Deepen, de-silt or re-profile ponds

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Two before-and-after studies in France and Denmark found that pond deepening and enlarging or re-profiling resulted in the establishment of a breeding population of great crested newts or translocated garlic toads. Two studies (including one replicated, controlled study) in the UK and Denmark found that pond deepening and enlarging or dredging increased a population of common frogs or numbers of calling male tree frogs.
  • Four before-and-after studies in Denmark and the UK found that pond deepening, along with other interventions, maintained newt populations and increased populations of European fire-bellied toads or natterjack toads.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A before-and-after study in 1977–1992 of a pond in an abandoned sand-quarry in northwestern France (Arntzen & Teunis 1993) found that pond enlargement for great crested newts Triturus cristatus resulted in rapid colonization and fast initial population increase, followed by a dramatic decline. Newts were recorded in the pond the year after enlargement. The population increased to 346 adults within five years, but decreased to 16 newts two years later. However, by 1992 the population was estimated at 55 adults. Variation in the adult population was largely due to variation in juvenile recruitment. The juvenile cohort was estimated at 300 individuals in 1980, but zero by 1984. Juvenile survival varied from 7 to 45%. Before enlargement, the shallow pond (30 cm) supported a breeding population of natterjack toads Bufo calamita, but not great crested newts. In summer 1977, it was enlarged by 7 x 20 m, approximately doubling its area, and to a maximum depth of at least 1.2 m. Newts were monitored in 1979–1984 and in 1992 by torching the shallow part of the pond from dusk to midnight and dip-netting.

    Study and other actions tested
  2. A before-and-after study in 1972–1991 of ponds on heathland in Hampshire, UK (Banks, Beebee & Denton 1993) found that pond restoration by deepening, along with other interventions, tripled a natterjack toad Bufo calamita population. Spawn string counts (female population) increased from 15 to 43, with a maximum number of 48 in 1989 (see also Buckley & Beebee 2004). Nine small ponds (< 1,000 m2) were created and four restored by excavation to generate shallow, temporary ponds with gradually shelved margins. Scrub was cleared from 40 ha by cutting and uprooting, bracken was treated with herbicide over 12 ha and swamp stonecrop Crassula helmsii, which invaded six new ponds, was pulled up and treated with herbicide. Captive-reared toadlets raised from spawn were released in 1975 (8,800), 1979, 1980 and 1981 (1,000 each). Limestone was added to one naturally acid pond (735 m2) annually in April 1983–1989. Toads were monitored annually, once every 10 days in March and August.

    Study and other actions tested
  3. A before-and-after study in 1981–1993 of 20 restored ponds in Middlesex, UK (Williams & Green 1993) found that the population of common frogs Rana temporaria increased as the number of ponds restored by deepening increased (see also Williams 2005). Egg clumps increased from 40 in one pond in 1983 to 584 in 1992 (1–370/pond). However, numbers declined to 399 egg clumps in 1993, which was considered by the authors to be due to drought. Many ponds within a country park had dried up and so were restored by deepening and enlarging (4–1,680 m2) in 1981–1993. Eight ponds were also created in the area increasing the total pond area from 2,248 m2 in 1983 to 4,965 m2 in 1993. Egg clumps were counted in restored ponds in February–March as an index of numbers of breeding females.

    Study and other actions tested
  4. A replicated, controlled study in 1991–1994 of 29 restored ponds on the island of Lolland, Denmark (Hels & Fog 1995) found that numbers of calling male tree frogs Hyla arborea increased significantly and larvae increased and then decreased after dredging. Numbers of calling males increased significantly in dredged but not undredged ponds from 1991 to 1994. The year after dredging, numbers of larvae were significantly higher in dredged ponds compared to undredged ponds; numbers had been similar before dredging. However, two years after dredging, there was no significant difference between numbers of larvae in dredged and undredged ponds. In 1991–1993, 29 ponds that had at least three calling males were restored by dredging. Water was usually pumped out and mud removed from the bottom. Frogs were monitored by call surveys and dip-netting (30 minutes) in 1991–1994.

