Action: Restore or create inland wetlands
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- Of eleven studies captured, 11, from the mainland USA, Guam, Canada and Hawaii, found that birds used artificially restored or created wetlands. Two found that rates of use and species richness were similar or higher than on natural wetlands. One found that use rates were higher than on unrestored wetlands.
- Three studies from the USA and Puerto Rico found that restored wetlands held lower densities and fewer species of birds than natural wetlands.
- A replicated study from the USA found that least bittern productivity was similar in restored and natural wetlands.
- Two replicated studies examined wetland characteristics: one from the USA found that semi-permanent restored wetlands were used more than temporary or seasonal ones. A study from Hawaii found that larger restored wetlands were used more than smaller sites.
This section includes studies describing the effects of wetland restoration or creation for all wetlands which are not coastal, or do not receive regular influxes of salt water.
Supporting evidence from individual studies
A study in 1958-1967 on Squaw Creek National Wildlife Refuge, Missouri, USA (Burgess 1969), found that annual use of the 2,772 ha wetlands (created in 1935) varied from 6-27 million duck-days and from 7-19 million goose-days each year. Management included winter water removal to aerate the soil and eradicate carp, and spring flooding.
A study in 1986 at the Savannah River Site, South Carolina, USA (Coulter et al. 1987), found that up to 94 wood storks Mycteria americana and over 210 other wading birds were seen on specially constructed and managed ponds at once. Ponds were created in a 14 ha depression and stocked with fish between 1985 and 1986.
A before-and-after study in 1992 on Guam, South Pacific (Ritter & Sweet 1993), found that Mariana common moorhens Gallinula chloropus guami colonised a newly-created wetland within five months of its creation, with two adults and at least four chicks being seen. The wetland was 20-60 cm deep, 45 m long and up to 27 m wide and created using an excavator in January 1992. Spikerush Eleocharis dulcis, water lettuce Pistia stratiotesm, taro Colocasia esculenta and rusty flatsedge Cyperus odoratus were planted, although the taro died, probably because of excessive flooding.
A replicated, controlled study in 1992-1994 in wetlands in the Lake Ontario and St. Lawrence River plains, New York State, USA (Brown & Smith 1998), found lower species richness and densities of wetland birds on restored wetlands compared with natural wetlands (6 species/ha and 15 birds/ha for 18 restored sites vs. 8 and 20 for eight natural sites). This pattern was stronger for wetland dependent species. Restored sites also had community compositions more similar to other restored sites than to natural wetlands. Birds were surveyed with an unlimited-radius point count within each wetland each year during the breeding season.
A replicated study from April-June in 1985-1991 in a 13 ha wetland site in South Carolina, USA (Post 1998), found that least bitterns Ixobrychus exilis nested at high densities (12 pairs/ha), had a 50% hatching rate and 55% of nests produced fledglings. The author points out that this rate is only slightly lower than that reported for natural wetlands. An average of 2.7 fledglings/nest were produced from an average 3.8 eggs/clutch. Most egg mortality was caused by nest instability.
A replicated, paired site study from May-July in 1997-1998 in 39 pairs of restored and natural wetlands in North and South Dakota, USA (Ratti et al. 2001) found that restored wetlands exhibited equal, and often greater, avian abundance, species richness and diversity. There were no significant differences in overall bird abundance, species richness or diversity; waterfowl breeding pair density or upland species richness between restored and natural wetlands. However, Canada goose Branta canadensis, mallard Anas platyrhynchos, redhead Aythya americana, and ruddy duck Oxyura jamaicensis exhibited significantly higher densities on restored wetlands. Total area bird counts were performed four times on each wetland.
A replicated, controlled study from May-July in 2000 in Virginia, USA (Snell-Rood & Cristol 2003), found that bird species richness and diversity in artificially created wetlands were significantly lower than in natural wetlands (average of 11 species/site for six artificial wetlands vs. 17 for five natural wetlands). Although total bird abundance, and the abundance of wading birds, waterfowl, raptors, aerial feeders or woodpeckers were similar, natural wetlands had significantly higher songbird abundance. In addition, created wetlands exhibited bird communities with significantly lower conservation value (based trophic level and migratory status) but similar average habitat specificity and wetland dependency. All wetlands had similar surrounding habitats and were of similar ages (time since planting for created and since logging for natural wetlands), and sizes (5-15 ha).
