Chloroplast DNA sequences and recently established morphological characters were used to confirm the widespread invasion of California's vernal pools by European low mannagrass. Morphological similarities between low mannagrass and western mannagrass have led to different taxonomic treatments, depending on the geographical extent of a particular flora. When California's flora was last revised, the two species were combined as western mannagrass, which was then considered to be a native species. Unfortunately, the revised flora was published just as low mannagrass began to rapidly expand its range within the state and, because it was considered to be a native species in the new flora, no actions were initiated to limit the invasion. Our data show that low mannagrass was present at all localities in the Central Valley of California that were investigated, indicating a widespread and undetected invasion. The invasion has led to a degradation of the vernal pool ecosystems, which are the habitat of many federal and state protected endangered and threatened species.
Nomenclature: Low mannagrass, Glyceria declinata Bréb.; western mannagrass Glyceria occidentalis (Piper) J. C. Nelson.
Interpretive Summary
Changing taxonomic treatments can lead to confusion or uncertainty in the native status of plant species. This can prevent timely management action. A delay during the early establishment stage can permit a nascent invasive species to expand its range to the point where eradication is impossible and the only reasonable management action is local control.
Morphological similarities between low mannagrass (Glyceria declinata) and western mannagrass (Glyceria occidentalis) have led to different taxonomic treatments, depending on the geographical extent of a particular flora. When California's flora last was revised, the two species were combined as western mannagrass, which was then considered to be a native species. Unfortunately, the revised flora was published just as low mannagrass began rapidly to expand its range within the state and, because it was considered to be a native species in the new flora, no actions were initiated to limit the invasion.
Vernal pools along the east side of the Central Valley are especially prone to being invaded by low mannagrass, which has been found in very high densities in some surveyed pools. Vernal pools are considered a major conservation priority because they contain many endemic plant and animal species, of which several have been given formal protection as threatened or endangered species.
This incident shows that it is important to consider the possibility of the introduction of foreign populations or of unrecognized species when rapid range expansion of a putative native species occurs. This is especially true if, as was the case with low mannagrass, revised taxonomic treatments lead to the inclusion of a foreign species into a native one.
Taxonomic confusion can be a serious barrier to the early detection, risk assessment, rapid response, and implementation of control methods. While there may be many technical causes for taxonomic confusion, here we use the term to describe a situation in which changing taxonomic treatments lead to confusion or uncertainty in the native status of plant species. This can prevent timely management action. Ultimately, a delay during the early establishment stage can permit a nascent invasive species to expand its range to the point where eradication is impossible and the only reasonable management action is local control.
Mannagrass (Glyceria), a genus of approximately 40 species, is represented in North America by approximately 15 species. During their evaluation of the genus for the Poaceae section of the Flora of North America, Barkworth et al. (2007) determined that it was impossible to distinguish some Glyceria species, including low mannagrass and western mannagrass from each other using published morphological characteristics. These morphological similarities have led to the species being either separated or combined depending on the geographical extent of a particular flora. Western mannagrass [Glyceria occidentalis (Piper) J. C. Nelson] is distributed in the western United States and Canada, south to northern California, and low mannagrass (Glyceria declinata Bréb.) is a Eurasian species. The first herbarium specimen of low mannagrass in California was collected in 1946 (Calflora 2000). When California's flora was revised in 1993 (Hickman 1993) the two species, which were previously treated as separate species (Munz and Keck 1968), were combined as western mannagrass, which was then considered to be a native species.
After the 1993 revision of the flora, the “western” mannagrass began a period of rapid range expansion in California's Central Valley (CH2MHill 1998; Pacific Mutual Consultants 2005). Vernal pools along the east side of the Central Valley are especially prone to being invaded by mannagrass, which has been found in very high densities in some surveyed pools. Vernal pools are considered a major conservation priority because they contain many endemic plant and animal species, of which several have been given formal protection as threatened or endangered species.
