INVASIBILITY OF EXPERIMENTAL HABITAT ISLANDS IN A CALIFORNIA WINTER ANNUAL GRASSLAND

In an experimental test of plant community invasibility, we introduced seeds of a native ruderal, California poppy (Eschscholzia californica ), at fixed density into experimental plots in a California winter ann ual grassland. Each of the 42 plots, which ranged in size from 2 m(2) to 32 m(2), had been studied for 4 yr previous to the introduction, wi th the common observation that a subset of plots of each size consiste ntly held more species than others. It was primarily in these more spe cies-rich plots that establishment and reproduction by the experimenta l invader occurred. Success of the invader per plot. measured as the t otal number of plants germinating, producing seeds, or perennating, va ried with plot size, but the statistical contribution of plot size was secondary to that of local species number. Contributing variables wer e the extent of small mammal disturbance (positive) and the degree to which a single resident plant species (in particular, Bromus diandrus) dominated a plot (negative). In contrast to theories of competitive e xclusion via niche partitioning, species-rich plots were more invasibl e.