BC Conservation Data Centre: Conservation Status Report
Callitriche marginata
winged water-starwort
| Scientific Name: | Callitriche marginata |
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| Scientific Name Synonyms: |
Callitriche longipedunculata
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| English Name: | winged water-starwort |
| Provincial Status Summary | |
| Status: | S3S4 |
| Date Status Assigned: | April 30, 2019 |
| Date Last Reviewed: | April 30, 2019 |
| Reasons: | Limited and fragmented in distribution, Callitriche marginata is only known from twelve populations in British Columbia. With specific habitat requirements (seasonal flooding), fluctuating populations, and annual life cycle, this species may be vulnerable to extirpation. Three subpopulations found in flooded agricultural field locations suggest this species may be less rare, vulnerable and at risk than previously considered. A study on the Saanich peninsula in flooded fields concluded that while potential habitat appears plentiful, Callitriche is still limited in distributon in this habitat type. Further studies up island will be important for informing the status. |
| Range | |
| Range Extent: | D = 1,000-5,000 square km |
| Range Extent Comments: | Callitriche marginata extends from Mitlenatch Island to Rocky Point near Victoria in southwestern B.C. The extent of occurrence for Callitriche marginata is approximately 2,615 square kilometres, calculated using GIS tools (minimum convex polygon). However, suitable habitat is rare and fragmented within this range (M. Miller, pers. comm. 2008). |
| Area of Occupancy (km2): | D = 6-25 |
| Area of Occupancy Comments: | The estimated area of occupancy is 60 square kilometres or 15 - 4 square kilometre grid cells. Using a 1 x 1 km grid cell for these primarily tiny populations, the area of occupancy is 11 square km. Excluding agricultural areas, the actual area of habitat is <0.08 square km; (M. Miller, pers. comm. 2008). |
| Occurrences & Population | |
| Number of Occurrences: | B = 6 - 20 |
| Comments: | Twelve populations of Callitriche marginata are currently known in B.C. Eight of these were confirmed in 2002 and 2003 during field work conducted in preparation for a COSEWIC status report (M. Miller, pers. comm. 2008). Since that field work, this species has been discovered in three agricultural fields (J. Fenneman, pers. comm. 2007; J. Penny, pers. comm. 2009), and on Portland Island (M. Fairbarns, pers. comm. 2009). Populations separated by greater than 500 m were considered to be distinct (M. Miller, pers. comm. 2008). Some of the populations were comprised of subpopulations (separated by greater than 50 m but less than 500 m). A total of 16 subpopulations were recorded, including one that is now considered to be extirpated. Other locations are very likely - agricultural fields from Victoria to Nanaimo should be checked. Surveys in the Victoria area uncovered three subpopulations but inventory further north is needed. A potential report from Harewood Plains, Nanaimo in natural habitat also exists (R. Batten, pers. comm. 2010). |
| Number of Occurrences with Good Viability / Ecological Integrity: | C = 4 - 12 |
| Comments: | There may be four viable populations based solely on population size. Excluding the agricultural field locations, using patch size as a surrogate for population size, only four element occurrences had populations larger than 1 square metre (or larger than 200-500 stems [M. Miller, pers. comm. 2008]). This number of viable populations agrees with the number of element occurrences considered to have good estimated viability using B.C. Conservation Data Centre (CDC) Element Occurrence reports (2005); however, the CDC lumped two populations that Miller considered to be distinct, which resulted in a total of three viable element occurrences. The yearly fluctuations in population size for annuals such as Callitriche marginata as well as the difficulty in counting individuals in mat-forming species makes it difficult to estimate viability from a single population count. |
| Number of Occurrences Appropriately Protected & Managed: | B = 1 - 3 |
| Comments: | Three populations occur in provincial parks and are protected under the provincial Park Act. Another four populations occur on federal lands: two occur in National Parks (Parks Canada), and two occur on properties owned by the Department of National Defense. The latter are protected since public access to the properties is restricted. Another two populations occur on Indian Reserve lands, which may, along with the four other populations on federal lands, receive legal protection under the Species at Risk Act if Callitriche marginata is listed by COSEWIC. None of the populations of C. marginata is being managed appropriately for long-term persistence. The agricultural field locations are not protected though land-owners are aware of their existence. Callitriche marginata has persisted in these sites through different management regimes and thus, may be able to persist under a variety of conditions, as long as flooding regimes are maintained. |
