WETLANDS, Vol. 21, No. 2, June 2001, pp. 199±209 q 2001, The Society of Wetland Scientists IS PURPLE LOOSESTRIFE (LYTHRUM SALICARIA) AN INVASIVE THREAT TO FRESHWATER WETLANDS? CONFLICTING EVIDENCE FROM SEVERAL ECOLOGICAL METRICS Elizabeth J. Farnsworth New England Wild Flower Society 180 Hemenway Road Framingham, Massachusetts, USA 01701 E-mail: [email protected] Donna R. Ellis Department of Plant Science, Unit 4163 University of Connecticut Storrs, Connecticut, USA 06269 E-mail: [email protected] Abstract: Con¯icting interpretations of the negative impacts of invasive species can result if inconsistent measures are used among studies or sites in de®ning the dominance of these species relative to the com- munities they invade. Such con¯icts surround the case of Lythrum salicaria (purple loosestrife), a widespread exotic wetland perennial. We describe here a 1999 study in which we quanti®ed stand characteristics of L. salicaria and associated vegetation in arrays of 30 1-m2 plots in each of ®ve wet meadows in Connecticut, USA. We explored linear and non-linear relationships of above-ground plant biomass, stem density, and indices of species richness, diversity, and composition to gradients of L. salicaria dominance, including stem density, percent cover, and biomass. Species richness, other diversity metrics, and stem density of other species were not signi®cantly correlated with the density or percent cover of L. salicaria stems. The relative importance values (number of quadrats in which they were found) of co-occurring species in low-density L. salicaria quadrats were signi®cantly correlated with their relative importance in high-density L. salicaria quadrats, indicating that only modest shifts in abundance occurred as L. salicaria increased in density. No individual species were consistently associated with or repelled by the presence of L. salicaria across sites. In contrast to density and diversity features, however, the total biomass of species other than L. salicaria was signi®cantly, negatively correlated with the total biomass of L. salicaria at each site surveyed. Lythrum salicaria in pure, dense stands maintained a greater above-ground standing biomass on invaded sites than uninvaded vegetation of similar physiognomy. This study demonstrates that hypotheses about L. salicaria effects can vary depending upon the ecological metric that is examined. Where one-time, correlative studies are the most feasible option, data taken on a range of metricsÐespecially biomassÐshould be taken to inform us about mechanisms by which L. salicaria invades and predominates in wetlands. Key Words: Lythrum salicaria, purple loosestrife, invasive species, Connecticut, diversity, richness, wet- land, plant ecology INTRODUCTION tifying the threats posed by these species can we de- termine which invasive taxa demand immediate atten- Invasive species, especially those that are rapidly expanding their habitat range, are increasingly recog- tion from managers (Parker et al. 1999). It can be chal- nized to be signi®cant drivers of biotic interactions and lenging to quantify the effects of invasive species, es- resource ¯uxes in biological communities (Mack et al. pecially in sensitive ecosystems of conservation 2000). In developing coherent strategies to address the signi®cance where direct, manipulative experiments or control of these species, it is imperative to characterize planned introductions are unfeasible. Thus, many stud- thoroughly their actual ecological impacts on the com- ies have inferred negative impacts of invasive species munities they invade, particularly their capacity to dis- from correlative data linking increasing prevalence of place existing species and alter species richness of a given invasive species with a decrease in abundance plants and their associated consumers (Mack 1996, of other associated species. Likewise, these data fre- Blossey et al. in press). Only by systematically iden- quently are collected at a single sampling date and thus 199 200 WETLANDS, Volume 21, No. 2, 2001 cannot capture temporal changes in community struc- its capacity to displace other wetland species remain ture. Con¯icting interpretations of ``impact'' can result in doubt. if inconsistent ecological metrics measuring diversity, We suggest that part of this uncertainty is due to density, and other features are used from site to site in the fact that different studies employ different, and de®ning the relative dominance of these species. possibly incomparable, metrics to quantify the impacts Purple loosestrife, Lythrum salicaria L. (Lythra- of L. salicaria on co-occurring plant species. Numer- ceae), has been the subject of such con¯icting inter- ous ecological methods exist for quantifying the spa- pretations. Reportedly