DDI086.fm Page 217 Monday, January 29, 2001 11:08 AM Diversity and Distributions (2000) 6, 217–231 BIODIVERSITY RESEARCH TheBlackwell Science, Ltd impact of the invasive alien grass Cortaderia jubata (Lemoine) Stapf on an endangered mediterranean-type shrubland in California JOHN G. LAMBRINOS Department of Environmental Science and Policy, University of California, Davis, One Shields Avenue, Davis, CA 95616, U.S.A. Abstract. Since its introduction in the late 1800s, tween vegetation types. Arthropod abundance and the perennial tussock grass Cortaderia jubata order diversity were lower in plots dominated (Lemoine) Stapf has become an increasingly fre- by C. jubata than in adjacent chaparral. Insect quent member of coastal plant communities in traps in C. jubata plots contained a significantly California and Oregon. In this study, the com- smaller proportion of Hymenoptera and Homoptera munity changes associated with C. jubata inva- and a significantly greater proportion of Aràneae sion into the mediterranean-type vegetation of than traps in maritime chaparral. Rodent activ- Vandenberg Air Force Base, California were exam- ity was significantly lower while rabbit activity ined. Pristine plots of maritime chaparral were was significantly greater in jubata grassland com- compared with spatially and topographically pared to maritime chaparral. This study indicates matched plots dominated by C. jubata. Aerial photo- that the presence of C. jubata can dramatically graphs indicated that the invaded plots had previ- alter the mediterranean-type landscape of central ously been shrubland. C. jubata invasion created a California. While it is likely that the initial establish- structurally less complex perennial grassland that ment of C. jubata is associated with disturbances was markedly depauperate in native shrub species. that are common in this ecosystem, the ability of The absence of native shrubs depressed native C. jubata to expand from founding populations richness in jubata grassland, but the greater rich- and to persist for long periods of time pose a seri- ness of both native and alien herbaceous spe- ous threat to the native diversity of these unique cies made overall richness in jubata grassland systems. indistinguishable from maritime chaparral. Vegeta- tional differences were associated with differences Key words. Biological invasions, chaparral, in arthropod and small mammal populations be- mediterranean-type ecosystems, shrubland conversion. Introduced species have the potential to signific- INTRODUCTION antly alter population dynamics, community struc- Mediterranean-type ecosystems cover less than ture and ecosystem level processes (Elton, 1958; 5% of the Earth’s surface yet contain nearly 20% Drake, 1988; Macdonald et al., 1989; D’Antonio of the planet’s known plant species (Cowling et al., & Vitousek, 1992). While understanding the precise 1996). A number of anthropogenically induced nature of these changes is critically important for changes threaten this remarkable plant diversity the effective management and conservation of (Rundel, 1998). In particular, the threat posed by mediterranean-type systems, it is often difficult introduced non-native species has generated increas- to quantify the direct community-level effects of ing concern (Groves, 1986; Kruger et al., 1989; di introduced species independent of other landscape Castri et al., 1990; Groves & di Castri, 1991). level disturbances such as habitat fragmentation. © 2000 Blackwell Science Ltd. http://www.blackwell-science.com/ddi 217 DDI086.fm Page 218 Monday, January 29, 2001 11:08 AM 218 J. G. Lambrinos Introduced plants have unquestionably altered In this study, the community level changes the mediterranean-type vegetation of California. caused by the expansion of C. jubata in a geographic- Over the last 230 years, more than 1025 plant ally restricted formation of maritime chaparral species have been added to the flora of the state along the central coast of California are described. (Rejmánek & Randall, 1994). Entire systems such Specifically are examined: (1) how C. jubata alters as the native perennial grassland have been largely the vegetation structure, plant species composition replaced by alien plants (Heady, 1977). Additionally, and plant species diversity of invaded chaparral; the anthropogenic influences of habitat frag- (2) how C. jubata invasion affects the composi- mentation, altered disturbance regimes and the tion and abundance of insect and small mammal high frequency of new introductions leave the state’s populations. remaining mediterranean-type ecosystems highly susceptible to degradation by invasive plants (Mooney & Dunn, 1972; Mooney et al., 1986; Soulé et al., METHODS 1992; Alberts et al., 1993). Surprisingly few studies, Study area