Eric Seabloom, The role of seed limitation, resource competition, and community complementarity in invasions and restoration

When we look around the grasslands on California’s hillsides, almost all the plants we see are exotic species from the Mediterranean region of Europe. In the 200 years since Europeans arrived, California’s native perennial bunchgrasses have been almost completely replaced by exotic grasses such as wild oats, soft brome, and medusahead, and exotic forbs such as yellow starthistle. This is one of the most dramatic conversions of a native plant community that has ever occurred in the world, and the reasons for it are still not well understood. The conventional wisdom blames some combination of livestock grazing and the intrinsically greater competitive ability of the exotic species over the natives.

Since most of the exotic species have an annual life-cycle, unlike the native perennials, this grassland conversion has had potentially major ecological effects. Not only California’s native plant and animal biodiversity, but also ecosystem processes such as nutrient cycling and fire regimes, have almost certainly been greatly altered. Ongoing grassland invasions continue to erode the value of grasslands for livestock grazing. So far, attempts to use managed fire or grazing to control invasions and restore grasslands have met with limited success.

To better understand grassland invasions, we initiated a study in 1999 at UC Santa Barbara’s Sedgwick Reserve. By tilling the soil and replanting it, we were able to create pure stands of either native bunchgrasses or exotic annual grasses. Then we “invaded” stands of full-grown native grasses with seeds of exotics, and vice versa. The results were a great surprise. We found that native grasses could invade and establish themselves in exotic grasslands, but exotics could not invade and establish in stands of natives. This was true even under a full range of additional treatments we tried, such as disturbance and fertilization. This result seemed to contradict the conventional wisdom about the competitive superiority of exotic species over the natives they displace.

Our interpretation is that native bunchgrasses are actually competitively superior to exotic annual grasses, but that the recovery of native grasslands is strongly limited by the lack of sufficient seeds. If this is true, it implies that the decline of native grasslands was caused by past events, such as drought and overgrazing in the nineteenth century. It also implies that restoration may not be as forbidding a task as has often been thought. However, we need to expand and strengthen our findings using parallel experiments at multiple sites.

We have recently set up field experiments at the Quail Ridge, McLaughlin, and Hastings (UC Berkeley) Reserves, as well as Sedgwick, where we will test the ability of communities of various mixes of native species to re-invade exotic annual grasslands. The reserves offer us the opportunity to assess the robustness of our results across a 700 km latitudinal gradient. They also offer us places where we can conduct long-term monitoring of naturally occurring stands of perennial bunchgrasses. Our hope is that through combining the rigor of controlled field experiments with geographic breadth, long-term observations and modeling, we can contribute a better conceptual and practical understanding of Californian grassland invasions. We could not do this without the secure highquality field sites provided by the UC Natural Reserves.

Photo Credits: Title, Research (Mike Benard), Survey and Grass pictures courtesy of Eric Seabloom