Biting the hand that feeds: the invasive grass Schismus barbatus (Poaceae) is facilitated by, but reduces establishment of, the native shrub Ambrosia dumosa (Asteraceae)

We present a study of positive and negative interactions between the invasive grass Schismus barbatus (Poaceae) and Ambrosia dumosa (Asteraceae). Ambrosia facilitates seedling establishment, and such facilitation may accelerate invasion of exotic species, which, in turn, may reduce establishment of native plants. Joshua Tree National Park, California, USA. During 2003-2004, we used field surveys to characterize the natural spatial distribution of Schismus in relation to native shrubs, and experimentally manipulated seed rain of Ambrosia and Schismus at three distances from adult Ambrosia canopies. We measured percentage germination and individual performance of both species. Field data were complemented by a greenhouse experiment that measured competition between Ambrosia seedlings and Schismus planted at three densities and five relative abundances under controlled conditions. Field surveys showed that the density of Schismus is independent of Ambrosia shrubs, but growth is enhanced near shrub canopies. In our field experiment Schismus is facilitated by adult Ambrosia. Under controlled conditions, Schismus does not respond to the density of Ambrosia seedlings, but changes in density of Schismus decreased performance of Ambrosia seedlings. Schismus invasion may be detrimental to native perennial plant populations. Although a reduction of seedling establishment is not usually expected to slow population growth of long-lived perennials, recent unprecedented adult mortality in this community, and the well-documented facilitative role of Ambrosia, suggest that Schismus invasion may be of high ecological significance.