Biting the hand that feeds: the invasive grass Schismus barbatus (Poaceae) is facilitated by, but reduces establishment of, the native shrub Ambrosia dumosa (Asteraceae) Rodrı´guez-Buritica´ , Susana 1Ã & Miriti, Maria N. 1,2 1 Department of Evolution, Ecology, and Organismal Biology, The Ohio State University. 318 W 12th Avenue room 300, Columbus, OH 43210, USA; 2 E-mail miriti.1@osu.edu; Ã Corresponding author; Fax 11 614 292 2030; E-mail rodriguez.219@osu.edu Abstract Question: 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. Location: Joshua Tree National Park, California, USA. Methods: During 2003-2004, we used field surveys to characterize the natural spatial distribution of Schismus in relation to native shrubs, and experimentally manipu- lated 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 green- house experiment that measured competition between Ambrosia seedlings and Schismus planted at three densities and five relative abundances under controlled conditions. Results: Field surveys showed that the density of Schis- mus is independent of Ambrosia shrubs, but growth is enhanced near shrub canopies. In our field experiment Schismus is facilitated by adult Ambrosia. Under con- trolled conditions, Schismus does not respond to the density of Ambrosia seedlings, but changes in density of Schismus decreased performance of Ambrosia seedlings. Conclusion: Schismus invasion may be detrimental to native perennial plant populations. Although a reduction of seed- ling establishment is not usually expected to slow population growth of long-lived perennials, recent unprece- dented adult mortality in this community, and the well- documented facilitative role of Ambrosia, suggest that Schismus invasion may be of high ecological significance. Keywords: Population ecology; Desert; Island of fertility; Competition; Facilitation; Seedling survival. Nomenclature: Anon. (2009). Introduction Invasive grasses have changed the composi- tion and functioning of western arid ecosystems in the United States. Notable among these is the Mediterranean grass Schismus barbatus (Loefl. ex L.) Thellung, which is one of the most abundant and widespread exotic species in the Mohave and Sonoran Deserts (Brooks 1999; Brooks & Berry 2006). Native to Eurasia, Schismus was first re- ported in central California (Hoover 1936) and has since become widely distributed in US southwestern deserts (Brooks 1999; Brooks & Berry 2006). Schis- mus abundance is negatively correlated with abundance and diversity of native annuals (Brooks 2000), reflecting, in part, the ability of this grass to grow at very high densities (up to approx. 10 000 plants m  2 ; Venable 2007) and dominate areas with a high-nitrogen content (Brooks 2003). Most damaging among the documented impacts of Schismus is its positive association with increases in fire frequency (Brooks & Pyke 2001; Brooks & Berry 2006). To date, little is known about the relative fre- quency of competitive and facilitative interactions between Schismus and perennial shrubs. In general, perennials create spatial heterogeneity that promotes coincident establishment and growth of annual and perennial seedlings (Tielbo¨rger & Kadmon 1995; Facelli & Temby 2002; Holzapfel et al. 2006). In contrast to inter-shrub spaces, areas close to shrubs serve as ‘‘resource islands’’ (Schlesinger & Pilmanis 1998) because they are nutrient-rich (Schlesinger et al. 1996; Facelli & Temby 2002; Titus et al. 2002), have favorable soil properties, higher water avail- ability, and reduced irradiance (Forseth et al. 2001). These conditions promote denser and more diverse assemblages of annuals under shrubs than in inter- Journal of Vegetation Science 20: 241–250, 2009 & 2009 International Association for Vegetation Science