large fraction of the introduced plant species persisting in any given area. Still, ecologists have a poor understanding of how details of the disturbance regime influence the success with which invaders spread through a community. In this paper, we report results from an experiment that investigates how the spatial distribution of bare ground influences the rate at which offspring of an introduced invader spread through a perennial ryegrass community. Artificially created gaps of three sizes were positioned along linear transects according to one of two spatial distributions. These six transects (one for each treatment combination) were positioned as spokes on a wheel and Senecio vulgaris was introduced into the center. After controlling for the total amount of gap area, we monitored the number ard position of Senecio vulgaris establishing for two subsequent generations. Our results showed that the rate of spread of Senecio vulgaris was sensitive to both the gap size and the gap distribution; plants moved a greater distance when the gaps were large and underdispersed. Simulation models confirm that this pattern follows from the way in which gaps sample from seed shadows. We derive an analytical expression to explain why the mean seed dispersal distance does not predict the spread of Senecio vulgaris adults, and show how rates of spatial spread will change under different spatial patterns of disturbance. Finally, a model of spatial point processes confirms that the distribution of gaps mediates the rate of weed spread in two dimensions as well.