Background information and definitions
Loss and degradation of wetlands are two major factors contributing to the global decline of amphibians. Many wetlands have been drained and altered to allow for agriculture and urban development. It has been estimated that the number of wetlands had declined by 33–90% depending on the region of the world (Mitsch & Gosselink 2007).
Creation of wetlands may help to replace some of the habitat lost and therefore help to maintain and increase amphibian populations. However, wetland dynamics are complex and the success of created wetlands depends on many factors such as vegetation, geomorphology and hydrology.
There is additional literature describing the presence of amphibians in agricultural and mine-drainage treatment wetlands designed to intercept and process pollutants (Lacki et al. 1992; Smiley et al. 2011). Use by wildlife is not typically considered in the design of treatment wetlands and wildlife may be restricted using exclusion barriers, trapping or other habitat modifications. Therefore, these studies are not included here.
Study ‘ponds’ and ‘pools’ have been referred to as ‘ponds’ within this section. Studies investigating the creation of individual ponds are discussed in: 'Create ponds for amphibians', 'Create ponds for frogs', 'Create ponds for toads', 'Create ponds for natterjack toads', 'Create ponds for green toads', 'Create ponds for salamanders (including newts)' and 'Create ponds for great crested newts'.
Lacki M.J., Hummer J.W. & Webster H.J. (1992) Mine-drainage treatment wetland as habitat for herptofaunal wildlife. Environmental Management, 16, 513–520.
Mitsch W.J. & Gosselink J.G. (2007) Wetlands. Wiley, New York.
Smiley P.C. & Allred B.J. (2011) Differences in aquatic communities between wetlands created by an agricultural water recycling system. Wetlands Ecology and Management, 19, 495–505.
Supporting evidence from individual studies
A site comparison study in 1994–1996 of a created forested wetland in a Research Refuge in Maryland, USA (Perry, Sibrel & Gough 1996) found that the created wetland supported a similar number of amphibian species to an adjacent natural forested wetland. Ten species were captured in the created wetland (284 individuals) and 11 in the adjacent natural wetland (87 individuals). Spotted salamander Ambystoma maculatum was only found in the created site and wood frog Rana sylvatica and marbled salamander Ambystoma opacum only in the natural wetland. As mitigation for loss of wetland, a 9 ha wetland was constructed in 1994, of which 5.5 ha was forested wetland. Amphibians were captured in pitfall and flannel traps along drift-fencing within the created and adjacent natural forested wetland. Trapping was conducted several times during the year.Study and other actions tested
A before-and-after study in 1996–1997 of a created wetland in Nairobi, Kenya (Nyakang’o & van Bruggen 1999) found that eight species of amphibians used the wetland. Seven species of frog and common toads Bufo bufo were recorded in the wetland. In 1996, a 0.5 ha wetland was constructed using a combination of a sub-surface horizontal flow system planted with Typha, followed by a series of three pond systems planted with a variety of species including local reeds and ornamental plants. Ponds were shallow near the shore with deep sections in the centre (1.5 m).Study and other actions tested
A before-and-after study in 1992–1994 of a constructed wetland (32 ha) in Florida, USA (Kent & Langston 2000) found that nine amphibian species used the wetland within the first two years. Seven species were already present as construction was completed in July 1992. Species richness continued to increase throughout the study. Wildlife was monitored quarterly from July 1992 to August 1994. Counts were undertaken on transect and perimeter walks and call counts were undertaken at night.Study and other actions tested
A replicated, site comparison study in 1999–2000 of nine wetlands in South Dakota, USA (Juni & Berry 2001) found that amphibian and reptile species richness did not differ significantly between created and natural wetlands. A total of 11 amphibian and reptile species were recorded in the wetlands. Four wetlands had been created during the previous 10 years by excavation or enclosing small streams. Five natural wetlands were used for comparison. Monitoring was undertaken using drift-fences and pitfall traps and visual surveys around wetland perimeters in spring and autumn in 1999–2000.Study and other actions tested
A site comparison study in 1995–1996 of a created wetland in Maryland, USA (Toure & Middendorf 2002) found that all but one amphibian species present in an adjacent forest were recorded in the created wetland. Spotted salamander Ambystoma maculatum was the only species not recorded in the wetland. Nine of the species were recorded in all four wetland terraces created. The 52 ha wetland was constructed in four terraces and was surrounded by regenerating forest. Monitoring was undertaken in March–September 1995–1996 using transects, call counts, drift-fencing with pitfall and funnel traps and dip-netting. The adjacent forest was used as a reference site.Study and other actions tested
