Invasion resistance in California grasslands: the effect of native species abundance declines on ecosystem functioning.
Hulvey, Kris*,1, Zavaleta, Erika1, 1 University of California, Santa Cruz, Santa Cruz, CA, USA
ABSTRACT- Biodiversity loss in the form of species abundance declines can alter the ways ecosystems function. Examples of altered ecosystem functioning due to loss of abundance include, modified food web dynamics through declines of top predators, and changes in successional patterns due to shifts in abundances of seed and pollen dispersers. An ecosystem function of current conservation interest that may be reduced by species abundance shifts is invasion resistance. Recent studies using California grassland species indicate that the native species Hemizonia congesta subsp. Luzulifolia decreases the biomass of invading yellow starthistle, Centaurea solstitialis. We examined the effect of H. congesta abundance declines on invasion resistance of California grasslands to starthistle. We created simplified outdoor grassland microcosms at Jasper Ridge Biological Preserve, California. Microcosms consisted of varying abundance levels of the native tarweed H. congesta, and a functionally contrasting, dominant grass, Bromus diandrus. We invaded half of the microcosms with starthistle and measured plant biomass, soil moisture, nutrient availability, starthistle flower number, and H. congesta seed number. Declining H. congesta abundance increased microcosm susceptibility to starthistle invasion, with a non-linear relationship between H. congesta abundance and invasion resistance: small declines in H. congesta abundance produced little loss of function, but larger declines produced an accelerating loss of invasion resistance. H. congesta continued to contribute to invasion resistance at low abundances. This result indicates that even rare forbs may be important in protecting California grasslands from invasion.
Key words: diversity-ecosystem functioning, invasion biology, grasslands
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