What is ‘special’ about Cors Erddreiniog SSSI?

Cors Erddreiniog has 10 special features:

As well as the features listed above, Cors Erddreiniog has other habitats/earth science features that contribute to the special interest. These include woodland (a wet birch /mixed wood and hazelwood with primroses) together with hedgerows, springs, streams, ponds, small rock outcrops and individual trees. The peat and marl deposits also represent an irreplaceable historical record of environmental changes since the last ice age This mixture of habitats is important for much of the wildlife including otter, brown hare, great crested newts, adder, water vole, lapwing, snipe, curlew, hen harrier, marsh harrier, linnet, willow tit and redpoll and these too are key components of the special interest of the site. Unless specified below, management of this site should aim to look after these habitats and animals as well as the listed features of interest.

What do we want Cors Erddreiniog to look like?

At least 70% of the site should be wetland. In this we would expect to see a range of different plant communities characteristic of low fertility wetland soils – the main elements of this vegetation would include great fen sedge, mixtures of black bog rush and blunt flowered rush, areas of lesser tussock sedge and common reed, and mosses typical of these conditions including sphagnum and ‘brown mosses’. Purple moor grass should be present but should not be dominant over large areas. Patches of willow and birch scrub should remain but should not be permitted to exceed 10 % of the wetland area.

Heathland, with heather, cross-leaved heath and western gorse, should cover at least 7% of the site. Within the heath there should be areas of differing ages and therefore different structure and height. Areas of bare ground should also occur in the heathland to allow the marsh gentian, pale heath violet and petty whin to flourish. At least 8% of the site should comprise species-rich grassland with plants such as ragged robin, common spotted orchid, frog orchid, moonwort (a primitive fern) and yellow rattle. Hazel woodland should cover much the eastern escarpment above the spring line. The mixed woodland in the centre of the site should be managed to promote indigenous tree species.

The lake should retain its clear water quality (without excessive algal growth) and continue to support indigenous water plants and wildfowl. The site should support great crested newts and a rich variety of invertebrates including southern damselflies, geyer’s whorl snail and the marsh fritillary butterfly. Lapwing, snipe and curlew should nest here with hen and marsh harrier overwintering and the latter possibly breeding.

What management is needed on Cors Erddreiniog SSSI and why?

Although Cors Erddreiniog is an excellent place for wildlife it will only remain so if the necessary management continues. CCW’s aim is to work with you to ensure that this management is carried out.

What does this mean in practice?

There are many factors that could damage the special features at Cors Erddreiniog if they are not properly managed. These are the ones we regard as most important:

Water level: A high water table level is essential for the survival of wetland plants and animals. It is therefore important that no work is carried out which would lower or change in any other way water levels on the site – for example by widening or deepening ditches. However, some wetland plants and animals require very shallow surface water or moving groundwater, so deep or prolonged flooding can destroy these. Raising water levels should not be undertaken without careful assessment. Equally important is the need to maintain the water supply to the site through springs, groundwater seepage, ditches and surface run off. Any actions that would reduce the amount of water entering Cors Erddreiniog would be damaging to the site.

Water quality: Good water quality is essential for maintenance of the characteristic assemblage of wetland plants and animals at Cors Erddreiniog. Nutrients such as nitrogen and phosphorus encourage the spread of strong growing plants such as floating sweet-grass and common reed, which can out-compete the less common (and more desirable) species of Cors Erddreiniog. This in turn would have a negative effect upon the animals that depend on these plants. Such nutrients in the lake stimulate growth of filamentous and planktonic algae, causing the loss of species that need clear and clean water, including fish. Toxins such as sheep dip could also damage the site’s wildlife, particularly in the aquatic environment. Measures to enhance soil fertility within the catchment of the site are therefore likely to harm the special interest features of Cors Erddreiniog.

Water movement: Gradual movement of water from springs through the site is essential for the survival of the spring-water flushed wetland. In order to maintain this important passage of water through the site, great care in the management of water supply and levels is needed.

Grazing: Light grazing of wetlands and heathland removes excess plant material. The most appropriate grazers here are cattle or ponies as they can maintain open areas in wetland by browsing scrub and by light poaching of the ground. Cattle, sheep or ponies can be used to graze areas of grassland, but a rest period to permit flowering and seeding is required. Animal dung also provides an important food resource for some invertebrates. There should be no supplementary feeding with silage as this increases soil nutrient levels. Hay may be fed in severe weather, mineral licks used to enable the animals to digest coarse material and small quantities of concentrates to keep livestock tame.

Scrub control: Scrub provides nesting places for birds and shelter for other animals. However too much scrub can alter the special qualities of the wetland site. Grazing alone may not be enough to prevent scrub expansion and it may sometimes be necessary to carry out control manually.

Fire: Fire was regularly used in the past to clear the land, promote new growth and attract grazing animals, particularly on dry heathland. However uncontrolled fires can damage some plant and animal communities e.g. Sphagnum mosses and marsh fritillary and its use should be very strictly limited on wet heath and fen. Small, controlled fires in winter may be used to rejuvenate patches of littered fen and dry heath and to create more diversity of structure. Some species such as marsh gentian will rapidly colonise burnt ground. Firebreaks may be needed to enable control and to prevent the spread of wildfires.

Peat cutting There is evidence of past small-scale peat cutting for domestic use. These areas now contain many important plants and animals. To mimic this process and revive these areas small-scale peat removal will be carefully used where appropriate.

Invasive aquatic plants: A number of invasive non-native aquatic plants such as water fern are now present on Anglesey. Should any of them be detected within the SSSI steps should be taken immediately to eradicate them.

Nutrients and fertilisers: Low soil fertility is important to the maintenance of typical heathland and grassland communities, enabling heather and western gorse to compete against more vigorous agricultural grasses. Low soil fertility helps the characteristic native grassland species to compete against more vigorous agricultural grasses. The application of any fertiliser or slurry should be avoided and animals should not be fed with silage on the site.

Woodland: In the past exotic trees (from other countries) have been planted in the birch woodland. Over time these non-native trees should be removed leaving a native mixed deciduous woodland. The hazel woodland on the eastern escarpment should be restored to create a natural habitat link to adjacent woodlands and to help protect the water quality of the aquifer, which feeds the wetland.


Our knowledge and understanding of wildlife is continually improving. It is possible that new issues may arise in the future, whilst other issues may disappear. This statement is written with the best information we have now, but may have to change in the future as our understanding improves, in particular, of the possible/probable impact of climate change. Any information you can provide on the wildlife of your site, its management and its conservation would be much appreciated.

If you would like to discuss any aspect of your SSSI, or have any concerns about your SSSI, please contact your local CCW office.

Your local office is:

Countryside Council for Wales

North Wales Region

Llys y Bont

Ffordd y Parc

Parc Menai



LL57 4BN

Telephone: 01248 672500

Fax: 01248 679259