Ranunculus hederaceus L., Ivy-leaved Crowfoot

Account Summary

Fermanagh occurrence

In Fermanagh, Ivy-leaved Crowfoot has been recorded in 89 tetrads, 16.9% of those in the VC although, as the distribution map indicates, it is widely scattered throughout the county; ten of the tetrads contain pre-1976 records only, which could be argued as indicating some degree of local decline during the past half-century.

Growth form and habitat preferences

R. hederaceus regularly occurs in Fermanagh as something of an invading, ephemeral, companion species of a number of quite different submerged floating or emergent vegetation communities. The habitats where this happens include the muddy banks of slow-flowing rivers, streams (together with springs and upland flushes) and drainage ditches (especially in late autumn after ditch cleaning) and beside relatively still waters in smaller lakes and ponds and on the sheltered shores of backwater bays of the larger loughs in the VC. It does not occur, or is only occasional and sparsely developed, where water stagnates. Typically, R. hederaceus requires sufficient seepage of ground water to create at least a slight fluctuation in levels and a consequence of this will be an additional in-flow of oxygen and mineral nutrients.

R. hederaceus was also once found in muddy ground by temporary pools in an old quarry beside Keenaghan Lough and it also appeared on an urban waste tip in Enniskillen, which mirrors Segal's experience in Holland (Segal 1967). However, the most predictable habitat in Fermanagh of this small, prostrate annual or short-lived perennial appears to be by field gateways and along wet, muddy tracks where water lies in shallow quagmire pools and puddles in mud that has been trampled, poached and dung-enriched by cattle. We find it both where the parent rock of the soil is acidic or of a calcareous nature, and it clearly tolerates a rather wide range of pH and appears indifferent to lime (Cook 1966b).

The authors of the critical Flora Nordica regard R. hederaceus as a calcifuge throughout their northern European region (Jonsell et al. 2001). In Great Britain, Hill et al. (1999), summarising the preferred environmental growing conditions of species, gave R. hederaceus an indicator value of '5' on a nine point scale, indicating that while it mainly occupies moderately acid soils, occasionally it ranges wider and it can occur on both very acid and more neutral to basic (alkaline) substrates. In the latter situation, due to our very wet climate, a leached calcareous soil usually develops. This generally becomes overlain with a thin, acid, peat horizon, which forms the substrate over which Ivy-leaved Crowfoot spreads and shallowly roots at its nodes.

Cook (1966b) made a significant point when he speculated that if R. hederaceus and the closely related species R. omiophyllus Ten. (Round-leaved Crowfoot) are not directly competing (the latter does not occur anywhere in N Ireland), they may exhibit wider ecological amplitudes than when they overlap with one another. The habitat description of R. hederaceus is rather similar to that of R. sceleratus (Celery-leaved Buttercup), but the latter is more frequently found by lakes, or in somewhat less disturbed, more nitrogen-enriched, perhaps slightly better drained mud which is more liable to drying out temporarily than that of R. hederaceus. Also, R. sceleratus never takes on a truly aquatic 'submerged' – or more realistically – 'floating' existence, in the way that Ivy-leaved Crowfoot is capable of doing in water even as shallow as 5 cm!

Apart from its freshwater habitats, R. hederaceus can also be found in sheltered coastal parts of both Britain and Ireland, usually on the upper edges of salt-marshes (Preston & Croft 1997).

Variability and identification

R. hederaceus is classified within the Subgenus Batrachium of the genus Ranunculus. Most species of this subgenus are notoriously variable, frequently hybridize and backcross, and thus are often very difficult to identify (Cook 1966a; Holmes 1979; S.D. Webster In: Rich and Jermy 1998). R. hederaceus and its close relative R. omiophyllus (Round-leaved Crowfoot) (the latter apparently absent from Northern Ireland), differ from all the other members of the subgenus by being essentially semi-terrestrial and by invariably having un-dissected, laminar, floating leaves. Unlike all other water-crowfoots in the subgenus, these two species do not possess thread-like submerged leaves (Cook 1963, 1966b). Thus, in our Fermanagh survey area, Ivy-leaved Crowfoot is very easily recognised, despite the species being remarkably variable in form in response to environmental changes, ie it has a very plastic phenotype with respect to many characters. This high level of variability is quite characteristic of emergent aquatic species (Segal 1967; Cook 1966a, b).