A replicated, site comparison study of 11 mitigation wetlands in West Virginia, USA (Balcombe et al. 2005) found that amphibian species richness and abundance was significantly higher in created and partially restored wetlands than natural wetlands. Mitigation wetlands had 2.0 species/point compared to 1.5 in natural wetlands and 4.8 amphibians compared to 4.7 per wetland. Seven species were recorded in both wetland types. Abundance of American bullfrog Rana catesbeiana, northern green frog Rana clamitans and pickerel frog Rana palusris were higher in mitigation than natural wetlands (0.2–7.8 vs 0.1–3.6/wetland). Abundance of northern spring peeper Pseudacris crucifer, gray tree frog Hyla chrysoscelis, wood frog Rana sylvatica and eastern American toad Bufo americanus were similar between wetland types (mitigation: 0.4–22.9; natural: 0.1–28.4/wetland). Mitigation wetlands were 3–10 ha, had depths of 5–57 cm and had been constructed 4–21 years previously. The four reference wetlands were 7–28 ha, had depths of 5–17 cm and were near mitigation sites. Amphibians were monitored using nocturnal call surveys once a month in April–June 2001–2002.Study and other actions tested
A replicated, site comparison study in 2000–2001 of 42 wetlands constructed to replace lost wetlands in the Tillplain ecoregion, Ohio, USA (Porej & Hetherington 2005) found that created wetlands supported 13 species of amphibians and had an average species richness of four per site (range 1–7). Occurrence of species varied from 2–76%. Species richness was positively associated with presence of a shallow shoreline (on average two species more than wetlands without) and negatively associated with predatory fish (average one species less). Wetland age, size, emergent vegetation cover and surrounding forest cover did not affect species richness. Wetlands tended to be in areas with little or no forest cover and so amphibian species associated with forested wetlands were rare or absent. Amphibians were sampled three times in March–July 2001–2002 using aquatic funnel traps, dip-netting and visual surveys. Four call surveys were undertaken at the end of each month from March–June.Study and other actions tested
A site comparison study in 2004–2005 of eight created and 14 restored wetlands associated with hardwood forests in Louisiana, USA (Barlow 2007) found no significant difference in amphibian species richness between created/restored and natural wetlands. Twelve of 13 species in the area were found within the wetlands, one of which was only found in created/restored wetlands (upland chorus frog Pseudacris feriarum). Species richness was higher in 2004 (created/restored: 3.7; natural: 4.2) than 2005 (created/restored: 2.4; natural: 2.2). Richness was positively associated with water depth, canopy cover, flooding, aquatic vegetation and surrounding forest. Temporary and permanent wetlands were 1–174 ha and had been created or restored 1–18 years previously. Restoration had included replanting trees, water management and dredging. Eight natural wetlands within a wildlife refuge were used for comparison. Amphibians were monitored by call surveys (2/season), egg mass counts (1/season) and dip-netting (monthly along a 100 m transect).Study and other actions tested
A before-and-after study in 2000–2004 of constructed wetlands in southern Illinois, USA (Palis 2007) found that amphibians rapidly colonized wetlands and restored surrounding terrestrial habitat. A total of 17 species were recorded with one new species each year. There were 12–15 species and 5,216–8,462 animals recorded at each wetland. Wetlands were created in 1999–2000 by enclosing water behind earth dams at the end of valleys. Hardwood tree seedlings were also planted. Wetlands were surveyed in April–June each year. Monitoring was undertaken using drift-fencing (four/wetland and three/adjacent habitat) with funnel traps (4/fence), artificial coverboards (0.7 m2), visual encounter surveys, dip-netting and frog call surveys.Study and other actions tested
A before-and-after study in 1998–2004 of created wetland on a golf course in Long Reef, Sydney, Australia (Pyke et al. 2008) found that captive-bred green and golden bell frog Litoria aurea tadpoles released into a created wetland did not establish a self-sustaining population. Once releases had stopped, the number of frogs declined to zero. Only 45 adult frogs were recorded. A few males were heard calling, but breeding was not recorded. Releases did not result in any metamorph or immature frogs if they occurred during autumn or involved low numbers of tadpoles, if ponds dried out soon after release or if fish were present. Successive releases into fish-free ponds were less successful in terms of numbers of metamorphs and immatures. Sixteen ponds, 12 of which were inter-connected (20–200 cm), were created in 1996–1997, with aquatic emergent vegetation and shrubs planted. A total of 9,000 captive-bred 3–4 week old tadpoles were released into the ponds over 11 occasions in 1998–2003. Amphibian monitoring was undertaken at 1–4 week intervals using artificial shelters around ponds, dip-netting and visual count surveys.Study and other actions tested
A replicated, site comparison study in 2007 of a complex of 10 created temporary ponds in central Ohio, USA (Korfel et al. 2010) found that amphibian biomass was similar in the created and a natural complex of temporary ponds, although natural ponds supported more species (7 vs 4). There was no significant difference between created and natural ponds in overall biomass (dip-net: 3 vs 1; funnel trap: 3 vs 6 g/pond) or family biomass (hylidae: 1 vs 1; ranidae: 1 vs 4; ambystomatidae: 1 vs 2 g/pond). Created ponds had higher taxa diversity than the natural ponds (0.95 vs 0.70 Shannon-Weaver index) due to a more even distribution between the three families. Eleven years after construction, significant differences between created and natural ponds were found for hydrology, dissolved oxygen, conductivity and temperature. Wetland construction was completed in 1996. Amphibian larvae were sampled in May-July 2007 using dip-netting (all ponds) and funnel traps (in one of each pond type). Hydrology and physiochemistry were recorded for each pond in April–July. Comparisons were made with six natural temporary ponds in a nature preserve.Study and other actions tested
