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Ranunculus aquatic species agg., a water-crowfoot

Account Summary

Native, rare or occasional, but difficult to identify and therefore over-looked and under-recorded.

1988-91; NI Lakes Survey; Lakeview Lough, 1 km N of Drummully.

June and August.

Since we have relatively few records of Batrachian Ranunculi for Fermanagh, these 20 undetermined water-crowfoot records from 17 tetrads, which need to be followed up, are simply included for the sake of completeness.

The first seven records were made by the NI Lakes Survey between 1988 and 1991. In addition to the first record listed above, the site details of their other finds are: Lough Ora, 1 km SSW of Ora More, Ballintempo Forest; Green Lough turlough, near Fardrum; and Fardrum Lough turlough. The remaining three records made by the Lake Survey were all made on unimproved neutral to calcareous grasslands on the shores of Lower Lough Erne shore as follows: E of Castle Hume, 11 June 1991; Inner shore, Dulrush Peninsula, 2 August 1991; mainland shore opposite Dulrush Peninsula, 4 September 1991.

All the subsequent 13 records were made by RHN either alone or accompanied by HJN. The sites and dates are: Lough Skale, Clogtogle Td, 5 June 2006; Lackboy, S shore of Boa Island, 1 July 2010; Mill Lough near Ballinaleck, 13 October 2010; Castle Archdale, 31 October 2010; all the remainder were around the shores of Lower Lough Erne in November and December 2010: mainland shore NW of Portinode Bridge; jetty near car park, Castle Archdale; Hill's Island; Rossigh Bay; riverfoot, just N of Rossclare jetty; near Rosscor Viaduct; mouth of Garvary River; Carrickreagh Bay; N shore of Tully Point, Sand Bay.

Growth form and habitat preferences

This aquatic species, which only produces dissected capillary leaves, is a spreading to erect perennial when growing in permanent water, but it becomes greatly dwarfed and behaves as a small annual if its muddy substrate dries out during the summer. It is most frequently found in still or almost still shallow water less than 50 cm deep in small lakes or ponds and water-filled holes in disused quarries.

Thread-leaved Water-crowfoot roots mainly in bare clayey bottoms and it can tolerate a wide range of water chemistry, from oligotrophic to eutrophic, lime-rich or otherwise. It chiefly frequents mesotrophic to eutrophic conditions in lowland water bodies (C.D. Preston, in: Preston et al. 2002). R. trichophyllus also occurs in brackish waters around the coastline of Great Britain and Ireland (particularly in Scotland and Wales), eg in dune-slacks and around sheltered estuarine bays (Preston & Croft 1997).

Like its close relatives, R. aquatilis (Common Water-crowfoot) and R. peltatus (Pond Water-crowfoot), this species tolerates disturbance such as trampling by stock animals, human activities and/or a substrate which seasonally dries out. It can therefore behave as a pioneer colonist of bare mud around temporary water bodies, including recently dug or cleaned pools and drains (Cook 1966a, p. 134). Later in the season, as the vegetation in such muddy habitats matures, R. trichophyllus and its pioneer associates tend to be ousted by increased shade and other forms of competition produced by rhizomatous aquatic plants such as Potamogeton species (Pondweeds), Hippuris vulgaris (Mare's-tail), or by any of the numerous vigorous emergent plants of swampy ground.

Fermanagh occurrence

In Fermanagh, there were only two records for R. trichophyllus prior to 1980 and there have been just eight additional finds of the species since then. There are records from nine Fermanagh tetrads, seven with post-1975 dates. The species is undoubtedly under-recorded here, as is also the case elsewhere in Britain and Ireland. In the New Atlas, Preston suggests that part of the reason for the paucity of records is the early flowering season of the species, and this, together with the well-known discouraging taxonomic difficulties of the Batrachian group of Ranunculus, is very probably responsible for the dearth of observations (Preston & Croft 1997; New Atlas).

Most of the Fermanagh records for this species will have been determined on the basis of the flower size being less than 12 mm in diameter, with the petals being non-contiguous. Non-flowering plants in this group are almost impossible to identify, contributing to their all being under-recorded. Regrettably, none of the ten Fermanagh records of R. trichophyllus is supported by a voucher.

