Action: Manage water level in wetlands
- Of six studies, one replicated, controlled study from the USA found that bird diversity was affected by maintaining water levels at different levels.
- A study from the USA found that ducks were more abundant when high water levels were maintained on a wetland site. Geese were more abundant when lower levels were maintained. Three studies from the USA and Canada, two replicated, found that different species showed preferences for different water levels in wetlands.
- A replicated study from the UK found that great bitterns established territories earlier when deep water levels were maintained, but this had no effect on productivity.
- A review from Spain found that management successfully maintained water near a greater flamingo nesting area, but the effects of this were not measured.
Supporting evidence from individual studies
A study in 1958-1967 on Squaw Creek National Wildlife Refuge, Missouri, USA (Burgess 1969), found that, in general, ducks increased when the largest expanses of marshes were flooded, and geese were most abundant when the largest areas of marshes and forage plants were available in response to lowered water levels. The use of the wetlands is discussed in ‘Habitat restoration and creation’.
A trial in Janurary-May 1991-1992 at coastal impoundments and intertidal mudflats on South Island, South Carolina, USA (Boettcher et al. 1995), found that significantly higher numbers of American avocets Recurvirostra americana used the impoundments as water was drawn down over the spring, compared to mudflats, despite mudflats being significantly larger. Avocet distribution within impoundments was highest where water was 10-17 cm deep and 1-30% of the area was exposed; but lowest where there were high daily fluctuations in water depth. Water was slowly drawn down from impoundments from November to April, creating a wide range of water depths, before reflooding in June.
A review of management at a coastal wetland in 1978-1982 in Andalusia, Spain (Martos & Johnson 1996), found that water management was successful in ensuring that there was always an area of water close to a greater flamingo Phoenicopterus roseus nesting area. However, the impact of this could not be quantified.
A replicated study at Delta Marsh, Manitoba, Canada (Murkin et al. 1997), found that different species preferentially used areas of prairie wetlands with varying amounts of open water. Responses to water level (and associated vegetation) changes in ten adjacent prairie wetlands (150 x 300 m) created in 1980 were assessed. Censuses were conducted 1 May to 31 October (1980-1989). Yellow-headed blackbird Xanthocephalus xanthocephalus used shallow-flooded areas with a mix of open water and emergent vegetation, red-winged blackbird Agelaius phoeniceus preferred denser vegetation. American coot Fulica americana preferred deep water with interspersed vegetation. Dabbling ducks generally occupied marsh with equal amounts of vegetation cover and open water. Diving ducks used deeper water but there was variation between species and season, as to whether open or more densely vegetated areas were preferred.
A replicated partially-randomised, controlled study compared waterbird use of four experimentally drawndown wetlands with flooded wetlands at the Grasslands Ecological Area in California's Central Valley, USA (Taft et al. 2002), found that maximum bird diversity and abundance occurred at average depths of 10-20 cm on wetlands with a 30-40 cm difference between deepest and shallowest zones. There was limited availability of shallow-water habitat in winter but not spring, allowing waders, cinnamon teal Anas cyanoptera and American green-winged teal A. carolinensis to use the site. Use by deeper-water dabbling ducks and diving waterbirds declined during later stages of drawdown. Birds were monitored over winter and spring 1994-1995.
A replicated study in 1997-2001 in ten reedbed sites across England (Gilbert et al. 2007) found that male great bitterns Botaurus stellaris established territories significantly earlier in four sites with water levels maintained at 19-27 cm, compared to six with lower water levels (4-9 cm, four managed and four unmanaged). However, there was no effect on chick survival or overall productivity (1.3 chicks/female on high water sites vs. 1.2 on low water sites). Reeds at sites with low water levels were also cut in spring (March-April), compared with winter (completed by December) for high water level sites, but the effect of cutting was not specifically investigated.
- Burgess H.H. (1969) Habitat management on a mid-continent waterfowl refuge. The Journal of Wildlife Management, 33, 843-847
- Boettcher R., Haig S.M. & Bridges W.C. Jr. (1995) Habitat-related factors affecting the distribution of nonbreeding American avocets in coastal South Carolina. The Condor, 97, 68-81
- Martos M.R. & Johnson A.R. (1996) Management of nesting sites for greater flamingos. Colonial Waterbirds, 167-183
- Murkin H.R., Murkin E.J. & Ball J.P. (1997) Avian habitat selection and prairie wetland dynamics: a 10-year experiment. Ecological Applications, 7, 1144-1159
- Taft O.W., Colwell M.A., Isola C.R. & Safran R.J. (2002) Waterbird responses to experimental drawdown: implications for the multispecies management of wetland mosaics. Journal of Applied Ecology, 39, 987-1001
- Gilbert G., Tyler G.A., Dunn C.J., Ratcliffe N. & Smith K.W. (2007) The influence of habitat management on the breeding success of the great bittern Botaurus stellaris in Britain. Ibis, 149, 53-66