Why do wetlands need management?
All wetlands have their own destruction built into the nature processes through which they pass and Whisby's lakes and ponds are no exeception.
The bare sand, gravel and clay substrates left behind from the extractions, when filled with ground water, are ideal for rapid colonisation by a variety of marginal vegetation and willow scrub. Both are remarkably efficient at in-filling small shallow ponds with organic detritus, and this has already happened in parts of non-intervention areas bordering Magpie Walk. The larger water bodies simply take longer to reach this stage.
Whilst it should be remembered that each succession from open water to closed woodland has its distinctive associated wildlife which is worth preserving for its own sake, the sheer variety of niches in the gravel pit complex allow degrees of management of wetlands to provide diverse habitats and an attractive landscape for visitors.
The biological result of some of this management is to retain elements of the species associated with a range of successional stages. In places, former gravel pit excavations were used as sediment lagoon during extraction and liquid mud from the sorting process was pumped into the lagoon so that clean water could be reclaimed after settlement. These pits effectively passed through decades of succession in just a few years under this regime and now yield damp willow carr or, in one case, wet clay grassland.
How can man-made wetlands help to conserve wildlife?
The location of wetlands in Whisby Nature Park is un-related to the nearby Witham and Trent drainage systems, and their existence is merely an accident of geology and regional industrial exploitation of resources. As such their value to wildlife might seem to be small. However, in the context of massive wetland drainage in the fen areas of Lincoln over the last 200 years, the gravel pits and associated wetlands do compensate in a small way for this loss. The value would have been much greater if the extractions had occurred in one of the river valleys as the colonising water-dependent organisms would have had a corridor to follow. The fact that Whisby has indeed become important for a range of species, demonstrates the powers of dispersal that can be found among plants and animals.
One specific characteristic of the Whisby wetlands is that being separated from almost all natural land drainage, the water quality is high and the nutrient levels are moderate. As a result, aquatic plants which dominate nutrient-rich environments to the detriment of less vigorous species, are held in check, and animals with relatively high requirements for dissolved oxygen in water can thrive.
In general over-enriched water courses are responsible for the restricted distribution of many aquatic species in Lincolnshire. The problem stems from intensive use of the land in farming and the high applications of nitrogen, ammonia and phosphates that are required to keep productivity high on over-worked soils, some of which finds its way into streams and rivers.
What specific policies are used to increase biodiversity in wetlands?
In maintaining wetlands for wildlife, many land managers are constrained by the long-history of established lakes or river courses in their care. At Whisby Nature Park, the wardens can be more adventurous because there is obviously no long history of wetland species on the site.
The installation of a sluice system allows a water-level regime which benefits a specific range of plants and animals. The severe maintenance of a drainage dyke can be undertaken to study the aquatic plant species that colonise fresh cuts.
Gravel pit islands can be cleared of developing scrub for the benefit of nesting water-birds before the scrub becomes a significant feature in its own right. Newly disused gravel pits can be bulldozed into configurations which will provide sympathetic marginal shallows if they do not occur naturally.
Grazing with sheep is used to maintain certain pits without the characteristic willow margin in order to encourage migrant wildfowl in winter. Grazing with sheep can also help to hold back marginal growth to benefit pioneer plant species which depend on low-competition in shallow water.
Meanwhile, the establishment of some non-intervention areas allows the intrinsic interest of the natural long-term succession to be observed by future generations despite the loss of the water bodies themselves.
