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.

Exotic dominance of CA grasslands: don't judge a grassland by its cover.

Harpole, W. Stanley^*,1, Seabloom, Eric², ¹ University of Minnesota, St.Paul, MN² National Center for Ecological Analysis and Synthesis, Santa Barbara, CA

ABSTRACT- Observations of plant community dominance can lead to erroneous conclusions regarding the mechanisms controlling community assembly. Community assembly mechanisms can be divided into two general groups: local mechanisms (e.g. competition and disturbance) and regional mechanisms (e.g. dispersal limitation). Tests of local competitive dominance must control for seed limitation. For example, the scale and persistence of the invasion of California grasslands by exotic annual species suggests that annual grasslands are a new stable state or that the exotic species are competitively dominant to the native flora. Furthermore, while long term grazing has been suggested to be a causal factor in the original invasion of this grassland system, the removal of grazing does not cause invaded grasslands to return to a perennial state. In contrast, we show that the proportion of exotic annual species biomass in a restored native grassland decreases significantly from 0.49 to 0.09 with increased time since mowing when native seed limitation is overcome through seed addition. Thus, the abundance of annual species is dependent on frequency of disturbance, as predicted by community theory. We also show that frequency of disturbance has strong effects on microbial processes that may also affect invasibility. These results demonstrate the importance of controlling for dispersal limitation when testing the importance of local mechanisms such as competition and disturbance.

KEY WORDS: Invasion, Seed Limitation, Disturbance, California