As the tetrad map indicates, most of the records are confined to the Lough Erne basin, with just four outlying stations, one at Green Lough turlough and three scattered further east. Although more frequently found by smaller water bodies, it does also occur in well-sheltered areas around larger lakes, especially where shallow backwater bays are protected by islands close to shore. Local examples of the latter habitat are the West Point islands and Lowery Island, both at the W end of Lower Lough Erne.

Flowering and pollination

Thread-leaved Water-crowfoot is the first Batrachian Ranunculus species to flower in Britain and Ireland, which it does from April through to July. The flowers are habitually self-pollinated, this often (but not always) taking place in the bud (ie the flowers are cleistogamous). As a result of this, seed set is perfect (100%) and completely assured (Cook 1966a, p. 183; Hong 1991, p. 53).

Seed output

Statistics of mean seed production per plant do not appear to be available, or have not been located.

Germination

The seeds can germinate in wet conditions at any time of year, but in winter seedlings require the protection of submerged conditions. If frozen under terrestrial conditions they die, but they can survive being frozen in ice for several months (Cook 1966a).

Survival in the soil seed bank

Seed longevity is unknown, but it is probably similar to that of R. peltatus which is considered to be only short-term persistent, ie the seeds survive in soil for periods ranging between one and five years (Thompson et al. 1997).

Vegetative reproduction

Like most other aquatic macrophytes, R. trichophyllus can reproduce vegetatively from plant fragments after disturbance or uprooting of an original established individual. An experimental study in France examined the fate of five different types of plant fragments of R. trichophyllus and five other wetland species. The plant parts used ranged from roots, to stems with or without nodes, to apical buds. After ten weeks greenhouse incubation floating in pans of shallow water (7-8 cm deep) and sediment, the R. trichophyllus results showed that within two or three weeks, between 30% and 50% of the aerial parts rapidly rooted and established into the sediment, while the remainder of the fragments of the species died off without showing any root development. None of the fragments of R. trichophyllus developed new buds before they rooted (Barrat-Segretain et al. 1998). The French workers recognised that the six aquatic macrophyte species they studied reacted to disturbance by exhibiting one of two survival tactics: 1. the fragments either develop roots and establish rapidly into the sediment (eg R. trichophyllus and Sparganium emersum (Unbranched Bur-reed), or 2. the fragments develop many new propagules that may be dispersed, but they fail to establish within the 10 weeks of the experiment (eg Hippuris vulgaris (Mare's-tail) and Elodea canadensis (Canadian Waterweed)) (Barrat-Segretain et al. 1998).

The species examined in the French study appeared to present a trade-off between the plant's regeneration and colonisation abilities: each species practiced both tactics, but high levels of regeneration implied low colonisation ability and vice versa (Barrat-Segretain et al. 1998). One of the most interesting and unexpected conclusions of this work was that the different survival tactics involving vegetative reproduction were not related to the morphological types of the species and thus they were unpredictable. At the same time, it appears that species possessing narrow ecological amplitude promote dispersion of propagules, thus increasing opportunities of finding suitable habitats, whereas other species (including R. trichophyllus), that are more tolerant of varying ecological conditions, try to anchor themselves as soon as possible at the nearest sites to the original population (Barrat-Segretain et al. 1998).

A subsequent similar experimental study showed that some aquatic species behaved quite differently with respect to these two vegetative strategies in the spring and in the autumn, but this did not apply to either R. trichophyllus or Sparganium emersum (Unbranched Bur-reed) (Barrat-Segretain & Bornette 2000).

British and Irish occurrence

In Ireland, R. trichophyllus is occasional in the C & SE, and rare and widely scattered elsewhere including NI (Webb et al. 1996). In Britain, it is much more frequent in the SE of England, below the line from Hull to Bristol, but it is widely scattered in the lowlands elsewhere, becoming distinctly coastal in the W and throughout Scotland (Preston & Croft 1997; Preston et al. 2002). Although the New Atlas contains a high proportion of pre-1970 records, the authors consider that this does not indicate any real decline in what is a difficult enough species to identify.