Whipple et al. (2007) sequenced chloroplast DNA to separate the species and to identify unique morphological features for taxonomic keys. During their molecular analysis they tested two samples of low mannagrass from Denmark; one sample from Oregon; and seven California herbarium samples from Pacific coastal areas, the Sierra Nevada foothills, and a degraded vernal pool in the Central Valley. All samples were determined to be low mannagrass and the clearest morphological characteristic identifying low mannagrass was found to be the presence of two lobes on either side of the lemma tip (see Fact Sheet images at http://utc.usu.edu/). Whipple et al. (2007) found that low mannagrass contained chloroplast sequence haplotypes that differ from those found in western mannagrass; this indicates that the 1993 taxonomical treatment of the species was misleading. In addition, their determination that low mannagrass was present at one site in the Central Valley of California revealed that it is likely that the observed invasion of vernal pools in California is caused by low mannagrass, an exotic species. Because of the importance of the California's Central Valley vernal pool ecosystems for threatened and endangered species, we undertook a study to confirm the taxonomic status of Glyceria occurring in the vernal pool system in California by sampling a larger number of vernal pool sites, including those associated with herbarium specimens.
Materials and Methods
Herbarium specimen data were consulted to identify all Central Valley locations of Glyceria species. Distribution information for either low mannagrass or western mannagrass in vernal pools in California's Central Valley and in two other nearby vernal pool regions that are closer to the Pacific coast was taken into account. Most vernal pools are located on private ranches or in vernal pool preserves. We used the collection data of herbarium specimens as well as publicly available information from conservation agencies to determine accessible localities.
Central Valley specimens of Glyceria deposited in the University of California Davis herbarium were examined. Low mannagrass has a clearly wavy to dentate lemna tip whereas western mannagrass does not, and low mannagrass has the more evidently bifurcate palea (Munz and Keck 1968; Whipple et al. 2007; M. Barkworth personal communication; see figures at http://herbarium.usu.edu/webmanual/). Lemna tip morphology is a character that easily is accessible in the field and effectively differentiates low mannagrass from every other congeneric species. Using these characters, the occurrence of mannagrass in 24 accessible localities have been confirmed during botanical surveys (Tables 1 and 2). One individual from each of six locations was collected for DNA sequence analysis to verify the result of the field surveys. This collection took place across a variety of vernal pool types in the central portion of the distribution. Fresh leaf samples were frozen in liquid nitrogen, freeze dried, and ground. DNA extraction was performed using the ChargeSwitch kit1 following the manufacturer's protocols. Voucher specimens of each occurrence evaluated with our molecular markers were submitted to the University of California Davis Herbarium.
Table 1
List of herbarium records that were used to infer the distribution of low mannagrass in Central Valley vernal pools in California. Determination was performed in the field using the characters described in the text. The status of six accessions as low mannagrass was verified using DNA sequence analysis of chloroplast loci according to Whipple et al. (2007).
Table 2
List of surveys in the field (Visited localities) that were used to infer the distribution of low mannagrass in Central Valley vernal pools in California. For localities visited in the course of this study, UTM coordinates are listed. Determinations were performed in the field using the characters described in the text. The status of six accessions as low mannagrass was verified using DNA sequence analysis of chloroplast loci according to Whipple et al. (2007).
A 600-bp sequence of the trnK(AAA)-rps16 spacer was sequenced comparatively. Amplification and sequencing were performed using the trnK-rps16 primer set and methods described in Whipple et al. (2007). Sequences were aligned manually and compared with their results (Genebank accession numbers DQ665551 to DQ665637). Phylogenetic and distance analysis was performed using the program PAUP (Swofford, 2002) with the indels included and coded as presence/absence data.
Results and Discussion
All specimens of the genus Glyceria of the University of California Davis Herbarium, that had been collected from Central Valley vernal pools, showed lemna morphology that is characteristic of low mannagrass. In addition, specimens at all localities that we surveyed were determined to be low mannagrass. We were not able to visit reported field survey locations at the northernmost site, the southern group of sites, and the two northwestern sites, but the presence of low mannagrass seems likely because local botanical surveys, California State University Chico Herbarium collection location data, and/or other nongeoreferenced surveys in those areas found low mannagrass to be present (Figure 1). None of the occurrences in vernal pools that we surveyed or present in collections in the University of California Davis Herbarium were found to be western mannagrass. Including recently determined herbarium specimen from other herbaria (Tables 1 and 2), a total of 52 locations in the Central Valley for low mannagrass were documented (Figure 1).
Figure 1
Map of the Central Valley vernal pool system in California, indicating the localities investigated. Identification of Glyceria declinata was performed by sequencing of chloroplast DNA, morphological examination of collected specimen, and examination of herbarium specimen.
