Plant Ecol DOI 10.1007/s11258-006-9232-2 ORIGINAL PAPER Competition between Lythrum salicaria and a rare species: combining evidence from experiments and long-term monitoring Madlen Denoth Æ Judith H. Myers Received: 25 April 2006 / Accepted: 23 October 2006 Ó Springer Science+Business Media B.V. 2006 Abstract The rare endemic plant Sidalcea Removal of Lythrum significantly improved the hendersonii (Henderson’s checker-mallow) oc- vegetative performance of Sidalcea compared to curs in tidal marshes of the Pacific Northwest the removal of randomly selected native plants and may be threatened by Lythrum salicaria and the control treatment in the first year. In the (purple loosestrife), a European invader plant. second year, the performance of Sidealcea did not We compared the abundances of Lythrum and differ significantly with treatment. Removals did Sidalcea in a wetland in British Columbia (Can- not influence the reproductive performance of ada) in 1999 to those measured in 1979 to track Sidalcea in either year. A one-year additive changes in both species. Although the frequency experiment, carried out in pots, compared the of Sidalcea decreased by more than 50%, and that competitive effect of Lythrum on Sidalcea with of Lythrum increased by almost 20%, there was that of two native species. Lythrum’s impact on no significant relationship between the changes of Sidalcea was not consistently stronger than that of the two species. We assessed the potential effects the native species. Collectively, these results do of competition by Lythrum on Sidalcea in field not indicate a strong impact of Lythrum on the and patio experiments. In the field, we measured reproduction or abundance of Sidalcea. the response of Sidalcea to the removal of Lythrum over a two-year period and compared Keywords Invasive plant Á Purple loosestrife Á this to the response of Sidalcea to the removal of Removal experiment Á Sidalcea hendersonii Á native species and in unmanipulated control plots. Tidal wetland M. Denoth (&) Á J. H. Myers Department of Zoology, The University of British Introduction Columbia, 6270 University Boulevard, Vancouver, BC, Canada V6T 1Z4 e-mail: [email protected] The ecological and economic impacts of invasive species have been widely documented in terms of J. H. Myers e-mail: [email protected] their detrimental effects on native ecosystem processes, communities and populations (Vitousek Present Address: et al. 1996; Mack et al. 2000). Invasive plants could M. Denoth be particularly detrimental to rare and endemic De´partement de biologie, Unite´ d’e´cologie et e´volution, Universite´ de Fribourg, Chemin du species with small geographic ranges and popula- Muse´e 10, 1700 Fribourg, Switzerland tion sizes, and high ecological specializations. 123 Plant Ecol However, it has been pointed out that the Lythrum is thought to be a strong competitor detrimental effect of invasive species may be due to its high total biomass, tall plant size, and exaggerated (Gurevitch and Padilla 2004) due to fast growth (e.g., Gaudet and Keddy 1988; the failure to disentangle the often simultaneous Farnsworth and Ellis 2001), and the rapid spread effects of species invasions and other problems, and high abundance of Lythrum in many invaded such as habitat destruction, exploitation, pollution, areas have led to concerns that the plant may inbreeding, pollen limitation and herbivory (e.g., outcompete native plant species and degrade the Kery and Matthies 2004; Lavergne et al. 2005). To value of infested areas for wildlife (e.g., Mal et al. date, only few studies have quantified the effects of 1997; Weihe and Neely 1997; Blossey et al. 2001). invasive species on rare native species (but see Consequently, a biological control program Walck et al. 1999; Thomson 2005). against Lythrum was initiated in North America Sidalcea hendersonii S. Wats., Henderson’s in the 1990s (Malecki et al. 1993). While there is checker-mallow (henceforth Sidalcea) is native evidence that the progam is successful in many to wet meadows and tidal marshes from south- circumstances, the tidal conditions in Ladner eastern Alaska to northwestern Oregon. The marsh prevent the establishment of the most majority of populations occur in Washington promising biological control organism, the leaf- state, with a few outlying occurrences in Oregon, feeding, chrysomelid beetle Galerucella calmari- British Columbia and Alaska (NatureServe 2005). ensis L. (Denoth and Myers 2005). Only 21–100 scattered populations of the plant The aim of this study was to measure the impact persist, and the species is therefore vulnerable to of Lythrum on Sidalcea in Ladner marsh in order extirpation or extinction across its range (Natu- to assess the risk posed by the invader on the rare reServe 2005). In a recent survey, 90% of the plant, and to evaluate the need for efficient control previously located populations in Oregon could measures. Our approach was three-tiered: