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Priority Habitats | Wetland | Fens

Description of Fens

Fens are a mosaic of different habitats such as open-water, raised bog, wet grassland, reed beds or even wet woodland. It is simply a wetland that permanently has a high water level at or just below the surface.

Fens receive the majority of their water and nutrients from soil, rock and ground water. They occur in river valleys, poorly drained basins or inter-drumlin hollows, along lake margins or on river flood-plains. Due to the complexity of the habitat it is always going to be a rich in its diversity of plants and animals. Fens are covered by mats of floating plants and moss which start to slowly decompose to form the basis of the layers of peat.

Historical info about Fens

Between 10,000 and 7,000 years ago the great stretches of water that were established after the cold conditions of Ireland had ended were progressively being reduced by the growth of fens and marshes.

Algae were probably the first free-floating plants to invade the water, followed by small planktonic animals of a great variety, the debris of which added to the mud accumulating on the lake floor. If the water contained some amount of calcium - and most early waters did - then the submerged stoneworts (Chara/Nitella) were abundant. Aquatic molluscs fed off the stonewort and other debris and the mud became rich in their small coiled shells, of which thick deposits accumulated. This build up then lead to the colonising of floating plants such as the bur-reed (Sparaganium) and the water-lily (Nuphar).

Other reed based plants also invaded; plants such as the bulrush (Scirpus lacustris), the reed (Phragmites communis) and many others. The accumulating vegetable debris gradually consolidated into fen-peat, which built up to water level and replaced open water with a fen. (Mitchell et al, 1998)

Species living in and around Fens

Within fens more than two hundred different species of plants have now been recorded, however the vegetation found within the fens is usually divided into two areas, those found in ‘poor-fens’ and those found in ‘rich-fens’.

‘Poor-fens’ mainly occur in upland areas and the base of the fen is mostly sandstone or granite, they are dominated by a moss layer including or dominated by bog mosses Sphagnum spp., purple moor-grass Molinia caerulea, bottle sedge Carex rostrata and the smaller sedges, such as star sedge C. echinata and common sedge C. nigra.

‘Rich-fens’ are very restricted and mostly occur in lowland areas, such as south-east Co. Fermanagh and Lecale (Co. Down), where they are fed by mineral enriched waters. The vegetation that you can find is usually much more diverse and can include, bog pimpernel Anagallis tenella, meadow thistle Cirsium dissectum, saw sedge Cladium mariscus, marsh helleborine Epipactis palustris, blunt-flowered rush Juncus subnodulosus, grass-of-parnassus Parnassia palustris, common butterwort Pinguicula vulgaris, black bog-rush Schoenus nigricans, and bladderworts Utricularia.

As well as supporting a large variation of plant species, fens also support a wide range of animal species. These species include European (or common) frog Rana temporaria and the smooth newt Triturus vulgaris and many Northern Ireland species of conservation concern.

Breeding birds include reed bunting Emberiza schoeniclus, breeding waders and shoveler Anas cyptela.

Northern Ireland fens are particularly important for invertebrates, several of which are absent or threatened in Great Britain. These include dragonflies such as the Irish damselfly Coenagrion lunulatum, beetles such as the whirligig beetle Gyrinus natator, the water beetle Haliplus variegates, the pond skater Limnoporus rufoscutellatus and the carabid beetle Pterostichus aterrimus.

It is also an important habitat for butterflies and moths such as the marsh fritillary butterfly Euphydryas aurinia, as well as other priority species including curlew Numenius arquata, redshank Tringa totanus, lapwing Vanellus vanellus, grasshopper warbler Locustella naevia, holy grass and the ground beetles Carabus clatratus and Lebia crux-minor.

A number of locally rare plant species are associated with fens, such as the Northern Ireland species of conservation concern fen bedstraw Galium uliginosum, greater water-parsnip Sium latifolium, holy grass Hierochloe odorata, Irish lady’s-tresses Spiranthes romanzoffiana, marsh helleborine Epipactis palustris and marsh pea Lathyrus palustris.

Threats to Fens

There are a number of different factors that will threaten the existence of fen areas, these may include drainage; fens have declined as a result of drainage and control of water levels. Many extensive areas of fen associated with large lakes have disappeared.

Unbalanced levels of grazing will also affect this habitat, too much can be just as harmful as too little, fens are very sensitive to changes in grazing levels.

Land infill for agricultural land reclamation, waste disposal or development affects a large number of fens. This directly results in the loss of extent and quality of fen vegetation, and can affect water-levels and water quality over a wide area.

Peat-cutting has also affected the character of many important fen sites. Although this has largely ceased, without appropriate management, the biodiversity associated with many sites will continue to deteriorate due to vegetation succession resulting in the loss of open water and more open fen habitats.