Threats to UK Lowland Wetland Habitats

 

UK lowland wetland habitats and their associated species are threatened by a range of factors. The text below lists the major pressures and threats to UK lowland raised bogs and lowland fen – and provides a summary of each. This is based on information in the 3rd UK Report on Implementation of the Habitats Directive, the UK Biodiversity Habitat Action Plans, and Common Standards Monitoring for Designated Sites: First Six Year Report.

 

Birch, purple moor grass, bramble and heather invading dried out areas of Fenn's, Whixall and Bettisfield Mosses NNR – copyright Espresso Addict (licensed for use under the Creative Commons Attribution-Share Alike 2.0 Generic Licence, http://creativecommons.org/licenses/by-sa/2.0/, via http://www.geograph.org.uk/)Drainage, water abstraction and water pollution

One of the main causes of loss and poor condition on surviving lowland raised bogs and lowland fens is water management.

Both are waterlogged systems, so any major loss of water will damage them. Water abstraction, insertion of drainage ditches and peat cuttings within or around raised bogs and fens can cause them to degenerate. The importance of protecting the hydrological zone around raised bog margins is often not appreciated. Similar effects can be caused by excessive water abstraction from underlying aquifers, as this can inhibit spring-line flows, generally lower water tables, and affect the natural balance between the differing water qualities of ground water and surface water that fen vegetation is highly dependent upon. As raised bogs dry out, the central 'raised' dome collapses and normal bog vegetation is replaced by species associated with drier habitats, notably bracken and birch. Drying fens are readily invaded by alder-ash-willow-birch scrub/woodland.

Given that natural lowland raised bogs receive water inputs from precipitation alone, they are particularly sensitive to any inputs of surface or ground water, even more so if these are polluted or enriched. Alkaline fens in valley settings are particularly susceptible to pollution from agricultural run-off, and the status of surface and groundwater is crucial in providing the conditions associated with each type of fen vegetation. In both types of habitat, water pollution can have severe adverse effects on the resident vegetation.

 

Regenerating lowland raised bog at Thorne Moors National Nature Reserve following extensive drainage and peat extraction © Peter Rowarth, Natural England

Peat extraction & conversion

Many lowland raised bogs have suffered, and continue to suffer, from peat extraction.  Where extraction has been wholesale and the bog has been converted to agriculture, forestry or urban development, this has effectively destroyed the site potential. However, at sites like Fenns & Whixall Mosses and Thorne Moors, where areas of bog/peat have survived extraction/conversion, major restoration projects have been undertaken to re-flood sites and re-establish bog vegetation and the peat-forming process. Although peat extraction has not been such a major issues for lowland fens, given that peat depth is usually much shallower, loss of fen sites through conversion to agriculture has been very widespread.

 

Air pollution

Air pollution can result in the deposition of unwanted nutrients onto lowland raised bogs and fens, which can critically alter the pH and overall nutrient status of sites. Dry deposition of ammonia is still very high in most parts of England, Wales and Northern Ireland, and critical loads of sulphur are exceeded on some sites. Bisulphite is known to have an inhibitory effect on some Sphagnum moss species and deposition of nitrogen encourages rank competitors, such as the purple moor-grass. For further info: UK Air Pollution Information System (APIS).

 

Inappropriate site management

Konik ponies grazing at Wicken Fen, Cambridgeshire © Rob Noble (licensed for use under the Creative Commons Attribution-Share Alike 2.0 Generic Licence, http://creativecommons.org/licenses/by-sa/2.0/, via http://www.geograph.org.uk/)

A further major cause of unfavourable condition on lowland raised bogs and fens is inappropriate site management. Damage to bog surfaces, by machinery or other means, directly affects the vegetation and hydrological regime and ultimately encourages sites to dry out. Over-grazing by livestock can lead to poaching, compaction, surface contamination, and loss of grazing-sensitive plants. At the other extreme, a lack of grazing coupled with drier conditions on raised bogs usually favours widespread expansion of bracken, tall heather, birch and pine.

 

Comparable vegetation changes occur on lowland fens and here the open vegetation is generally less stable and tends to be more dependent on moderate amounts of grazing and cutting, without which coarse vegetation, scrub and woodland readily develop. Burning was once a management tool used to ‘open’ bog vegetation and create a diverse surface structure: it is now not encouraged as it leads to loss of bog vegetation.

 

Fragmentation

A growing concern is the degree to which surviving lowland raised bog and fen sites are fragmented. Most surviving sites are small in size and/or isolated from another comparable patch of wetland habitat. This means that opportunities for species to disperse between sites and re-colonisation sites under-going restoration are inevitably limited.