UC Agriculture & Natural Resources California Agriculture Title Predicting invasive plants in California Permalink https://escholarship.org/uc/item/2s20g5k8 Journal California Agriculture, 68(3) ISSN 0008-0845 Authors Brusati, Elizabeth D Johnson, Douglas W DiTomaso, Joseph Publication Date 2014 Peer reviewed eScholarship.org Powered by the California Digital Library University of California RESEARCH ARTICLE Predicting invasive plants in California by Elizabeth D. Brusati, Douglas W. Johnson and Joseph M. DiTomaso Preventing plant invasions or eradicating Joseph M. DiTomaso incipient populations is much less costly than confronting large well-established populations of invasive plants. We devel- oped a preliminary determination of plants that pose the greatest risk of becoming invasive in California, primarily through the horticultural industry. We identified 774 species that are invasive elsewhere in Mediterranean climates but not yet invasive in California. From this list, we determined which species are sold through the horticulture industry, whether they are Giant reed (Arundo donax) infesting a wetland area in Southern California. Giant reed was introduced as both an ornamental and erosion control species and is now one of the most invasive species in sold in California and whether they have the state. been reported as naturalized in California. The geographic diversity of California cause, or have the potential to cause, We narrowed the list to 186 species with the has led to broad evolution in native economic damage to the state’s agricul- greatest potential for introduction and/or plants. California has approximately 3,400 tural industry; CDFA has legal authority invasiveness to California through the hor- species of native plants, of which 24% are to regulate plants on this list through found only in the state (Baldwin et al. Section 4500 of the California Code of ticultural trade. This study provides a basis 2012). However, California is also some- Regulations (CDFA 2013). Because the cri- for determining species to evaluate further thing of a hotspot for nonnative plants, teria for these lists have a different focus, through a more detailed risk assessment with over 1,500 nonnative species natural- the listed species overlap but are not the that may subsequently prevent importation ized, weedy in agricultural systems or same. Few species derived from the hor- invasive in natural areas (DiTomaso and ticultural trade are included on the state via the horticultural pathway. Our results Healy 2007). As a result, California not Noxious Weed List. can also help land managers know which only faces a high species to watch for in wildlands. risk of escape, The high number of invasive plants with establishment and invasion of horticultural origin makes it critically important lants have been transported around introduced or- to natural resource managers, ecologists and Pthe world for centuries, as agricul- namental plants, tural commodities, ornamental species but also has a policymakers to predict which newly introduced or inadvertent contaminants of imported high proportion species pose the greatest risk of escape and invasion. materials. Naturalized plants are those of native species that have spread out of cultivated areas, threatened by invasive plants. The horticultural trade is one of the including gardens, into more wild areas, Within California, there are two lists major pathways for invasive plants in and invasive plants are the subset of that identify invasive plants. First, based California and elsewhere (Drew et al. naturalized species that cause ecologi- on 13 questions that assess impacts, inva- 2010; Okada et al. 2007; Reichard and cal or economic harm. In general, only a siveness and distribution, the California White 2001). For example, higher market small proportion of plants introduced into Invasive Plant Council’s list includes frequency (as measured by availability a new region have been invasive plants. 214 species that cause ecological harm in seed catalogs) and lower prices were However, the number of invasive plants in the state’s wildlands (Cal-IPC 2013). shown to be good predictors of a plant’s with horticultural origin is high, making Approximately 63% of these species were probability of invasion in Britain (Dehren- it critically important to natural resource deliberately introduced to California, Schmutz et al. 2007). Horticulture is also managers, ecologists and policymak- mostly as ornamental plants (Bell et al. ers to predict which newly introduced 2007). Second, the California Department Online: http://californiaagriculture.ucanr.edu/ species pose the greatest risk of escape of Food and Agriculture (CDFA) Noxious landingpage.cfm?article=ca.v068n03p89&fulltext=yes and invasion. Weed List primarily lists plants that doi: 10.3733/ca.v068n03p89 http://californiaagriculture.ucanr.edu • JULY–SEPTEMBER 2014 89 established invasive plants reported from states neighboring California, including Arizona (Northam et al. 2005), Nevada (Nevada Department of Agriculture 2005) and Oregon (Oregon Department of Agriculture 2006). We included spe- cies on the California Noxious Weed List (CDFA 2007) as well as those that have Joseph M. DiTomaso Joseph M. DiTomaso been shown to invade wildlands (Cal-IPC After being introduced as an animal forage species, kudzu (Pueraria montana) escaped to invade forested areas in the southern United States. Kudzu is neither naturalized nor sold in California. 2013; personal communications with land managers in California). a major agricultural sector in California, considered the single most important Of the plants that have invaded other accounting for $2.5 billion in sales in 2011 factor to be a species’ invasiveness in Mediterranean regions, we first removed (CDFA 2012). other areas of the world with a similar species native to California and those The ability to predict potential inva- Mediterranean climate or in a state neigh- already known to be invasive in wildland siveness is important both for species that boring California. While we recognize areas within the state. Then for each of have already been introduced to a region that this list is not comprehensive, we the remaining plant species, we evaluated but are not yet invasive and for species believe that it provides a good starting the Mediterranean-type region(s) invaded, that may be introduced through the hor- point for subsequently conducting risk location of origin, human uses (especially ticultural industry in the future. In both assessments that could reduce the threat in horticulture) and whether the species cases, prediction of invasiveness before it of introducing new invasive ornamentals was native, cultivated, naturalized or in- occurs can, through collaborative efforts to the state. This approach might also vasive in California (Baldwin et al. 2012; with the nursery industry, lead to volun- help determine which naturalized species Cal-IPC 2013). For species already natural- tary restrictions in sales, preventing the should be monitored to see if they will ized but not yet invasive in California, we potential for damage should the species become truly invasive. determined the year they were first re- escape cultivation. ported as naturalized based on the online Identifying potential invaders Knowing that a plant is invasive in Consortium of California Herbaria data- one region can give insight into whether Invasive plant data were collected base (ucjeps.berkeley.edu/consortium/). it might be problematic in another through online databases and pub- In addition, we determined if plants are region, particularly if the two regions lished lists from other regions with currently sold in the horticultural and have similar climates. For woody or- Mediterranean climates. We also used ornamental trade in California using the namental species, for example, being invasive elsewhere was the single best 774 predictor of potential invasiveness in a Plants invasive in other Mediterranean regions or adjacent new region of introduction (Reichard U.S. states but not yet invasive in California and Hamilton 1997). In addition, Caley and Kuhnert (2006) showed that four variables were most important for screen- ing potential invasive plants: human dispersal, naturalized elsewhere, inva- 408 366 Native or already naturalized or Not naturalized in California siveness elsewhere and a high degree of invasive in California domestication. Two of these variables, human dispersal and high degree of domestication, are characteristics of horticultural species. California is one of five Mediterranean 116 70 318 Ornamentals Ornamentals Eliminated climate regions in the world, along with the Mediterranean Basin of Europe and northern Africa, central Chile, the Cape Region of South Africa and western Australia. All these regions are char- 22 94 10 60 acterized by a winter rainy season and Not sold in Sold in Not sold in Sold in California California California California a summer dry season and are likely (table 1) (table 2) (table 3) (table 4) to share invasive species due to their similar climates. The primary objective of this study Fig. 1. Process used to determine the species with potential to become invasive in California through was to identify ornamental species at surveys of other Mediterranean climatic regions and adjacent U.S. states, with a focus on species sold as ornamentals. Tables 1 to 4 list these 186 species (22 + 94 + 10 + 60). The 318 eliminated species were high risk of becoming newly invasive eliminated because