This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Characteristics and Consequences of Invasion by Sweet Resin Bush into the Arid Southwestern United States Elizabeth A. Pierson and Joseph R. McAuliffe1 Abstract.-Eutyops multifidus (sweet resin bush), a shrubby composite native to South Africa, was introduced to the arid southwestern United States in 1935 by the USDA Soil Conservation Service. The spread of this shrub represents one of the most serious threats to the ecological integrity and economic value of several ecosystems in the semiarid Southwest. In southern Arizona, this shrub readily invades vegetationally intact, semi-arid grasslands and eventually forms virtually uninterrupted monocultures from which native grasses, shrubs, and succulents are almost completely excluded. Study of plant responses at the advancing front of the areas occupied by sweet resin bush demonstrated that death of native species is linked to the spread of this exotic shrub. These dramatic vegetation alterations are persistent and lead to a variety of additional detrimental changes, including marked increases in soil erosion. We have identified the climatic, vegetation, elevation, and soil characteristics of sites that are susceptible to invasion by E. multifidus. In southern Arizona sweet resin bush can occur in vegetation ranging from Sonoran Desert at low elevation (ca. 850-1060 m) to grasslands, chaparral, and woodland at higher elevation (ca. 1300+ m). In grassland, this shrub can invade a wide variety of different soil types, ranging from loamy calcic soils typically occupied by stands of black grama (Bouteloua eriopoda) to heavy clay soils occupied by tobosa (Hilaria mutica) and curly mesquite (H. belangeri). A significant amount of public and private southwestern rangeland is within the range of this invasive species. Our results demonstrate the catastrophic consequences of invasion by sweet resin bush in the arid southwestern United States. INTRODUCTION not been affected by invasions of exotic terrestrial plants, and purposeful or accidental introduction Invasion of natural plant communities by in­ combined with urbanization and land usage have troduced plants constitutes one of the most been a major causal factor in these invasions. serious threats to natural ecosystems worldwide. At a global scale, grasslands are among the The resulting transformations can permanently most vulnerable ecosystems to extensive vegeta­ decrease native diversity, and can produce perma­ tion change due to plant invasions. According to nent, self-perpetuating changes in ecosystem Mack (1989), in less than 300 years (and in most properties including hydrology, biogeochemical­ cases, little more than 100 years) much of the tem­ cycling, and disturbance regimes (especially fire). perate grassland outside Eurasia (a collective area 6 2 There are few ecosystems in the world that have of 2.0 x 10 km ) has been irreparably trans­ formed by human settlement and the concomitant introduction of alien plants. In contrast to temper­ ate grasslands, semi-arid grasslands have been 1U.S.Geological Survey, 1675 W. Anklam Rd., Tucson, Arizona, 85745, USA and Desert Botanical Garden, 1201 N. Galvin Parkway, viewed as less vulnerable to invasion. The percep­ Phoenix, Arizona, 85008, USA. tion is based on the notion that because of the 219 greater enyironmentallimitations placed on exist­ ence in arid and semi-arid climates, few species are assumed capable of invading and the few that are capable are unlikely to bring about extensive vegetation change. The paucity of information available on the consequences of plant invasions in semi-arid grasslands has perpetuated the no­ tion that these grasslands are less vulnerable to invasion. For the arid southwestern United States, this notion may prove to be dangerously inaccurate. The biseasonal rainfall and subtropical tempera­ tures characteristic of the Sonoran Desert and adjacent semi-arid grassland make them vulner­ able to invasive species tolerant of both Mediterranean and neotropical climate regimes (Burgess, et al. 1991). Species of exotic plants tol­ erant of both climatic regimes have already become naturalized in the semi-arid vegetation of the Southwest; many were deliberately intro­ duced by the United States Soil Conservation Service (SCS) for their climate tolerance (Table 1). The consequences of these invasions are just be­ ginning to be recognized and further vegetation change due to invasion is likely to be only a mat­ ter of time. Examination of the characteristics of success­ ful invading species and the consequences of their spread for the invaded ecosystem provides an ex­ Figure 1.