1 CIPOLLINI AND SCHRADIN: IMPACTS OF RANUNCULUS FICARIA
3 Guilty in the Court of Public Opinion: Testing Presumptive Impacts and Allelopathic 4 Potential of Ranunculus ficaria
16 KENDRA A. CIPOLLINI AND KELLY D. SCHRADIN
7 Wilmington College, Wilmington, Ohio 45177
9 ABSTRACT.—Information about invasive species can be based primarily on anecdotal evidence, 10 indicating the need for further information. Ranunculus ficaria is an ephemeral riparian plant
11 species that is presumed invasive in the United States, despite the lack of any published 12 information on its impacts. Mechanisms by which R. ficaria may affect native plant species
13 include competition and allelopathy. We examined if R. ficaria negatively affected the growth
14 and reproduction of the native Impatiens capensis and, if so, whether it is by allelopathy, nutrient 15 competition or some combination thereof. We performed a fully-factorial field experiment in the 16 field, manipulating the presence of R. ficaria, nutrients, and allelopathy with the use of activated 17 carbon. The presence of R. ficaria tended to negatively affect survival of I. capensis. In the
18 absence of carbon, R. ficaria significantly decreased seed production, illustrating the negative 19 impact of R. ficaria. In the presence of carbon, there was no effect of R. ficaria, suggesting that
20 carbon may have ameliorated the negative allelopathic effect of R. ficaria. Nutrient competition
21 did not show strong effects. Despite its widespread identification as an invasive species, this is 22 the first study to demonstrate the negative impact of R. ficaria on a native species and the
1 Corresponding author: Phone: 937-382-6661 ext. 367, Fax: 937-383-8530, e-mail: KAL143@alumni.psu.edu
23 possible role of allelopathy in its success. Further, the negative impacts of this ephemeral species 24 persist well beyond its early growing season, which calls into question previous assumptions
about R. ficaria exerting effects primarily on other ephemeral species. 25
30 Invasive species threaten biodiversity on a global scale (Wilcove et al., 1998; McGeoch et al.,
31 2010) and are defined as those species that cause or have the potential to cause economic or 32 environmental harm, weighed against their benefits (NISC, 2006). Most naturalized plants are 33 introduced through the horticultural industry (Mack and Erneberg, 2002) and some can still be 34 purchased in some instances despite their invasive status (Harris et al., 2009; Axtell et al., 2010).
35 However, only a portion of naturalized species are actually categorized as invasive (Milbau and 36 Stout, 2008). As a result, there is much interest in characterizing the species traits and 37 introduction routes that make a species invasive (Lambdon et al., 2008; Milbau and Stout, 2008;
38 van Kleunen et al., 2010). Yet, in many of these studies the methods by which species are 39 identified as invasive are vague and based on expert opinion and anecdotal evidence, with little 40 scientific evidence (Blossey, 1999). Further, even when there is some published information, the 41 impacts of an invasive species can be overstated by the popular press (Lavoie, 2010), which may 42 lead to inappropriate response strategies or undue focus. The lack of information on the impact 43 of invasive species and on the possible mechanism of impact is an obstacle to effectively 44 prioritizing the control of invasive species during a time of dwindling resources.
45 Invasive plant species can negatively impact native species through a variety of mechanisms 46 (Levine et al., 2003). Invasive species can simply outcompete native species for above- and/or
below-ground resources (e.g., Kueffer et al., 2007; Cipollini et al., 200847 a; Galbraith-Kent and
48 Handel, 2008). Enhanced nutrient acquisition can lead to invasive species success. For example, 49 Centaurea maculosa acquired more phosphorus than surrounding native species which may have 50 increased competitive success (Thorpe et al., 2006). Additionally, Leishman and Thomson (2005)
51 found that invasive species had greater responses to nutrient addition than native species, thus 52 outcompeting the natives in nutrient-rich environments. Another mechanism by which invasive 53 species affect native communities is allelopathy (Hierro and Callaway, 2003; Ens et al. 2009).
