Response of invasive species to functional group removal and disturbance in Argentine grassland.

SEIDLER, T; TOGNETTI, P.M.; CHANETON, E.J.

Portland, Oregon, USA

Congreso; Ecological Society of America Meeting; 2004

ESA

While acute invasions by single plant species have been the focus of many studies, communities invaded at relatively low intensity by multiple exotic species are increasingly common. The seasonally inundated grasslands of Argentina contain up to 30 percent exotic species, but although none dominates in biomass terms, their cumulative effects on community structure and nutrient cycling may be considerable. In order to decouple the effects of exotic species from those of native species on community structure, we manipulated functional group composition in the field. We distinguished among three functional groups of native, and two of invasive species: native C4 grasses, native C3 grasses, native forbs, exotic grasses and exotic forbs. Functional groups were removed singly or in combinations from grazed but otherwise undisturbed grassland plots, resulting in 18 selected treatments containing from 1 to 5 functional groups. In additional plots we controlled for disturbance by removing plants without regard to species at four levels: 5, 20, 40 and 80 percent ground cover. Random disturbance reduced biomass of native grasses while increasing biomass of native and exotic forbs. This increase failed to compensate for the decrease in biomass of grasses, but removal of forbs resulted in compensation by native grasses. Removal of exotic species had little effect on biomass of natives, but removal of native C3 grasses increased biomass of invasive species, even though C3 grasses represent less than 5 percent of grass production. This effect was likely due to overlapping emergence phenologies. These early results of a large-scale study show that relative abundance of native and invasive fractions of the community depend on multiple interacting factors. Ecosystem effects of invasive species will depend on not only on how much they functionally differ from, but also how much they alter the relative abundance of, the native species that they replace.