PARENT SESSION
Oral Session #51: Invasions: Population and community interactions.
Presiding: T. Stohlgren
Wednesday, August 7. 8:00 AM to 11:30 AM. Graham Meeting Room, TCC.
Resource competition, seed-limitation, and invasion in California grasslands.
Seabloom, E*,1, Harpole, W2, Reichman, O3, Tilman, D2, 1 National Center for Ecological Analysis and Synthesis, Santa Barbara, CA2 University of Minnesota, St. Paul, MN3 University of California, Santa Barbara, CA
ABSTRACT- Predicting the outcome of invasions and restorations both entail determining the conditions under which small populations have positive growth rates in the face of competition from another species. We used a mutual invasibility approach to investigate one of the most dramatic plant invasions worldwide, the invasion of California grasslands by annual grasses and forbs introduced from Mediterranean Europe. We used seeding experiments to test whether the dominance of exotic annual species is due to local interactions mediated by resource competition, seed limitation, or the presence of multiple stable equilibria (MSE). We reject the resource competition hypothesis. Native grasses were better competitors for water and light than were exotic annuals. Furthermore, undisturbed stands of native grasses were resistant to invasion by exotic annuals. We found strong support for the seed-limitation hypothesis. Native grasses invaded experimental annual grasslands under a wide range of environmental conditions. We were not able to find conditions under which the annual community was resistant to perennial invasion, a necessary condition for MSE. Seed limitation, the most likely mechanism, also holds the greatest hope for conservation and restoration of the native perennial grass community, because it suggests that viable populations of native grasses can be established by seeding at sufficiently high densities. In contrast, if the invasive species were the competitive dominants, the reestablishment of the native flora would depend on the eradication of the exotic species.
KEY WORDS: restoration, invasion, multiple stable equilibria, community ecology