A replicated, controlled study in April 1998-1999 on Prince Edward Island, Canada (Stevens et al. 2003), found that six out of eight wildfowl species were found in significantly higher numbers in 22 restored wetlands than in 24 control (unrestored) wetlands. Four species also had significantly more broods at restored sites. Large wetlands, close to rivers and with a large proportion of cattails Typha spp. held more species than other sites. All sites were freshwater wetlands, 0.3-6.0 ha in size and restored sites were dredged, starting in 1990, to remove excess organic material.
A small controlled study from 2004-2005 in Toa Baja, Puerto Rico (Acevedo 2007) found that fewer bird species were recorded in an 18 ha restored forested wetland than in a natural forested wetland (nine records of five species in restored site vs. 65 records of 16 species in the natural site). In addition, only one species (yellow-faced grassquit Tiaris olivaceus) was observed foraging in the restored wetland; 40% of records in the natural site were of foraging birds. Only two records, both of northern waterthrush Seiurus novaboracensis were made at a 14 ha control (unrestored) grassland site. The restored wetland was planted with 7,000 Pterocarpus officinalis and Annona glabra trees during 1997-2000.
A replicated, randomised study in spring from 2004-5 in 28 small restored wetlands in Illinois, USA (O'Neal et al. 2008), found that semi-permanent wetlands were used more frequently by waterbirds than temporary or seasonal wetlands and held more waterfowl broods (semi-permanent wetlands were used on 56% of days and held 1.1 broods/ha vs. 37% and 0.2 for seasonal and 7% and zero broods for temporary wetlands). Hydrologic management (passive restoration and management; active restoration through hydraulic engineering but passively managed or actively restored and managed through regulation of hydrologic regime) was the most important variable in explaining bird abundance and distributions. Of the 28 wetlands, 25 were <5 ha in size and 17 were <1 ha. Water birds included waterfowl, wading birds and shorebirds. Wetlands were classified as semi-permanent if there was surface-water throughout growing season; seasonal if there was surface water at the start, and for long periods of the growing season, but not by the end of it; and temporary if surface-water was only found for brief periods throughout the growing season.
A replicated, site comparison study from March 2002 to July 2003 in wetlands in Kohala-Mauna Kea, Hawai’i (Uyehara et al. 2008) found that Hawaiian ducks Anas wyvilliana used 16 restored wetlands more often than 32 agricultural wetlands, despite the greater availability of the latter. Restored wetlands had a significantly higher occupancy rate than agricultural wetlands (81 vs. 41% of sampled sites) and higher consistency of occupancy (13 vs. 7% of all surveys). Hawaiian ducks preferred wetlands that were larger (>0.23 ha), further from houses and surrounded by more wetland habitat. No wetland within 600 m of a house was occupied. Wetland occupancy was not affected by presence of invasive species or grazing intensity. Wetlands ranged from 0.01-1.30 ha and were surveyed every two months.
- Burgess H.H. (1969) Habitat management on a mid-continent waterfowl refuge. The Journal of Wildlife Management, 33, 843-847
- Coulter M.C., McCort W.D. & Bryan Jr A.L. (1987) Creation of artificial foraging habitat for wood storks. Colonial Waterbirds, 203-210
- Ritter M.W. & Sweet T.M. (1993) Rapid colonization of a human-made wetland by Mariana common moorhen on Guam. Wilson Bulletin, 105, 685-687
- Brown S.C. & Smith C.R. (1998) Breeding season bird use of recently restored versus natural wetlands in new york. The Journal of Wildlife Management, 62, 1480-1491
- Post W. (1998) Reproduction of Least Bitterns in a Managed Wetland. Colonial Waterbirds, 21, 268-273
- Ratti J.T. (2001) Comparison of Avian Communities on Restored and Natural Wetlands in North and South Dakota. The Journal of Wildlife Management, 65, 676-684
- Snell-Rood E.C. & Cristol D.A. (2003) Avian communities of created and natural wetlands: bottomland forests in Virginia. The Condor, 105, 303-315
- Stevens C.E., Gabor T.S. & Diamond A.W. (2003) Use of restored small wetlands by breeding waterfowl in Prince Edward Island, Canada. Restoration Ecology, 11, 3-12
- Acevedo M.A. (2007) Bird feeding behavior as a measure of restoration success in a Caribbean forested wetland. Ornitologê Neotropical, 18, 305-310
- O'Neal B.J., Heske E.J. & Stafford J.D. (2008) Waterbird Response to Wetlands Restored Through the Conservation Reserve Enhancement Program. Journal of Wildlife Management, 3, 654-664
- Uyehara K.J., Engilis A. Jr. & Dugger B.D. (2008) Wetland features that influence occupancy by the endangered Hawaiian duck. Wilson Journal of Ornithology, 120, 311-319