Islands, both natural and created, are a valuable habitat within large ponds and lakes. During the breeding season they provide safe nesting sites, particularly favoured by ducks, terns and waders. Waterfowl in moult, when they are unable to fly, will choose the safety of an island for roosting. Wintering and passage species are particularly attracted to islands in brackish water near the coast. Islands are not totally secure from predators however (see below), and vegetation has to be carefully managed to maintain the favoured conditions for particular species.
Islands also increase the amount of valuable edge and shallow water habitat, on which many species of plant and animal depend. To maximise the edge effect, and to allow diversity to develop, several smaller islands are usually better than one large island. Islands can also be effective wave barriers against erosion, although they themselves are liable to erode, especially with increasing distance from shore.
Islands can either be remnant islands, which are areas left unexcavated during pond or lake construction, or they can be spoil or deposit islands, created from material dumped into position. Site, size, shape, orientation and vegetation cover will all affect the value of the island as a habitat.
Islands should not normally be made in ponds less than about 0.5ha size, or the island is likely to prove a constraint on the design of the rest of the pond.
Regional patterns, together with the landform and vegetation of the island, will determine which type of birds are attracted. In general, terns and waders tend to favour sparsely vegetated islands, only just above water level, in the south and coastal areas. Ducks, geese, swans and divers are more numerous on well-vegetated islands on inland and northern waterbodies.
Location
In general, the more distant from shore the safer the island will be from predators, and the more attractive it will be to birds. Stoats and weasels may visit islands less than about 25m from shore, and rats will swim short distances. Foxes are capable of swimming some way, but generally don’t bother. No island is safe from mink predation if they are in the area. Ice provides a bridge for all mammalian predators, and stoats, weasels and rats may stay on the island and wreak havoc during the next breeding season.
In areas open to the public, islands can be situated to provide good viewing conditions for bird watching, or sited well away from public areas. Islands can also be used to screen areas beyond from visual disturbance.
Sheltered locations, to the upwind end of large areas of water, will prove more resistant to erosion.
Shape and profile
This will largely be determined by the depth of water, the material available for construction, and the purpose of the island. Terns and waders require low islands with gently sloping banks and shallow margins, which can normally only be provided in shallow water. Ducks, geese, swans and divers can cope with steeper banks, but still require gently sloping beaches to give young birds access to the water, and sheltered shallow areas for feeding.
Changes in water level and erosion factors need to be taken into account. Low flat islands may be flooded or washed by waves in winter, which can be an advantage in controlling vegetation growth, although shingle may be washed away (see below). Steeper banks are more subject to erosion than shallow banks, and should be aligned away from the prevailing wind.
Where steeper slopes are necessary due to water depth, they should be a maximum of 1:4 both for stability, and for use by wildfowl. A shallow landing beach in the most sheltered location is needed for ducklings. A steep drop to deeper water on one side will prevent the establishment of emergent vegetation, and provide sheltered open water favoured by diving ducks. On reserves open to the public, where good views of birds are important, ‘tear-drop’ shaped islands have been found popular with birds and onlookers. They should be aligned as shown, with the exposed shore sloping gently at about 1:10.
Convoluted edges and bays increase the edge effect and provide shelter and feeding areas for birds, but are more difficult to construct and more liable to erode than simple shapes. The direction of the prevailing wind should be taken into account.
In deep water, island creation is more difficult, as a vast amount of material is needed below water level. Where water levels are fairly constant, revetment techniques can be used to make islands which are steep sided below water level, thus saving on fill, but which slope gently up to the water line and above. However, revetments above about 2m height are beyond the scope of voluntary involvement.
On a miniature scale, sewer pipes can be used to make islands suitable for small ponds. Otherwise, rafts are the only solution for safe nesting sites in deep water.
Remnant islands
Remnant islands are those made by leaving material in situ during excavation of the pond or lake. Where excavation is being done only for pond creation, this lessens the amount of spoil which has to be disposed of. Conversely, for gravel extraction or other commercial operations, leaving an island may represent a large loss, and may hinder the efficient working of machinery. In the latter case, later dumping of spoil or other waste material may be the preferred method.