| Population Size: | F = 10,000 - 100,000 individuals |
| Comments: | The number of mature individuals was estimated as possibly more than 50,000 fruiting stems in May 2008 using patch size as a surrogate for population size (1 square metre = 200-500 stems [M. Miller, pers. comm. 2008]). It is unclear how many genetically distinct individuals this represents. The total population size, including total emergent individuals and dormant seeds in the seed bank, is likely much larger but can not be estimated until more information on seed dormancy rates is known (M. Miller, pers. comm. 2008). In 2009, ca. 20,900 plants were enumerated in three agricultural fields (2 occurrences). As an annual plant, the population size should be de-emphasized in determining status. |
| Threats (to population, occurrences, or area affected) | |
| Degree of Threat: | Substantial, non-imminent threat |
| Comments: |
Mike Miller (pers. comm. 2008) identified five major potential threats to the persistence of Callitriche marginata: habitat loss due to urbanization and secondary succession, alterations to hydrological regimes, competition for light and water from invasive woody shrubs and introduced annual grasses, water contamination by seabird guano, and disturbances from recreational foot and bicycle traffic. The primary threat is invasive species especially woody ones encroaching on vernal pools. Vernal pools have likely declined in quality and area with the Garry oak ecosystems and continued decline is expected. Threats that apply directly to vernal pool micro-ecosystems include changes to drainage patterns as a result of land management modifications, secondary succession or livestock grazing, competition from invasive plants (including buildup of organic litter associated with them), overgrazing by waterfowl, eutrophication (e.g., due to seabird guano), agricultural runoff, herbicide exposure, and mechanical disturbance caused by heavy foot and bicycle traffic. In terms of recreational pressure, some light trampling or mowing/grazing appears to benefit C. marginata by inhibiting the invasion of weedy competitors. Because of their proximity to the ocean, another threat to C. marginata populations is marine pollution, such as oil spills (M. Miller, pers. comm. 2008). Many of the vernal pools in B.C. occur just above the intertidal zone and are affected by salt spray during storms. There is likely a pervasive moderate threat based on degradation of habitat (probably continuing since European settlment). The agricultural field sub-populations, the largest in the province are in the ALR designation but that does not protect this population from the impacts of farming on floodplains and sensitive ecosystems. However, all the land-owners are now aware of the species existence, and the largest population is likely to be afforded at least some protection. If flooding regimes are significantly disturbed at any of these site either on purpose or inadvertantly, Callitriche marginata could be lost from these areas. |
| Trend (in population, range, area occupied, and/or condition of occurrences) | |
| Short-Term Trend: | G = Relatively Stable (<=10% change) |
| Comments: | Little is known about recent changes in population numbers, size and area of occupancy. No monitoring of population trends (e.g. year-to-year variation in population abundance and density) for Callitriche marginata has been done. Mike Miller (pers. comm. 2008) suggested that the extent of occurrence has been relatively stable over the past 10 years, while population size and area of occupancy have apparently been increasing. It is uncertain how the trend of an apparent increase was calculated. One population was extirpated in the past 10 years, likely due to seabird guano contamination of the vernal pool site (M. Miller, pers. comm. 2008). In the past 10 years, the number of known localities has increased from 4 to 11, but this is likely attributed to increased search intensity in recent years (M. Miller, pers. comm. 2008). Miller also suggested (pers. comm. 2008) that C. marginata was likely under-collected in the past owing to its small size, non-descript appearance and morphological similarity to more common Callitriche species. The largest population (15,000-60,000 plants or up to 80% of the current population) could be affected by future changes to the land use. However, the land owners are aware of its existence and the overall goals for the parcel are conservation-oriented. |
| Long-Term Trend: | U = Unknown |