54 Most plants produce secondary compounds (Ehrenfeld, 2006) that can affect an adjacent plant 55 either directly (Cipollini et al., 2008b) or indirectly through changing soil ecology (Stinson et al.,
56 2006; Mangla et al., 2008). Some allelopathic chemicals that have no negative impact in their 57 native environment may have negative effects in an invaded community, a mechanism coined the 58 ―novel weapons hypothesis‖ (Callaway and Aschehoug, 2000; Callaway and Ridenour, 2004;
59 Callaway et al., 2008; Thorpe et al., 2009).
60 Discovering impacts due to allelopathy can be done with careful experimentation (Inderjit and 61 Callaway, 2003). Allelopathic effects have been studied using activated carbon (e.g., Ridenhour 62 and Callaway, 2001; Cipollini et al., 2008a). Activated carbon adsorbs organic compounds,
63 including allelochemicals (Ridenour and Callaway, 2001). Addition of carbon can also has 64 effects on soil properties and plant growth in potting soil (Lau et al., 2008; Weisshuhn and Prati,
65 2009). Addition of nutrients is thought to help ameliorate any fertilizing effects of the addition of 66 activated carbon (Inderjit and Callaway, 2003). Activated carbon may also serve as a restoration 67 tool to change soil conditions in invaded soils (Kulmatiski and Beard, 2006; Kulmatiski, 2010).
68 Ranunculus ficaria L. (Ranunculaceae), or lesser celandine, is a groundcover, native to 69 Europe (Taylor and Markham, 1978; Sell, 1994), which appears to be affecting native plants in
forested floodplains in many US states (Swearingen 2005). There are five known subspecies, all 70
71 of which are found in the United States (Post et al., 2009). Ranunculus ficaria was first recorded
72 naturalized in the United States in 1867 (Axtell et al., 2010). As it is still being marketed by the
73 nursery industry (Axtell et al., 2010), it was likely imported for horticultural purposes. It was 74 recognized as a naturalized species in the Midwest in the 1980’s (Rabeler and Crowder, 1985)
75 and in southern states more recently (Krings et al., 2005, Nesom, 2008). Ranunculus ficaria is
76 currently documented in at least 21 US states, the District of Columbia, and 4 Canadian 77 provinces (USDA, 2010). It has been identified as invasive in 9 states and the District of 78 ). Columbia and is banned in Massachusetts and Connecticut as a noxious weed (Axtel et al., 2010
79 Ranunculus ficaria emerges before most native spring species, which may provide it with a 80 competitive advantage. Once established, it spreads rapidly across the forest floor to form a 81 dense monoculture, which native species seemingly cannot penetrate (Swearingen, 2005). 82 Hammerschlag et al. (2002) reported that R. ficaria created a monoculture in the Rock Creek
83 floodplain in Washington, DC and few native species re-colonized after its removal. It is thought 84 that R. ficaria, as a spring ephemeral, has impacts primarily on other spring ephemerals 85 (Swearingen, 2005). However, most all information on R. ficaria as an invasive species is
86 primarily anecdotal in nature. Unpublished and preliminary data indicate that presence of R.
87 ficaria is associated with reduced abundance and richness of native species (Hohman, 2005). 88 Ranunculus ficaria exhibits direct allelopathic effects on germination of some native species 89 (Cipollini, unpublished data), indicating that R. ficaria may have negative effects beyond the
90 spring time period through lingering allelopathic effects.
91 For our study, we examined if R. ficaria negatively affects the growth and reproduction of the 92 native annual Impatiens capensis and, if so, whether it is by allelopathy, nutrient competition or
some combination thereof. In the field we performed a fully-factorial experiment with the 93
94 treatments of R. ficaria (present and absent), carbon (present and absent), and nutrient addition 95 (present and absent). We expected that the presence of R. ficaria would have an overall negative
96 impact, that addition of nutrients would have an overall positive impact and that addition of 97 carbon would have no overall effect on the performance of I. capensis. If allelopathy were