14.5.6Hydrology of wet grasslands
Shallow (50 cm deep) flooding on grassland can be used to attract wildfowl by
providing both safe roost sites and suitable feeding conditions. Regular winter
flooding followed by grazing or cutting encourages perennial grasses and some
other plants important as seed sources for wintering wildfowl, but tends to produce
less ruderal vegetation than moist soil management (Section 14.9.2). Regular
inundation also encourages some grass species favored by herbivorous wildfowl,
notably Creeping Bent Agrostis stolonifera. Retention of shallow floods during the
breeding season will provide feeding areas for breeding wildfowl, although more
densely vegetated ditches are probably more valuable for brood rearing.
Waders on grassland feed on invertebrates in the soil (primarily earthworms
Lumbricidae and leatherjackets Tipulidae), amongst vegetation, and in shallow
pools. Flooding previously unflooded grasslands creates a short-term “flush” of
displaced soil invertebrates, which can attract waders and other species. However,
soil invertebrates are slow to re-colonize areas vacated during flooding. This
means that flooding large areas of wet grassland, either at the same time or on
rotation, is likely to greatly decrease the total abundance of prey for waders
(Ausden et al. 2001). One effective way of maintaining suitable conditions for
breeding waders in the long-term is to maintain a mosaic of unflooded grassland
with a high water table (if soils are suitable—see below) on which waders can nest
and feed on soil invertebrates, and shallow pools that sequentially dry out and
concentrate aquatic prey during the breeding season (Ausden 2001). This is eas-
iest to achieve on sites with varied topography, such as unleveled coastal grazing
marsh. Excavation can be used to create shallow creeks and pools at otherwise
uniform sites, but it can be difficult to create a natural-looking variation in
height. Disposal of unwanted spoil can also be a problem. Retention of surface
water is easiest on soils with low rates of water transmission, such as clays.
Maintaining a high water table within fields can benefit Snipe Gallinago
gallinagoand Black-tailed Godwits by keeping the upper soil moist and therefore
soft enough for them to probe for soil invertebrates (Green et al. 1990b). A high
water table also concentrates soil invertebrates close to the soil surface, particu-
larly on the margins of shallow floods. It may also retard vegetation growth and
thereby help maintain suitable conditions for breeding waders that prefer more
open conditions, such as Lapwings (Figure 14.5).
It is usually only possible to maintain a high field water table during the breeding
season on soils that have a high rate of water transmission, such as undamaged
peat. This can be done by maintaining high water levels in surrounding ditches,
particularly if these ditches are closely spaced, so that water can flow laterally
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