and plot selection however, have quantified the changes wrought by these introductions. Moreover, while many intro- This study was conducted on Vandenberg Airforce duced species have been identified as serious invas- Base, Santa Barbara County, CA, U.S.A. (34°41′N, ive wildland pests (see Robbins, 1940; McClintock, 120°36′W). The base contains most of the remain- 1985; Halvorson, 1992), few studies have attempted ing stands of Burton Mesa chaparral, a regionally to quantify how the continued expansion of these endemic variant of maritime chaparral. This unique invasive species will impact native communities. shrub community contains four endemic shrub spe- One invasive plant with the potential to signi- cies as well as other narrowly restricted plant ficantly alter mediterranean-type ecosystems in species. Less than 40% of the original Burton Mesa California is Cortaderia jubata (Lemoine) Stapf chaparral cover still exists, and the remaining acreage (jubata grass). C. jubata is a large perennial tus- continues to be threatened by development and sock grass native to the Andean regions of Ecuador, the invasion of non-native species such as C. jubata Peru and Bolivia. In the later half of the 19th (Davis et al., 1988). century it was introduced into California, along Stands of Burton Mesa chaparral are restricted with the congener C. selloana (Schultes) Asch. & to the Burton Mesa, a low elevation (30 m) coastal Graebner (pampas grass), for use as a landscap- terrace composed of thin sandy soils derived from ing ornamental. Both species have subsequently Orcutt sandstone underlain by a shale or clay hardpan escaped from cultivation and expanded in coastal (Shipman, 1972). The climate is mediterranean- habitats along the Pacific coast (Costas-Lippmann, type and strongly maritime. The average rainfall 1976; Lambrinos, 2000). Both species have also is less than 40 cm per year, almost all of this become naturalized in New Zealand (Connor, 1965; precipitation falling between October and April. Edgar et al., 1991), South Africa (Robinson, 1984) C. jubata was first recorded on the Burton Mesa and the Hawaiian islands (Chimera, 1997). in the mid-1970s (Davis et al., 1988), but aerial Cortaderia jubata has the more restricted dis- photographs taken during the 1970s and 1980s tribution of the two species in California, occurring indicated that large populations of C. jubata did primarily in a narrow band of coastal habitat from not develop until the mid-1980s. The initial establish- central California to southern Oregon (Lambrinos, ment of these large infestations seems to have been 2001). This restricted range, however, coincides with associated with large disturbances that occurred some of the region’s most diverse and restricted plant during the extension of an aircraft runway and the communities, and the prevalence of C. jubata in construction of several buildings (Chris Gillespie, these habitats has caused increasing management personal communication). concern (Cooper, 1967; DiTomaso et al., 1999). From these initial ruderal populations, however, Although considerable effort has been expended on C. jubata has since expanded into the surrounding the monitoring and control of C. jubata popula- relatively undisturbed chaparral. Using the aerial tions, no published data exist that describe how photographs, five locations were identified that these populations threaten the structure of native were currently invaded by C. jubata but that had communities. been uninvaded shrubland (Fig. 1). In these five © 2000 Blackwell Science Ltd, Diversity and Distributions, 6, 217–231 DDI086.fm Page 219 Monday, January 29, 2001 11:08 AM Impact of Cortaderia invasion 219 Fig. 1 Location of study plots on the Burton Mesa, Vandenberg Air Force Base, CA (34°41′N, 120°36′W). locations paired 50 m × 50 m plots were estab- cover at the time of invasion. The C. jubata indi- lished in stands dominated by C. jubata and in viduals composing invaded stands had a distribu- immediately adjacent stands of pristine maritime tion of sizes skewed slightly toward the smallest = = chaparral. size classes (g1 0.85, SEg1 0.34), although both The C. jubata-invaded portions of many of the small and large tussocks (0.002 m3–4.08 m3) were sites were associated with relatively small scale present within stands. A similarly skewed distri- disturbances: one site was bordered by an eroding bution of sizes was observed for C. jubata indi- stream channel, three sites were bordered by dirt viduals currently invading relatively pristine stands = = roads, while one site was located approximately of maritime chaparral (g1 0.86, SEg1 0.40; range: 100 m from a paved road. The only visible signs 0.002 m3–3.85 m3). This similarity, as well as the of disturbance within stands were narrow