-EuryOPS multifidus (Asteraceae), also known as sweet cellent opportunity to study the structure and resin bush, Is a perennial subshrub. It has ShOWY, yellow function of the native biological community and Inflorescences which bloom in Arizona from December to assess the vulnerability of these com-!punities to March. further invasion. In this paper we focus on E u- ryops multifidus (Asteraceae), commonly known as sweet resin bush (fig. 1). This woody subshrub Table 1.-Examples of exotic plant species that have become widely established in semi-arid and arid vegetation in was introduced into the arid southwestern United Arizona as a result of deliberate introduction for erosion States from South Africa in 1935 by the SCS. The control and range improvement prior to 1942. The table ecosystem-level changes that have occurred as a gives the family, genus, and species names, country of origin, and source of introduction. An asterisk (*) result of the range expansion of this species from Indicates those species that were either imported and experimental introductions by the SCS are among introduced for the first time by the USDA Soil the most dramatic examples of the consequences Conservation Service (SCS) or widely distributed by the SCS, having been initially introduced by other means. of plant invasions in the arid Southwest. Interest­ ingly, relatively little has been written previously Plant names Origin Source Asteraceae Euryops multifidus South Africa* about this serious ecological problem. In this pa­ Asteraceae Pentzia incana South Africa * per, we summarize what is presently known Chenopodiaceae Atrip/ex semibaccata Australia* about the A) taxonomy and B) ecology of E. multi­ Fabaceae Melilotus indicus Eurasia* fidus; C) describe the history of introduction of Fabaceae Melilotus officinalis Eurasia'" Geraniaceae Erodium cicutarium Europe* sweet resin bush and other exotic species by the Poaceae Agropyron cristatum Turkey* SCS to the southwestern United States, D) de­ Poaceae Andropogon ischaem Turkey* scribe some of the changes in the structure and Poaceae Avena fatua Europe function of vegetation communities that have Poaceae Bromus rubens Europe Poaceae Cynodon dactylon Old world* been invaded by sweet resin bush, E) describe the Poaceae Eragrostis curvula South Africa * potential mechanisms that can account for these Poaceae Eragrostis lehmanniana South Africa * changes, and F) delineate the geographic area in Poaceae Pennisetum ciliare South Africa * Poaceae Schismus barbatus Old World* North America at risk of future invasion by this invasive species. 220 TAXONOMY, PHYTOGEOGRAPHY, AND and several are toxic (Hegnaur, 1977). Almost all PHYTOCHEMISTRY OF EURYOPS members of the genus Euryops produce and se­ MULTIFIDUS crete resin. The nature of the distinctive phytochemistry of the entire tribe suggests that Euryops multifidus belongs to the tribe many of the chemicals produced serve a role in Senecioneae (Asteraceae), which includes the defense against herbivores and parasites, as prominent genera Senecio, Othonna, and Euryops chemical inhibitors involved in allelopathy, or (Nordenstam, 1968). Representatives of the tribe both. are most common in South Africa. There are 97 species of Euryops, all of which occur natively only on the African continent; 96 of them occur ECOLOGY OF EURYOPS MULTIFIDUS IN only in South Africa. Euryops multifidus has a ITS NATIVE RANGE western Cape distribution in South Africa, rang­ ing from the Malmesbury Flats in the south to E'uryops multifidus is listed in A Catalogue of Lake Namaqualand in the north (range includes Problem Plants in Southern Africa (Wells, et al.p the Cape Province, The Orange Free State, Basuto­ 1966), as a ruderal, agrestal, and pastoral weed. land, and Namaqualand States). It occurs in The undesirable characteristics of this weed in­ transitional communities between arid fynbos and clude a) its ability to replace "preferred karriod vegetation, in rhenosterveld and in succu­ vegetation" b) its unpalatability and c) its occur­ lent karroo. It often occurs on rocky outcrops rence as a contaminant of seed (Wells, et al., 1966). between sea level and 1500 m in elevation (Nor­ Sweet resin bush, like most members of the genus, denstam, 1969). The distribution of E. multifidus has showy, yellow flowers which bloom during strongly overlaps that of E. tenuissim us. Interest­ the winter and early spring. The 3-4 mm long, 1-2 ingly, the latter species has been introduced as an mm wide achenes are covered by a wooly in­ ornamental in the arid southwestern United dumentum of 3-7 mm long white or brown hairs States, but has not yet become invasive. and are easily transported on clothing or fur. The common name "resin bush" has been ap­ Three to seven achenes per inflorescence are com­ plied