14.9 Managing wetlands
14.9.1 Manipulating the proportions of open water, ruderal vegetation,
and swamp
The suitability of shallow (ca1m), nutrient-rich freshwater wetlands for birds
is primarily influenced by the proportions of open water, swamp, fen, and scrub
(Kaminski and Prince 1981; Linz et al. 1996 and Section 14.9.6), inputs of
nutrients, pesticides, and other pollutants and levels of human disturbance
(e.g. see Newton 1998). Each of the different habitats mentioned represents a dif-
ferent stage in the process of vegetation succession and each supports different
assemblages of birds. Long-term changes in the extent of open water, swamp/fen,
and scrub can be determined from aerial photographs. The effect of nutrients,
pesticides, and other pollutants is a complex subject and outside of the scope of
this chapter (see Newton 1998 for a review of their effects).
Nutrient-poor wetlands comprise bogs, base-poor fens, and some other types
of fen fed by oligotrophic groundwater. Where they persist in the lowlands, their
fragile plant and invertebrate assemblages are invariably threatened by low water
levels and eutrophication. There is little or no management that could be carried
out specifically to benefit birds in these types of wetlands without damaging their
existing plant and invertebrate interest. For this reason, their management is not
discussed further.
There are two approaches to manipulating the proportions of open water and
swamp in a wetland. One involves periodically lowering water levels (draw-
downs). The other involves maintaining a more constant water regime, while
preventing or reversing succession in specific areas by removing vegetation or
lowering the substrate. When installing control structures to manipulate water
levels, it is important to consider their potential impact on movement of fish
within the site. In particular, water control structures can impede upstream
migration of fish such as Common Eels Anguilla anguilla, which can be import-
ant prey for birds such as Bitterns (Gilbert et al. 2003). This problem can be
mitigated by installing passes (Knights and White 1998).
Periodic drawdowns during the growing season wetlands expose moist, bare
mud on which seedlings of ruderal and emergent plants can germinate.
Techniques for optimizing germination of ruderal plants to maximize seed pro-
duction are discussed in Section 14.9.2. Swamp can be created by re-flooding
these emergent plants in autumn, taking care not to completely submerge
them, and then allowing them to expand through vegetative growth. The ratio
of swamp to open water will be important in influencing bird use and can be
352 |Habitat management