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Nonnative Species and Bioenergy: Are We Cultivating the Next Invader? JACOB N. BARNEY AND JOSEPH M. D I TOMASO
Biofuel feedstocks are being selected, bred, and engineered from nonnative taxa to have few resident pests, to tolerate poor growing conditions, and to produce highly competitive monospecific stands—traits that typify much of our invasive flora. We used a weed risk-assessment protocol, which categorizes the risk of becoming invasive on the basis of biogeography, history, biology, and ecology, to qualify the potential invasiveness of three leading biofuel candidate crops—switchgrass, giant reed, and miscanthus (a sterile hybrid)—under various assumptions. Switchgrass was found to have a high invasive potential in California, unless sterility is introduced; giant reed has a high invasive potential in Florida, where large plantations are proposed; miscanthus poses little threat of escape in the United States. Each biofuel crop shares many characteristics with established invasive weeds with a similar life history. We propose genotype-specific preintroduction screening for a target region, which consists of risk analysis, climatematching modeling, and ecological studies of fitness responses to various environmental scenarios. This screening procedure will provide reasonable assurance that economically beneficial biofuel crops will pose a minimal risk of damaging native and managed environs. Keywords: biofuels, ethanol, invasive species, weed risk assessment, bioenergy
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rowing energy demands, a desire to reduce reliance on fossil fuels, and greater awareness of climate change have led both state and federal governments to pursue alternative energy sources. Biomass-derived energy has been pursued for decades in the United States and Europe, but recent renewed public and political interest has sparked explosive growth in the biofuel industry. The United States initiated a research program in the late 1970s to identify candidate crops for dedicated biofuel production, whereas Europe began biofuel research in the 1960s (Lewandowski et al. 2003). However, a recent surge in bio-based fuel research has incited concern regarding rapid adoption of novel crops that may become invasive pests (Raghu et al. 2006). Herbaceous and woody species are being selected, bred, and transformed for desirable agronomic traits, including tolerance to drought, salinity, and low-fertility soils, as well as increased aboveground (harvestable) biomass and enhanced competitive ability to reduce fertilizer, irrigation, and pesticide use. However, the very traits that characterize an ideal biofuel crop also typify much of our invasive flora. Indeed, the most promising biofuel crops are nonnative to the regions proposing cultivation, compounding the potential risk of future invasions. For example, California and the Pacific Northwest are pursuing switchgrass (Panicum virgatum L.), which is native to most of North America east of the Rocky Mountains; a private firm in Florida is initiating a biofuel program centered
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on the Eurasian giant reed (Arundo donax L.), so-called e-grass; and Europe and the United States are screening Asian miscanthus hybrids (Miscanthus × giganteus) (Lewandowski et al. 2003). Many invasive species have horticultural or agronomic origins with long periods of cultivation that precede their escape, naturalization, spread, and subsequent environmental impacts (Mack 2000). A classic example is kudzu (Pueraria montana [Lour.] Merr. var. lobata [Willd.] Maesen and S. Almeida), first promoted by the federal government as a forage species and later widely planted for erosion control (Forseth and Innis 2004). The rooting structure, perennial habit, and extraordinary growth rate of kudzu made for an ideal erosion mitigator, although these same traits fostered its eventual escape and dominance in the southeastern United States. The Southeast met a similar fate with johnsongrass (Sorghum halepense [L.] Pers.), introduced as a forage crop and now a noxious weed in 19 states. The sequence of
Jacob N. Barney (e-mail:
[email protected]) is a postdoctoral scholar interested in the factors determining invasion success, and Joseph M. DiTomaso (e-mail:
[email protected]) is an extension specialist interested in invasive weed ecology and management; both are in the Department of Plant Sciences at the University of California–Davis. © 2008 American Institute of Biological Sciences.