introduced in the early 1800s tial patterning of plant species and for detecting spe- via ships' ballast and livestock, and widely cultivated cies af®nities and competition (e.g., Mueller-Dumbois for its ornamental and pharmacological values, L. sal- and Ellenberg 1974, Whittaker 1975, Pielou 1975, icaria has more than tripled its areal extent in North Dale 1999). Innumerable past studies of plant com- America since 1940 and now occupies more than munities have made use of a variety of metrics, typi- 120,000 hectares of North American wetlands (Stuck- cally comparing two or more independent measures or ey 1980, Thompson et al. 1987, Galatowitsch et al. devising composite metrics (i.e., the relative impor- 1999). Due to its explosive range expansion and its tance value) from measures of several attributes of the increasingly common occurrence in wetlands, L. sali- system. By contrast, few studies of L. salicaria have caria has been identi®ed as a serious threat to the spe- employed more than one metric to characterize spatial cies richness of wetland ¯ora and fauna (Thompson et relationships between the invader and the invaded spe- al. 1987) and has been targeted for intensive biological cies (Table 1), nor have the metrics used to date re- control efforts (Malecki et al. 1993, Blossey et al. ¯ected the broad range of analytical tools available. In 1994, Muth and Hamburg 1998). The hypothesis that part, this may be due to the fact that the perceived L. salicaria is competitively dominant to other wetland urgency of the invasive threat has demanded very rap- species has been substantiated both by common-gar- id, often static, correlative ®eld studies. Likewise, so- den phytometer studies (Keddy et al. 1994, Gaudet and phisticated equipment, including ordination software Keddy 1995, Weihe and Neely 1997) and by long-term and geographic information systems technology, have data on L. salicaria performanceÐspeci®cally, estab- not necessarily been widely accessible to the com- lishment and percent coverÐin constructed wetlands munity of land managers who must address plant in- (Weiher et al. 1996). Another study of 12 Minnesota vasions head-on. wetlands (data re-examined from Emery and Perry We describe here a study of ®ve wet meadow sites 1995) shows that increased L. salicaria biomass was in which we quanti®ed multiple stand characteristics associated with decreased biomass of two Typha spe- of L. salicaria and associated vegetation and investi- cies. A common-garden experiment established in the gated linear and non-linear relationships among these Detroit River ¯oodplain demonstrated that L. salicaria variables across gradients of L. salicaria dominance. competitively displaces Typha angustifolia in terms of We used many of the methods employed in previous stem density, but only after several years of co-exis- studies, and added a few more simple measurements. tence (Mal et al. 1997). A gradual recovery of the den- We explored the null hypothesis that L. salicaria den- sity of graminoids (speci®cally grasses, sedges, and/or sity and biomass are not signi®cantly correlated with Typha) has also been noted following removal of L. the density, diversity, and biomass of other plant spe- salicaria using herbicides (Gabor et al. 1996) and cut- cies in these wetlands. We found, not surprisingly, that ting (Wilcox et al. 1988). Although we are concerned conclusions about L. salicaria effects vary widely de- with the relationship between L. salicaria and other pending upon the metric used to describe them. We plant species in this paper, we note that the species propose that several metrics should be examined composition of insect and bird species is also observed across a variety of wetland typesÐtogether with ma- to change with an increase in the prevalence of purple nipulative experiments where timing and conditions loosestrife (reviews in Thompson et al. 1987, Blossey permitÐin order to characterize these effects fully and 1999, Brown 1999). consistently. Other reviews, however, have found little conclu- sive, quantitative evidence for negative impacts of L. METHODS salicaria on plant species (Anderson 1995, Hager and McCoy 1998). Furthermore, a recent study of 41 plots We chose ®ve wetlands (Figure 1) throughout Con- along the Bar River, Ontario, Canada detected no re- necticut, each that contained a stand of L. salicaria lationship between the density of L. salicaria stems having a range of stem densities from 0 to between 30 and the richness of other riparian plant species (Tre- and 60 stems/m2. These sites also possessed the fol- berg and Husband 1999). Thus, despite two decades lowing characteristics: 1) a riparian wet meadow veg- of accounts, the actual dynamics of L. salicaria and etation assemblage (corresponding