The creeping or surface floating habit of the plant, its small white flowers, and ivy-shaped leaves with dark markings following the veins are all very distinctive identification features, and the typical plant species associates of Ivy-leaved Crowfoot include Montia fontana (Blinks), Callitriche spp. (Water-starworts) and Stellaria uliginosa (Bog Stitchwort).

Summary of environmental requirements

From the above, it is clear that R. hederaceus has the wide ecological tolerances that one would expect of a species inhabiting situations where slight but significant fluctuating water levels are the norm, and its requirements can be satisfied in a diverse range of habitats encompassing a mosaic of vegetation communities, moist, wet, semi-aquatic and shallow aquatic, lowland and upland, coastal and inland. The most constant requirements of the species are for high levels of illumination, wet, moderately acid, waterlogged soil, or very shallow water only a few centimetres deep, of medium (mesotrophic) fertility, or richer more productive eutrophic levels, plus shelter from strong water currents.

Temperature limits and distribution

An equable, fairly low water temperature throughout the year is recognised by ecologists and plant geographers as another very important environmental factor controlling the growth and occurrence of R. hederaceus. Temperature is sufficiently significant to actively govern the wider distribution of the species, confining it to the truly oceanic or Atlantic region of W Europe. A summer maximum of around 16C, and mild winter temperatures with little in the way of severe frost, characterises the required regime (Segal 1967).

In his study in Holland, Segal noticed that R. hederaceus tends to occur in specific landscape situations where small scale water bodies lie on the junction between hilly, acidic, infertile (oligotrophic), non-calcareous soils and much more fertile, mineral-rich (mesotrophic to eutrophic) conditions, on lower ground at the base of slopes. In the habitat gradients that occur when two very different ecological environments of this nature meet, and especially where the zone of contact is kept open by some form of disruption, either physical (eg trampling and grazing), or chemical (eg a moderate level of pollution, including manuring and other forms of farm effluent run-off), or both of these, then R. hederaceus and other interesting and quite scarce species such as Catabrosa aquatica (Whorl-grass) and Veronica catenata (Pink Water-speedwell) appear to find growing conditions to their liking. On the other hand, in excessively enriched sites containing high levels of nitrogen and phosphorus, ground that tends to be rapidly overgrown by algae and by Lemna species (often L. gibba (Fat Duckweed)), R. hederaceus cannot compete for very long under such conditions and becomes ousted.

Flowering and pollination

Ivy-leaved Crowfoot flowers early and for a prolonged period, often stretching from April to August. However, flowering is frequently curtailed by the habitat becoming overgrown later in the summer, or sometimes by it drying out. The small white flowers produce nectar, are sweet scented and are protogynous (ie the female parts develop first, followed by the anthers). This difference in sexual timing in the individual flower is generally considered an adaptation favouring or enabling cross-fertilisation between flowers, but in reality the blooms are highly self-compatible and they appear to habitually inbreed. Indeed, self-fertilisation often takes place at the unopened bud stage, making it obligatory. Despite this sexual behaviour, the timing of flower opening depends on the prevailing weather and some level of opportunity for cross-pollination does exist in other flowers. However, observation also indicates that the flowers attract few insect visitors (Cook 1966b).

Occasionally R. hederaceus flowers are produced underwater. When this occurs, a gas bubble is formed within the bud, allowing pollination to proceed as normal (Cook 1966b). Since R. hederaceus is seldom submerged and, if so, then usually only for short periods, submerged pollination is not likely to occur very often.

Fruiting and dispersal

As the fruit develops the flower stalk bends away from the light, forcing the developing achenes into the mud (Cook 1966b). This is another example of a negatively phototrophic movement by a fruit stalk (ie a growth movement away from the direction of light), similar to that of the fruit stalk of Cymbalaria muralis (Ivy-leaved Toadflax). It appears odd that both species which commonly show this unusual physiological feature, should possess ivy-shaped leaves!