All trnK-rps16 spacer sequences determined were identical to sequences obtained from low mannagrass by Whipple et al. (2007); gi: DQ665567 to 665576. Maximum Parsimony analysis of the spacer region of Glyceria samples investigated in this study in comparison to samples investigated by Whipple et al. (2007) indicated that, with the exception of marked mannagrass (Glyceria notata Chevall), all species differ at least in two positions from low mannagrass (Figure 2). Western mannagrass shares chloroplast haplotypes with two species: narrow mannagrass, (G. leptostachya Buckley, western United States) and water mannagrass (G. fluitans L., Eurasia), indicating a hybrid origin of western mannagrass from these two species (Whipple et al. 2007). However, all samples of western mannagrass included in the study of Whipple et al. (2007) are clearly differentiated from samples from low mannagrass using the trnK-rps16 spacer.
Figure 2
Maximum parsimony analysis of trnK-rps16 spacer sequences of samples investigated in this study (bold) and Glyceria species published by Whipple et al. (2007; DQ665551 to DQ665637). Only one sequence per species was included and each differed in at least in one position from the remaining samples. Shown is a phylogram of one randomly chosen tree from the 150 shortest trees of 83 steps. Bootstrap values from 1,000 replicates are indicated below the branches. All investigated Glyceria samples were equivalent to published low mannagrass (G. declinata) sequences and differed in at least two positions from western mannagrass (G. occidentalis) and from water mannagrass (G. fluitans).
The presence of chloroplast haplotypes from low mannagrass supports our analyses of lemna morphology. This suggests that only low mannagrass is present in the Central Valley (Figure 1). Low mannagrass is a known dominant of vernal pool ecosystems in Portugal and Spain (Gallego-Fernández et al. 1999; Molina 1996; Rivas-Martínez 2002). It is easily dispersed by seeds that are able to float and that become attached to waterfowl and grazing animals (Hubbard 1942). Our results indicate that this species has been introduced in California's Central Valley where it now is probably the only Glyceria species invading vernal pools.
The recent invasion of California's Central Valley vernal pools by low mannagrass provides an example of how taxonomic confusion can result in the widespread invasion of a rare ecosystem, despite close observation by the federal and state endangered and threatened species programs (Rogers 1998; Volmar 2002) and invasive species risk assessment programs (CalEPPC 1999). Many human activities in the invaded vernal pool ecosystems are regulated by the U.S. Army Corps of Engineers (Corps) under Section 404 of the U.S. Clean Water Act, and the U.S. Fish and Wildlife Service (USFWS) under Section 7 of the U.S. Endangered Species Act, because vernal pool ecosystems have been designated as critical habitat for federally listed threatened and endangered species. Because of these regulatory programs, the recent rapid range expansion of low mannagrass has been extensively documented under the Corps' wetland permitting process and reported in both federal (see CH2MHill 1998) and nonfederal environmental review processes (Pacific Mutual Consultants 2005).
Field collection labels associated with herbarium specimens (University of California Davis and California State University Chico) suggest that that the 1993 flora, which treated both species as conspecific, was published just as low mannagrass began rapidly to expand its range within California. The consolidation into a single native species effectively stopped all management pending further research. This incident shows that it is important to consider the possibility of the introduction of foreign populations or of unrecognized species when rapid range expansion of a putative native species occurs. This is especially true if, as was the case with low mannagrass, revised taxonomic treatments lead to the inclusion of a foreign species into a native one.
Acknowledgments
This research would not have been possible without the generous assistance of many individuals: J. Buck, J. Dittes, J. Gallego-Fernández, D. Gluesenkamp, F. Hrusa, G. Leppig, J. Marty, C. White, and C. Witham shared their distribution and survey information; J. Shepard (University of California Davis Herbarium) and K. Schierenbeck (California State University Chico Herbarium) provided invaluable access to herbarium collections and distribution data; B. Allen prepared Figure 1 and provided GIS assistance; K. Rice provided laboratory space and equipment; B. Rice posted a Red Alert on The Nature Conservancy Global Invasive Species site ( http://tncweeds.ucdavis.edu/alert/alrtgly2.html) which includes photographs of low mannagrass and its environment.
