| I examined the ecological processes influencing species diversity and biological invasions in the plant community bordering the South Fork Eel River in northern California. In this system, an average of 85% of the individuals of most plant species grow on tussocks formed by the sedge, Carex nudata. In a field transplant experiment, plants on sedges better survived winter floods and suffered less herbivory than plants on the adjacent streambed. Carex vegetation, however, competed with the associated species over the growing season. For some species, the net effect of this competition was positive. For example, while dense Carex vegetation interfered with the growth of the liverwort, Conocephalum conicum, it also benefited Conocephalum by suppressing more potent shared competitors.; Since the tussocks are mini-communities containing 2–18 associated species, some of which are exotic invaders, the system provides an ideal natural context in which to explore the relationship between species diversity and biological invasions. Based on a literature review of this relationship, I formulated the hypothesis that while diversity may enhance invasion resistance at small neighborhood scales, factors spatially covarying with diversity across the landscape, and not diversity itself, drive community-wide invasion patterns. At the scale of the 8 km study stretch of the South Fork Eel River, more diverse tussocks were more likely to contain invaders. However, a direct in situ manipulation of local diversity showed that these patterns emerge despite the intrinsic negative affects of diversity on invasion at tussock scales.; The most diverse and most invaded tussocks tended to be those downstream, leading to the hypothesis that increased seed supply downstream may explain these patterns. A simple patch model predicted that directional dispersal could drive increased abundance and diversity downstream, but only if the system were severely seed-limited, as might occur after a major flood. A subsequent seed addition experiment confirmed that tussocks are seed limited and that variable seed supply may explain downstream increases in diversity and invasions. Simultaneously exploring the ecological determinants and consequences of species diversity may enhance our understanding of both of these important issues in ecology. |
