This annual grows on acidic mud by small ponds where the habitat is kept open by fluctuating water levels and disturbance by grazing animals. Its sites are usually flooded in winter but may be wet or dry in summer. The seeds appear to persist in a long-lived seed bank. Lowland.
D. alisma declined catastrophically in the 19th and early 20th centuries following the decline of traditional grazing regimes. Most sites have become filled in, overgrown or reduced to suburban duck-ponds. Only one site was known in the 1980s, but the species has since responded to conservation management at some sites and has been planted at others.
Count of 10km squares in Great Britain: 54
Count of 10km squares in Ireland: 0
Count of 10km squares in the Channel Isles: 0
Atlas Change Index: -0.52
RDB Species Accounts
Damasonium alisma Miller (Alismataceae)
Status in Britain: ENDANGERED. WCA Schedule 8.
Status in Europe: Not threatened. Probably endemic.
D. alisma grows in shallow, saucer-shaped ponds with fluctuating water levels, located on acid soils. It requires open vegetation with areas of bare soil, being intolerant of competition from more vigorous species. This vegetation structure is generated by disturbance such as trampling by livestock, or periodic pond clearing. It is associated with species which are tolerant of both inundation and desiccation, including Apium inundatum, Bidens tripartita, Callitriche species, Juncus articulatus, J. effusus, Lythrum portula, Myriophyllum alterniflorum, Persicaria maculosa, Ranunculus flammula and R. peltatus.
In the wild, D. alisma generally behaves as an annual, but in cultivation, and possibly sometimes in the wild, it can live for two or three years. Its seeds germinate below water in early winter and develop into small plants with short linear leaves resembling a tuft of grass. In early summer long-petioled floating leaves are produced. If the water level falls to expose the plant above water, these leaves die and are replaced by stout, short-petioled alternatives. But if the water level remains high, the plant retains its aquatic growth-form. It flowers between June and August. The number of flowers produced by exposed plants is related to the time the mud remains moist: if it dries quickly only a single flower may be produced before the plant dies. Thus, reflecting the unpredictability of its habitat, the size and growth-form of starfruit are variable, ranging from tiny terrestrial plants with just a single flower, to large terrestrial or aquatic plants with several compound inflorescences bearing as many as 150 flowers. Plants are self-fertile and some self-pollination occurs (Vuille 1987). However, beetles and hoverflies visit the flowers and may effect cross-pollination. Fruits appear like 6-rayed stars. Only a small proportion of the seeds germinate in their first winter, the majority remaining dormant. Dormancy can be broken by a period of desiccation followed by hydration, or possibly by changes in the seed's environment following disturbance. D. alisma has made dramatic reappearances in ponds where it has apparently been absent for several decades, following the removal of dense stands of emergents, suggesting that dormant seeds can remain viable for very many years (Birkinshaw 1994).
Since first recorded, D. alisma has been found over 100 localities in 50 hectads, mainly in south-east and southern England, within an area bounded by Buckinghamshire, Hampshire, Kent and Sussex. It has never been common and, in any decade, occurred at only a few sites. However, the species continued to decline this century, until in the 1980s plants were seen in just one pond. The reasons for this decline were various: the neglect and mis-management of ponds, cessation of traditional use of ponds by livestock leading to excessive growth of emergent and submerged plants, the maintenance of constant water levels for angling, and the loss of ponds through infill. In the last few years, clearance of ponds where it was once known, including dredging out of accumulated silt and rotting vegetation, resulted in D. alisma appearing in six sites between 1990 and 1994 (Showler 1994; Rich, Alder, et al. 1994). However, from a high point of several hundred plants at these sites, it declined rapidly, to only fifteen plants detected in Britain in 1994, none in 1995, and about 40 at four sites in 1996. Details of this decline, and possible reasons are discussed in Rich, Alder, et al. (1995).
The maintenance of a healthy population of D. alisma in Britain is dependent on fluctuating water levels in ponds and, in the absence of livestock grazing, removal of excessive vegetation and scraping hard back to the substrate. With appropriate management, natural reappearance seems to be relatively predictable. However, D. alisma does not persist after restoration, and further management trials are needed to discover how to maintain populations. Currently, several ponds are being restored, and reintroduction has also been attempted. Four of its sites are in SSSIs.
Outside England, D. alisma occurs in southern and south-west Europe (France, Spain and Italy). The closely related D. bourgeai and D. polyspermum, which are considered by some to be subspecies of D. alisma, occur in North Africa, Asia Minor, southern Russia, Ukraine and southern Europe.
C. R. Birkinshaw
PLANTATT - Attributes of British and Irish plants. (.zip 1455KB) This dataset was compiled and published in 2004, and last updated in November 2008. Download includes an Excel spreadsheet of the attributes, and a PDF explaining the background and nomenclature. Note that the PDF version is the booklet as published, whereas the Excel spreadsheet incorporates subsequent corrections. A hardcopy can be purchased from the Centre for Ecology and Hydrology.