A spit of land can be turned into an island by excavating a channel to ‘cut off’ the headland. The channel can be fairly steep sided, provided the resulting island has gentle slopes around the remaining perimeter.
Spoil islands
Islands can be created in silted ponds by driving stakes around the planned perimeter of the island, and then piling in silt and other debris from the clearance. You may need to weave willow or other branches through the stakes to stop the silt oozing out. Debark the willow unless you want it to root.
Logs, stumps, rubble and other materials can be used as necessary, but do not use anything which may pollute the water. Organic material will rot down and enrich the water, as well as shrinking, so use inert material as much as possible. Use rubble or other material for the base of the island, and top it with earth, gravel or turf, depending on the purpose of the island. Avoid using nutrient-rich soil which will encourage vigorous growth and favour aggressive plant species, adding to the management work needed in future years. Allow for settlement, as you want to avoid the need to ‘top up’ later, which will be laborious if the only access is by boat.
Where machinery is available, the easiest method for existing lakes and flooded gravel pits may be for material to be tipped progressively from the bank to form a spit. The machine then works back to the shore, excavating the neck of the spit and using the material to build up shallows around the island.
Vegetation
Vegetation introduction and management is very significant in determining the type of species which will be attracted to the island.
In general trees are not a good idea, as in sheltered locations they will soon dominate the island and cast too much shade. They also provide perching places for predatory birds. In exposed locations trees are unlikely to thrive. Mature trees only become of value to waterbirds when they fall into the water, providing sheltered feeding areas and nesting sites.
Shrubs and dense, tussocky vegetation up to about 1.5m (4′) tall provides ideal waterfowl nesting sites, and reduces predation of nests and young ducklings. Remnant islands may have sufficient existing vegetation. Aim to do any necessary vegetation management before the island is cut off, when working conditions are easier.
Establishing vegetation on new wildfowl islands may be difficult, as ducks and geese will soon destroy seedlings and young shrubs, and turn grass to mud. Fencing around the margins, with netting stretched over the top to prevent landing, may be needed for a season or more to allow vegetation to get established.
In contrast, terns and waders require bare shingle and mud surfaces, which require special construction and management to prevent the growth of vegetation.
Bare shingle or mud islands
The RSPB have considerable experience of constructing shingle islands in shallow coastal lagoons for breeding terns and gulls. The main problem is keeping the island free of vegetation, whilst at the same time combating erosion which can rapidly attack the unvegetated surface. The islands are formed of clay or spoil, left to settle for a year, and then reprofiled with a JCB to a smooth flattened dome. The top is then covered with three layers of overlapping fertiliser sacks or similar, which helps suppress plant growth by reducing the supply of water. The sacks are then covered with shingle and sand to a depth of at least 100mm (4″). Alternatively, mud surfaces can be created by covering the plastic with a layer of mud, 50-150mm (2-6″) thick. The shingle or mud may need replenishing in spring every few years, according to the amount of erosion over the preceding winter.
Vegetation is removed from the shingle by hand pulling or raking in autumn, or by Rotavating or hand cultivation. Winter flooding also suppresses vegetation. Mud islands and islands without a plastic underlay rapidly become vegetated, needing weeding and cultivation in spring and autumn.
Low-growing vegetation
Islands which are covered with low-growing vegetation of grasses and shoreline plants may attract a variety of birds, including terns, gulls, waders, lapwing, moorhens and coots. Cutting by scythe or brushcutter may be needed before and after the breeding season (March and August) to encourage the growth of low-growing species. A cut in August also improves the grazing for winter wildfowl. Selective use of herbicides may be necessary if plants such as creeping thistle (Cirsium arvense) or dock (Rumex acetosa) take hold.
Tall-growing vegetation
Islands covered with tussocky vegetation and shrubs up to 1.5m (4′) tall provide ideal conditions for nesting waterfowl. Leave any dead growth over the winter as this provides nesting cover in the early spring.Areas may need managing in rotation in late summer to encourage thick growth near ground level by removing taller scrub and young trees which will otherwise shade out the growth beneath.