A replicated, site comparison study in 2006 of 49 constructed wetlands throughout northern Missouri, USA (Shulse 2010) found that 16 of 22 local amphibian species were recorded in the wetlands. The average number of species per wetland was five (range: 0–10). Cricket frogs Acris cepitans, bullfrogs Lithobates catesbeianus and leopard frog Lithobates blairi/sphenocephalus complex were each found in over 80% of wetlands. Green frogs Lithobates clamitans and gray tree frog Hyla versicolor/chrysoscelis complex were found in 53–55% of wetlands. Other frog and toad species were recorded in 29–37% and salamanders in 2–18% of wetlands. Species were positively associated with variables such as pond or stream density, grassland, wetland or vegetation cover. Fish were present at 43% of sites. Twenty wetlands were compensatory wetlands for road developments, and many others were farm ponds that were then managed by the agency for wildlife. Wetlands ranged from temporary ponds to large permanent ponds. Amphibians were sampled once or twice in March–August 2006 by visual and call surveys, dip-netting and funnel trapping.Study and other actions tested
A replicated, site comparison study in 2010 of 14 constructed ridge-top wetlands in a National Forest in Kentucky, USA (Denton 2011, Denton & Richter 2013) found that amphibian species richness was similar, communities different and abundance lower in created compared to natural wetlands. Species richness did not differ significantly in constructed (10) and natural wetlands (12). However, captures were lower in constructed (permanent: 650; ephemeral: 407) compared to natural wetlands (1, 315). Amphibian communities differed significantly between constructed and natural wetlands. Larvae of wood frogs Lithobates sylvaticus and marbled salamanders Ambystoma opacum were almost exclusively found in natural wetlands, whereas large frog species (Lithobates clamitans, L. catesbeianus, L. palustris) and eastern newts Notopthalmus viridescens tended to be in constructed (particularly permanent) wetlands. Wetland size and depth were positively and aquatic vegetation negatively associated with some species. Captures were not affected by wetland age. Constructed wetlands were either permanent damed wetlands (built 1988–2003; n = 7) or ephemeral wetlands with added woody debris (built 2004–2007; n = 7). Five natural wetlands were also monitored. Dip-net sampling was undertaken over three days/wetland/month in May–August 2010.Study and other actions tested
A replicated, site comparison study in 2009–2010 of nine constructed ridge-top wetlands in a National Forest in Kentucky, USA (Drayer 2011) found that amphibian communities in shallow, but not deep, constructed wetlands were similar to natural wetlands. Communities differed significantly in deep constructed and natural wetlands. Species richness was similar in created and natural wetlands (13 vs 12). Constructed wetland communities tended to reflect permanent pond-breeding amphibians, while those in natural wetlands contained temporary pond-breeding species. Abundance of individual species differed between wetlands types and a small number of species were found only in natural or constructed wetlands. Two predatory species American bullfrog Rana catesbeiana and eastern newt Notophthalmus viridescens were found in higher numbers in constructed wetlands and were considered by the authors to increase predation rates. Five shallow and four deep constructed wetlands and six natural (temporary) wetlands were monitored. Monitoring was undertaken four times/year in March–August and included visual perimeter counts, call surveys, minnow trapping and dip-netting.Study and other actions tested
A global review in 2012 of studies comparing created and restored wetlands to natural wetlands (Brown et al. 2012) found that amphibian species richness or abundance at created and restored wetlands tended to be similar to or greater than natural wetlands. Species richness or abundance of some or all species was greater at created or restored wetlands in 54% of studies, similar in 35% of studies and lower than natural wetlands in 11%. Created and restored wetlands tended to be larger, deeper and were wet for more of the year than natural wetlands. Species richness and abundance tended to be positively associated with abundance of emergent vegetation, proximity of source wetlands and the availability of wetlands with varying water levels. They were also influenced by upland habitat and tended to be negatively associated with fish presence. Only peer-reviewed studies were included (n = 37; 70% in USA). Only studies that converted existing upland or shallow-water areas to wetland habitat (created; n = 27), or restored wetlands (n = 14) were included. Wetlands built specifically for water quality improvement were not included. Twenty-six studies had controls, either natural reference wetlands or historic data.Study and other actions tested