Dispersal of the seeds (achenes – single-seeded dry fruits) is most probably achieved in mud by attachment to animals or vehicles. There might possibly be some degree of water dispersal too, if a fast current were to develop in the locality and dislodge seed shed onto the soil surface around the plant.

Germination and growth form

Seed germination is reported by Cook (1966b) to be very irregular if the achenes are kept wet – which one might imagine would be the normal condition of the habitat. On the other hand, if the mud dries out after the seeds ripen and the seeds themselves become dried, then Cook found that when they are re-wetted, the seeds gave nearly 100% germination. He also showed that, depending on the local regime of water levels and competing species, R. hederaceus (and also R. omiophyllus (Round-leaved Crowfoot)), can behave either as winter or spring annuals, or individual plants may persist and reproduce on multiple occasions for periods up to six years, thus achieving perennial status.

Overwintering and factors affecting survival

During the winter months, the resting plant survives as a small, tight, rosette of leaves. In this state, it is very resistant to freezing, desiccation and shade. However, in summer when individual plants develop their normal spreading habit, they becomes very susceptible both to the three mentioned physical factors and to pressure from taller, more aggressive competing plant species (Cook 1966b).

Ignorance regarding seed output, reproductive strategy and longevity in the soil seed bank

In common with many other species in the flora of Britain and Ireland, after careful searching, we do not appear to have any figures for typical plant seed output. Nor do we know anything regarding the relative significance of seed versus vegetative reproduction for perennial populations of R. hederaceus, nor even if a persistent soil seed bank exists (Thompson et al. 1997).

British and Irish occurrence

In Britain, R. hederaceus has quite a distinct northern and western distribution, partially created by the destruction of suitable habitats in much of the SE due to development, drainage and a shift from livestock to arable farming. The change to arable farming means that existing wetlands are no longer subject to the trampling and disturbance of grazing animals that previously opened the ground to colonisation by Ivy-leaved Crowfoot (Preston & Croft 1997; Preston et al. 2002). In Ireland, R. hederaceus, being essentially a lowland wetland species, has a somewhat scattered, almost disjunct distribution, featuring a heavy concentration of records in northern counties, but it is more coastal in the Republic and appears quite scarce in the Midlands.

European and world occurrence

R. hederaceus occurs throughout the Atlantic region of W Europe from Portugal to the S tip of Sweden (Jalas & Suominen 1989, Map 1884). It was previously present further N up the coast of Norway at Tronheim and considered native there by some, but it became extinct there in 1950 (Jonsell et al. 2001). Cook (1983) regarded the species as endemic to Europe (and indeed one of only a few ancient 'palaeoendemics'), and he felt that it has declined throughout Europe, probably due to changes in agricultural practices over the past 50 or so years which involved the destruction of wetland habitats. Preston reminds us that the species was losing sites in SE England before 1900 for the same reason and also because of urban expansion (Preston et al. 2002).

The species is also present in disjunct locations in eastern N America where it has been known since 1821 – in Newfoundland and the Chesapeake Bay region (Hultén 1958, Map 137). The general shape of the distribution of R. hederaceus in N America and its history discussed in Cook (1983, 1985), suggests that it was introduced from Europe. Against this view there is the existence of two fern species, Schizaea pusilla Pursh and Woodwardia areolata (L.) Moore, which have very similar N America distributions to R. hederaceus and they are certainly not European introductions.

Names

The Latin specific epithet 'hederaceus' simply translates as 'ivy-leaved' (Gilbert-Carter 1964) and the plant, having no herbal or folk-lore usages, does not appear to have any English common names other than the one chosen by the committee of the Botanical Society of Britain and Ireland (Dony et al. 1974).

Threats

None. The species may be benefitting, like R. sceleratus, from the current general eutrophication of water bodies, and considerable physical disturbance helps it colonise by keeping the habitat open.

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