| Comments: | Owing to the relatively recent discovery of Callitriche marginata in B.C. (1977 on Mitlenatch Island), there is no historic information about its range and population trends. It is known, however, that suitable habitats (i.e., shallow coastal vernal pools) for C. marginata have declined in abundance and quality during the last century (Fuchs 2001), but there is little specific information about the historic trends in the range and abundance of vernal pools on southern Vancouver Island and adjacent islands (Miller 2003). There are fewer than 100 functioning vernal pools in southwestern B.C. (H. Roemer, pers. comm. in Miller 2005). Vernal pools are one component of the Garry oak ecosystem (Fuchs 2001). Natural and anthropogenic fires (burning by First Nations peoples) have been suppressed since European settlement of southern Vancouver Island, which has reduced open savanna vegetation (and likely vernal pools) and increased shrubby and forested ecosystems. In the past 150 years, urbanization (agricultural, residential, and industrial development) of southern Vancouver Island has reduced the Garry oak ecosystem to <5% of its original extent, and even this remnant habitat has been modified by human activities (mostly introduced plant species) (Fuchs 2001). |
| Other Factors | |
| Intrinsic Vulnerability: | C=Not intrinsically vulnerable |
| Comments: | The life cycle of C. marginata is closely tied to annual precipitation and temperature patterns, which may make it vulnerable to global climate change, at the northern extent of its range in B.C. (Lesica and Allendorf 1995 in Miller 2003). |
| Environmental Specificity: | B=Narrow. Specialist or community with key requirements common. |
| Comments: | Callitriche marginata was once thought to have a very narrow environmental specificity. Formerly mostly found in vernal pools on low-lying coastal rocky knolls, bluffs, and outcrops within the Garry oak ecosystem in south coastal B.C. (Miller 2003), it is now known from some flooded agricultural fields in Saanich. Though the breadth of the habitat selection is now known to be wider, as with the natural sites, the agricultural field sites are still dependant on seasonal flooding and drying. The persistence of the coastal bluff populations is highly dependent on sub-Mediterranean-like climate cycles with winter/spring inundation and summer desiccation. |
| Other Rank Considerations: | Occurrence in flooded fields brings a new dimension to statussing this species. The discovery of one location prompted a inventory of suitable sites in Saanich. However, only two of the eight suitable locations surveyed (equivalent to one CDC occurrence) were positive detections (not including the known primary discovery)(Penny 2009). Eight other locations were considered (i.e., Viaduct Flats, Quicks bottom, Rithets bog, Swan Lake, sections of Martindale flats etc.), but these sites do not appear to have persistent enough flooding, or too high of water, or unsuitable for other reasons, and thus excluded as suitable sites to search. Four other locations considered suitable were not surveyed because land owner permission was not obtained. Only one of these where permission was denied is considered high potential, but if C. marginata were detected, it would constitute an additional subpopulation of a known occurrence. Another location has good potential, but the land has undergone a number of changes to prepare for blueberry planting which may have disturbed the hydrological regime enough to render the site less suitable. The other two are less suitable but still potential. |
| Information Gaps | |
| Research Needs: | Little information is available on the ecology, population biology, life history, and genetics of Callitriche marginata (Miller 2003). Research on the basic ecology of this species is highly recommended. Studies on seed bank dynamics are also recommended. An understanding of how long seeds are viable in the seed bank and what proportion of seeds germinate from dormant seed banks vs. immediately dehisced seeds is important. Another key research need is to determine the dispersal capability of C. marginata (e.g., is it as poor as believed) and what the main mechanism for dispersal is if it is occurring. Research should also be conducted on the effects of disturbances such as human foot traffic and guano contamination on population dynamics. Yearly population counts and estimates of area of occupancy are recommended to improve our understanding of short-term trends in population dynamics. Population viability analyses may also be required. Only a single intensive count of the 17 subpopulations has been done, and little is known about how population size fluctuates from year to year. This makes it difficult to determine whether populations are viable. Research into the effects of competition from introduced species would also provide valuable information. The level of inbreeding vs. outbreeding are not well understood, nor has the genetic variation within the B.C. population or the genetic distinctness of this disjunct population been determined. Research into restoration techniques such as removing invasive species or ex situ propagation and transplanting of C. marginata would be valuable for the long-term conservation of this species. |