www.biosciencemag.org
Forum Forum selection breeding horticultural, agronomic, selection andand breeding forfor horticultural, agronomic, or or soilsoil stabilization purposes, cultivation a naïve environment, stabilization purposes, cultivation in in a naïve environment, followed escape subsequent environmental ecofollowed by by escape andand subsequent environmental or or economic calamity, often describes many most invasive nomic calamity, often describes many of of ourour most invasive species (Reichard White 2001). Therefore, quandary species (Reichard andand White 2001). Therefore, thethe quandary is how to balance economic benefits of growing native is how to balance thethe economic benefits of growing nonnon native crops bio-based energy while minimizing of culcrops forfor bio-based energy while minimizing thethe riskrisk of cultivating next noxious weed invasive plant. tivating thethe next noxious weed or or invasive plant. effort to curtail introduction future invasive In In an an effort to curtail thethe introduction of of future invasive species, many researchers developing preintroduction species, many researchers areare developing preintroduction screening protocols nonnative species basis screening protocols thatthat sortsort nonnative species onon thethe basis risk-assessment criteria. These risk-assessment tools of of risk-assessment criteria. These risk-assessment tools areare science-based protocols designed to identify likely invaders science-based protocols designed to identify likely invaders andand benign species, reject accept them introduction, benign species, andand reject or or accept them forfor introduction, after consideration of the taxon’s biology ecology, climatic after consideration of the taxon’s biology andand ecology, climatic requirements, history, biogeography relative to the requirements, history, andand biogeography relative to the tar-tarregion (Pheloung et al. 1999). most widely adopted getget region (Pheloung et al. 1999). TheThe most widely adopted weed assessment (WRA) protocol designed Ausweed riskrisk assessment (WRA) protocol waswas designed forfor Australia New Zealand (Pheloung et 1999); al. 1999); derivatives tralia andand New Zealand (Pheloung et al. its its derivatives have been tested in Hawaii (Daehler Carino 2000), have been tested alsoalso in Hawaii (Daehler andand Carino 2000), Florida (Gordon et al. 2006), Czech Republic (Krivánek Florida (Gordon et al. 2006), thethe Czech Republic (Krivánek Pyšek 2006); a subsequent variation tested in Ausandand Pyšek 2006); andand a subsequent variation waswas tested in Australia (Caley Kuhnert 2006). Each protocol developed tralia (Caley andand Kuhnert 2006). Each protocol waswas developed validated using existing data of known invasive andand validated using existing data setssets of known invasive andand benign species, with their respective accuracy ranging from benign species, with their respective accuracy ranging from to 100 percent “major” invasives (i.e., rejecting invasive 96 96 to 100 percent forfor “major” invasives (i.e., rejecting invasive plants), to 100 percent noninvaders (i.e., acceptplants), andand 79 79 to 100 percent forfor noninvaders (i.e., acceptnoninvasive plants). However, accuracy of these proinging noninvasive plants). However, thethe accuracy of these protocols is much lower when considering introduced species tocols is much lower when considering introduced species of of presumably minor impact. Nevertheless, screening nonnative presumably minor impact. Nevertheless, screening nonnative species through a science-based risk-assessment protocol species through a science-based risk-assessment protocol be-beimportation produces a net bioeconomic gain (Keller forefore importation produces a net bioeconomic gain (Keller et et 2007), should become standard protocol a first al. al. 2007), andand should become standard protocol as as a first step in preintroduction evaluations of nonnative species. step in preintroduction evaluations of nonnative species. In response to the economic environmental incentives In response to the economic andand environmental incentives low-input biofuel crops desire to prevent future forfor low-input biofuel crops andand thethe desire to prevent future invasions, screened leading candidates biofuel invasions, wewe screened thethe leading candidates forfor biofuel feedstock crops United States—giant reed, switchfeedstock crops in in thethe United States—giant reed, switchgrass, miscanthus (a sterile hybrid)—using a validated grass, andand miscanthus (a sterile hybrid)—using a validated WRA protocol to qualify their of invasion under various WRA protocol to qualify their riskrisk of invasion under various assumptions (e.g., of domestication) scenarios assumptions (e.g., thethe rolerole of domestication) andand scenarios (e.g., sterile cultivars). Our to address following (e.g., sterile cultivars). Our aimaim waswas to address thethe following three questions: Would proposed biofuel feedstock species three questions: (1)(1) Would proposed biofuel feedstock species pass current standards entry into nonnative regions (i.e., pass current standards forfor entry into nonnative regions (i.e., earn “accept” rating)? Could potential invasibility earn an an “accept” rating)? (2)(2) Could potential invasibility be be reduced enhanced) through genetic modification? reduced (or(or enhanced) through genetic modification? andand (3)(3) How proposed biofuel feedstock species compare with How do do proposed biofuel feedstock species compare with other nonnative invasive or weedy species of similar form other nonnative invasive or weedy species of similar lifelife form habitat preferences? andand habitat preferences?
Weed risk assessment Weed risk assessment Giant reed, switchgrass, miscanthus were screened Giant reed, switchgrass, andand miscanthus were screened through original WRA protocol (Pheloung et al. 1999), through thethe original WRA protocol (Pheloung et al. 1999), which modified slightly each target region. WRA which wewe modified slightly forfor each target region. TheThe WRA contains questions a macro-driven spreadsheet, with each contains 49 49 questions in ainmacro-driven spreadsheet, with each question receiving points ranging from 4 (see www. question receiving points ranging from –3 –3 to to 4 (see www. www.biosciencemag.org www.biosciencemag.org
agric.wa.gov.au details). A final score is obtained adding agric.wa.gov.au forfor details). A final score is obtained by by adding individual scores. Thresholds “reject,” “accept,” “evalindividual scores. Thresholds forfor “reject,” “accept,” andand “evaluate further’” basis of minimizing number uate further’” areare setset onon thethe basis of minimizing thethe number of false positives (i.e., rejecting a benign species) false negof false positives (i.e., rejecting a benign species) andand false negatives (i.e., accepting invasive species) while maximizing atives (i.e., accepting an an invasive species) while maximizing accuracy. categorical thresholds unchanged accuracy. WeWe leftleft thethe categorical thresholds unchanged (> (> 6, 6, reject; 1, accept; 1–6, evaluate further), is standard pracreject; < 1,