European occurrence

Beyond the British Isles, R. trichophyllus is probably the most widespread Batrachian species in the world. The only reason for qualifying this with the word 'probably' is, of course, the great amount of variation and plasticity within the group, plus the infamous taxonomic difficulty of the subgenus which creates problems in terms of delimiting the species and their field recognition. For instance, when examining material of R. aquatilis (Common Water-crowfoot) and R. trichophyllus in S Sweden, Hong (1991) found that all the distinguishing characters for these two species, including nectary shape, were variable and readily modified. Also, sexual crosses between typical forms of the two species were found to be fully fertile, a feature undoubtedly allied to polyploidy within the complex. In S Sweden, both of these species are regarded as hexaploid taxa, whereas in his taxonomic review Cook (1966a) dealt entirely with tetraploid R. trichophyllus and hexaploid R. aquatilis material from C and W Europe. Reflecting this degree of variation, in Flora Nordica 2 (Jonsell et al. 2001), R. trichophyllus is downgraded from a species to a variety of R. aquatilis, referred to as var. diffusus With.

Apart from these problems of polyploidy and status change, R. trichophyllus is regarded as widespread throughout Europe, especially frequent in western regions and thinning eastwards. Unlike many other members of the subgenus, it is even frequent in the Mediterranean basin and is present on many of the smaller islands, for example, Majorca, Malta, Cephalonia, Zante and Crete (Jalas & Suominen 1989, Map 1895).

World occurrence

Beyond Europe, it is a very widespread species being both Arctic and circumboreal stretching across N Asia, the Himalaya, N and W China, Japan, SE Australia, Tasmania and central areas of N America (Cook 1966a; Hultén 1974, Map 80; Hultén & Fries 1986, Map 874). R. aquatilis is the only other water-crowfoot that approaches this extent of world distribution, but it is not as well represented as R. trichophyllus in both northern regions and in the Mediterranean region.

Dispersal of aquatic species

We have to ask ourselves why should R. trichophyllus be so much more widely distributed than all the other aquatic Ranunculus species? It is listed by Cook (1985, Table 1), along with Potamogeton pectinatus (Fennel Pondweed) and 24 other aquatic species widely distributed in the world, where their native ranges are impossible to distinguish from their areas of introduction. Prof. Cook points out that it is surprisingly difficult to get recent, accurate information on the distribution of aquatics and it is even more difficult to discover or decide where a particular species is native rather than introduced. In connection with dispersal ability, Cook also points out that aquatic plants in general lack very light wind-carried diaspores or propagules and they also lack large seawater-resistant ones; they are thus ill-equipped for long-distance dispersal. The widespread plants, listed in his Table 1 of unknown native status, have probably spread more or less naturally, or at least they dispersed before botanists were around keeping records!

The majority of aquatic plant species have become mobile as man has increased his own global mobility (Cook 1985). Reviewing the distribution of 172 aquatics, Cook found that marked imbalances existed between Old and New World regions, in terms of donor and recipient areas with respect to introduced species. He concluded that the problems of successful establishment in new lands are quite obviously very great and that it is extremely naive to regard the aquatic environment as globally uniform, thus allowing or expecting aquatic macrophytes to have a wide geographical range. It is known that some aquatic macrophyte genera (eg Brasenia and Dulichium), became extinct in Europe before or during the last Ice Age, but they persisted in N America. These plants have not re-established in Europe despite being in garden cultivation there, an observation, which lends strength to the argument that ecological niches in the aquatic environment are more complex than we commonly assume. Cook argues that we should not combine the terms 'introduced aquatic' and 'weed', pointing out that weeds (ie undesirable plants) essentially tend to flourish in sites that are disturbed by man. "Usually it is the nature of the disturbance that is more important for the establishment of the weed than its [powers of] mobility." (Cook 1985).

It is very clear that disturbance by man and his animals, and pioneer colonisation of bare muddy ground are two very important ecological features influencing the occurrence of many Batrachian Ranunculus forms. In many cases, these species could not successfully reproduce sexually and establish seedlings without the habitats they occupy temporarily drying out. Following Cook's advice, and maintaining a healthy scepticism regarding the real distribution of many difficult-to-identify Batrachian Ranunculi, it appears to follows that his remarks regarding introduced aquatics must apply quite closely to R. trichophyllus, since it does not appear any better equipped for long-distance jump dispersal than R. peltatus (Pond Water-crowfoot), R. penicillatus (Stream Water-crowfoot) or, indeed, any of the other seed- and vegetative propagule-producing members of this complex subgenus.

Names

The Latin specific epithet 'trichophyllus' is derived from the two Greek words for 'hair' and 'leaves', and thus translates as, "having leaves divided into hair-like segments" (Gilbert-Carter 1964). The English common name, 'Thread-leaved Water-crowfoot', is a straightforward invented 'book name', following the same line of thought.

Threats

None.