| Inventory Needs: | Additional occurrences of Callitriche marginata may be located in flooded fields from Victoria to Nanaimo and confirming status is highly dependant on such future inventories. A less pressing need is to search along remote or infrequently visited coastal areas of southern Vancouver Island and adjacent islands. Inventories of smaller Gulf Islands and islets between Mitlenatch Island and Victoria, as well as some remoter sections of the Vancouver Island coastline between Victoria and Rocky Point, may result in new element occurrences (A. Ceska, pers. comm., in Miller 2003). The rarity of vernal pools limits the likelihood that many new populations will be found in these areas. Miller (2003) did a thorough search of C. marginata in 2002 and 2003. Additionally, the Garry oak recovery strategy has enabled a concerted inventory of habitats such as vernal pools, preferred habitat for C. marginata, by Miller (2003) and other expert botanists in recent years. |
| Stewardship | |
| Protection: | Fifty-three percent of the total population and 9 of 17 subpopulations occur on public lands that provide some form of protection for Callitriche marginata (e.g., BC Parks, Parks Canada, Department of National Defence). An additional two populations occur on uninhabited islands off the coast of Vancouver Island. Nevertheless, two of the most substantial populations occur at Uplands Park, a heavily used area. Because alteration of hydrological regimes directly affects the survival of C. marginata, it is recommended that the seasonal hydrology of the vernal pool system be protected at Uplands Park (e.g., no irrigation, trenching, gravel deposition in the vicinity of pools) (Miller 2003). Protection of these populations from pollution and invasive plant species is also recommended. Protection needs for C. marginata at all locations should be based on information gained from careful monitoring of the populations (e.g., population trends, habitat condition, and status of threats), and should be adapted over time to respond to changes observed during monitoring. |
| Management: | There are no formal management policies in place that address Callitriche marginata conservation in B.C., although many of the recommendations for the recovery of plant species at risk in vernal pools (Miller 2005) may be relevant for C. marginata. Major management needs addressed in Miller (2005) that are pertinent to C. marginata are ecological research, inventory and monitoring, habitat restoration and site management, and public outreach and education. Further ecological research is critical for C. marginata. Because the basic biology of this species is poorly known, sound management will require additional information on demography, population dynamics, impacts of competition from invasive species, and effects of soil disturbance and fire management (Miller 2005). Once some of this fundamental information is known, inventory and monitoring programs that assess population trends, dynamics, and status of threats over time will be critical. Habitat restoration and site management and public outreach/education are dependent on findings from ecological research and inventory/monitoring, and have lower priority. Miller (2001) also presented four management recommendations specifically for C. marginata which focused on site hydrology, pollutants, access, and exotic plant invasion. Because C. marginata has a fragmented distribution, Miller (2001) recommended that management practices be based on the needs of particular sites. More specifically, he recommended the evaluation and management of hydrology in vernal pool systems. In sites close to roads and agriculture, he recommended that herbicide, fertilizer, and road runoff pollution be monitored. Limited access to vernal pool habitats in areas of heavy use was also suggested, as well as the monitoring and removal of introduced plant species, especially annual grasses. |
| Version | |
| Author: | Bartemucci, P. S. Hartwell and M. Donovan |
| Date: | April 29, 2015 |
| References | |
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Miller, M. 2001. Stewardship Account for Winged Water Starwort Callitriche marginata. Prepared for the B.C. Conservation Data Centre and the Garry Oak Ecosystems Recovery Team. Sponsored by the Habitat Stewardship Program, Gov. Can. and Nat. Conservancy Can. Victoria, BC. 26 pp.
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NatureServe. 2004b. Habitat-based plant element occurrence delimitation guidance, 1 October 2004. Available: http://www.natureserve.org/explorer/decision_tree.htm (accessed Mar. 17, 2005).
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Penny, J.L. 2009. Callitriche marginata Update Report Notes: Saanich Flooded Field Inventories. Conservation Data Centre. Victoria, B.C. 3pp.
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Please visit the website Conservation Status Ranks for information on how the CDC determines conservation status ranks. For global conservation status reports and ranks, please visit the NatureServe website http://www.natureserve.org/.
Suggested Citation:
B.C. Conservation Data Centre. 2015. Conservation Status Report: Callitriche marginata. B.C. Minist. of Environment. Available: https://a100.gov.bc.ca/pub/eswp/ (accessed Jun 5, 2026).