    Study and other actions tested
  5. A before-and-after study in 1986–1997 of 69 restored and created ponds at six sites in Funen County, Denmark (Briggs 1997) found that there was an increase in the population of European fire-bellied toads Bombina bombina. The total adult population increased from 82 in 1986–1988 to 542 in 1995–1997 (from 1–30 to 8–170/site). Numbers of ponds occupied by adults increased from eight to 62 and by tadpoles from one to 18 over the same period. The population declined at only one site that was flooded with salt water. Ponds were restored by dredging or created. Wild-caught toads were paired in separate nest cages in ponds and eggs collected and reared in aquaria. Metamorphs and one-year-olds were released into ponds. Ponds were monitored for calling males and breeding success (capture-recapture estimate) annually in 1987–1997.

    Study and other actions tested
  6. A before-and-after study in 1986–1995 of two ponds within a housing development near Peterborough, UK (Cooke 1997) found that pond deepening, fish removal and regulation of water levels resulted in the maintenance of great crested newt Triturus cristatus and smooth newt Triturus vulgaris numbers seven years after development. Pre-development numbers were variable for great crested (29–102) and smooth newts (10–18). Adults of both species returned to breed in 1989–1990 following development (crested: 51–67; smooth: 16–42) and until 1995 (crested: 55–123; smooth: 33–125). However, production of metamorphs failed in 1990 due to three-spined sticklebacks Gasterosteus aculeatus in one pond and drying of the other. Larval catches increased in 1991 following fish removal (crested: 37; smooth: 13) and maintenance of water level (crested: 62; smooth: 22) and then varied in each pond (crested: 1–15; smooth: 1–27). Development was undertaken in 1987–1989. Ponds (800 m2) were deepened in 1988, fish removed by pond drying in 1990 and water pumped to the pond that dried naturally from 1991. A 1 ha area was retained around ponds. Newts were counted by torch and larvae netted once or twice in 1986–1987 and 3–4 times in March–May 1988–1995.

    Study and other actions tested
  7. A before-and-after study in 1994–1997 of two restored ponds in Jutland, Denmark (Jensen 1997) found that translocated garlic toads Pelobates fuscus established breeding populations in both ponds. Breeding was recorded in one in 1996 and the other in 1997. Ponds were restored by removing surrounding willows and by levelling the banks of one pond. Forty-three toads were captured from a pond being eliminated by development. Four egg strings were produced and raised in captivity. The 43 adults and 1,000 tadpoles were released into one of the restored ponds in 1994. Toads were monitored by tadpole and call surveys.

    Study and other actions tested
  8. A before-and-after study in 1972–1999 of natterjack toads Bufo calamita at two sites in England, UK (Buckley & Beebee 2004) found that pond restoration and creation, vegetation clearance and captive-rearing toadlets resulted in population increases over 20 years. The continuation of a study in Hampshire, UK in 1972–1991 (Banks, Beebee & Denton 1993) until 1999 indicated that there was a doubling of the population. Egg string counts (female population) increased from 15 in 1972 to 32 in 1999, with a maximum number of 48 in 1989. At a second site, spawn string counts increased from 1 in 1973 to 8 in 1999, with a maximum number of 29 in 1997. Ponds were created and restored by excavation, scrub and bracken was cleared and captive-reared toadlets raised from eggs and released. Toads were monitored annually.

    Study and other actions tested
  9. In a continuation of a study (Williams & Green 1993), a before-and-after study in 1983–2004 of 31 ponds in Middlesex, UK (Williams 2005) found that pond restoration and creation resulted in a significant increase in total common frog Rana temporaria egg masses. Numbers increased from 40 egg masses in 1983 to 1,852 in 2002, although then declined to 1,000 in 2004. Numbers of egg clumps increased with pond size and eight ponds contained 89% of the spawn. The numbers of ponds used for breeding each year increased from one in 1983 to 20 in 2000. Breeding tended to occur two years after pond creation or restoration. Egg clumps were counted in restored ponds in February–March as an index of numbers of breeding females. An unmonitored number of eggs, tadpoles and frogs were introduced and removed from ponds by the public, particularly in 1984. Colonization may not therefore have been natural.

    Study and other actions tested
Please cite as:

Smith, R.K., Meredith, H. & Sutherland, W.J. (2020) Amphibian Conservation. Pages 9-64 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

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Amphibian Conservation

This Action forms part of the Action Synopsis:

Amphibian Conservation
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What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

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