First, not be refound (NatureServe 2005). Human we determined whether the abundance of encroachment into wetlands and displacement Lythrum had increased and was accompanied by by Lythrum salicaria L., purple loosestrife a decline of Sidalcea over a 20-year period in (henceforth Lythrum), a European invader plant, Ladner marsh; data from a previous study in are thought to contribute to the species’ rarity Ladner marsh (Bradfield and Porter 1982) allowed (Marshall 1998). However, the actual causes of a comparison of the abundance of the invader and the species’ decline are unknown. the native species in 1979 and in 1999. Second, we Only two large populations (>1,000 individu- measured the ability of Lythrum to suppress als) of Sidalcea are known from southwestern Sidalcea relative to co-occurring, native species British Columbia: one is at a tidal site near in Ladner marsh through a removal experiment. Duncan on Vancouver Island, where Lythrum Third, we carried out a competition experiment does not occur, and the other is in Ladner marsh with an additive design to evaluate the impact on (49°6¢ N, 123°5¢ W), a tidal wetland along the Sidalcea growth and reproduction of the exotic Fraser River (Marshall 1998). The water in species compared to that of two native species. Ladner marsh is mostly fresh except in the winter, The latter was done since removal experiments when brackish water may enter the marsh due to may result in biased measures of competition if the low discharge rate of the Fraser River. Tidal strong competition has led to spatial segregation of water levels may vary by up to 3 m during the the competitors (Freckleton and Watkinson 2000). growing season from May to September (Brad- field and Porter 1982). Fifty-five vascular plant species, mostly forbs and grasses, occur in Ladner Materials and methods marsh, and among them is Lythrum, which has invaded the wetland before 1979 (Bradfield and Natural history of Sidalcea Porter 1982); in some areas, the vegetation consists of high-density patches of the invader Sidalcea can grow conspicuously tall, reaching plant. 1.5 m in height, and can produce multiple stems 123 Plant Ecol with inflorescences in the first year. The plant is We applied three treatments: In the control gynodioecious, with both hermaphroditic, self- treatment, we carried out no manipulation; in the compatible flowers, and female flowers (Marshall Lythrum removal treatment, we killed all and Ganders 2001). Individuals flower from June Lythrum plants; and in the native-removal treat- through September and are pollinated by a ment, we killed randomly chosen native plants, variety of insects, including bumblebees and bees but not Lythrum, to compare the responses to (Marshall 1998). In 1995 and 1996, Marshall and removal of Lythrum to that of the removal of Ganders (2001) found that more than 20% of the native species. We used a randomized complete yearly seed production was consumed by two block design with nine blocks, totaling 27 exper- specialized weevils: by Macrorhoptus sidalcea imental plots. We established the blocks in eight Sleeper in Ladner marsh, and by Anthonomus different areas of the marsh that were at least melancholicus Dietz in Duncan, respectively. 100 m apart. Plots within blocks were separated by at least 5 m. Densities of Lythrum and Sidalcea in Ladner To remove plants, we wiped all leaves of shoots marsh in 1979 and 1999 selected for removal once with Round-upÒ,a systemic, non-selective herbicide in July of 2000 In 1999, we estimated the abundance of Lythrum only. Round-upÒ kills the root system, thereby and Sidalcea, following the methodology of reducing both above- and below-ground compe- Bradfield and Porter (1982). We recorded the tition. In the native-removal treatment, we wiped presence and the ground cover of the two species one shoot of a native plant with Round-upÒ for in 100 1-m2 quadrats in seven transects, which we every shoot of Lythrum present in the plot. To laid out in approximately the same areas of the imitate the spatial pattern of Lythrum in the plots, wetland as in the previous study (see Fig. 1 in we only selected shoots of native species adjacent Bradfield and Porter 1982). We compared the average frequencies of Lythrum and Sidalcea in a 8 the seven transects between years by permutation C LR tests (Lunneborg 2000) that analyze the outcome 7.5 NR of paired t-tests. We also performed a permuta- tion test to assess the relationship (Pearson 7 correlation) between changes in frequencies of LS Means 6.5 the two species in the seven transects. We ln (shootlength) performed both analyses using the software 6 Resampling, which is available at the website: b 30 http://www.uvm.edu/~dhowell/StatPages/Resam- pling/Resampling.html. The number of iterations 25 was 10,000. 1/2 20 i) ( R 15 Competitor removal experiment in Ladner LS Means marsh 10 5 12 Experimental design years Experimental plots were 1 m2 and contained both Fig.