Effects on habitat integrity

Effects on use by predators

Extensive areas of intertidal mud and sandflats have been removed through land-claim coupled in some areas with rising relative sea-levels (Davidson et al, 1991; Burd, 1992). Some estuaries have lost up to 80% of the available area, most of which has been the land-claim of intertidal mud and sand flats. In addition, sea level rise and the constraining of the upper shore boundary will produce ‘coastal squeeze’. Hence there is increasing potential for conflict arising from the conservation interests and the use of estuaries and other coastal areas by people for recreational and many other purposes.

A reduction in the area and biological integrity of these biotope complexes will reduce their carrying capacity for supporting bird and fish predator populations. For example, removal of intertidal areas for industrial developments such as those in the late 1980s in the Port of Felixstowe resulted in the loss of feeding grounds and subsequent reduction in foraging time for waterfowl (Evans, 1996). The remaining areas, in industrialised estuaries, are then often subject to a variety of pressures such as degradation through high levels of pollution and waste discharge, and damage to habitats and disturbance to wildlife by high levels of recreational pressure (McLusky, 1987).

The greatest impact of land reclamation is due to depletion of the main prey rather than simply to area loss and each prey and predator species will differ in their response (McLusky et al, 1992). Although the area of intertidal mudflats in estuaries is smaller than the subtidal area, it provides the dominant feeding area for the fish populations (Elliott & Taylor, 1989a). For example, land-claim in the Forth Estuary has removed 24% of the natural fish habitats in the estuary but 40% of their food supply (McLusky et al, 1992). The greatest effect of land claim in this area is on flatfish such as flounder and juvenile plaice.

Construction associated with land claim, for example in the Orwell estuary, destabilised sediments which may in turn have reduced populations of wading birds (Evans, 1996). The reduction in feeding densities coincided with the changes in the substratum, from mud to sands, a result of the infilling during land-claim. Species such as oystercatcher and dunlin may then lose feeding grounds as they prefer a muddy substratum whereas a sandier substratum would favour the bar-tailed godwit and ringed plover.

In the Forth Estuary (McLusky et al, 1992), numbers of most bird species, such as dunlin and bar-tailed godwit, declined or remained fairly constant after the loss of 20% of the intertidal area whereas shelduck and curlew increased. Similarly, dunlin and redshank were susceptible to changes in their environment on the Tees Estuary and this resulted in a population decline when intertidal mudflat feeding grounds were lost to industrial development (Pienkowski, 1993). The grey plover and the bar-tail godwit also showed reductions in numbers and marginal changes were seen in the knot and curlew whereas oystercatcher and shelduck showed no declines.

Although some species can tolerate or habituate to disturbance of their habitat (McLusky et al, 1992), in areas where land-claim narrows the width of intertidal flats available to birds for feeding, the effects are increased for some shy species which avoid feeding on narrow shores (Bryant, 1979). Shorebirds are seldom distributed evenly over their feeding grounds hence land claim may have a disproportionate effect in relation to its size (Goss-Custard & Yates, 1992) and site surveys are required to determine the number of birds whose feeding areas are directly affected. At a larger scale, land claim may reduce the carrying capacity (Goss-Custard, 1985) of the entire migration and winter feeding grounds. Finally, diminishing prey levels may intensify competition and increase winter mortality rates, with a consequent effect on equilibrium population size (Goss-Custard & Durell, 1990).

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