Excavate freshwater pools

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Seven studies evaluated the effects, on vegetation within pools or surrounding marshes/swamps, of excavating freshwater pools. Five studies were in the USA, one was in Guam and one was in Canada. Two of the studies in the USA were based on the same set of pools.

VEGETATION COMMUNITY

  • Relative abundance (2 studies): One replicated, paired, site comparison study in a freshwater marsh in Canada reported that a smaller proportion of individual plants around excavated pools were wetland-characteristic species, compared to the proportion around natural pools. The excavated pools were 1–3 years old. One replicated study in the USA reported that excavated pools became dominated by non-native plant species over eight years.
  • Overall richness/diversity (3 studies): One replicated, paired, site comparison study in a freshwater marsh in Canada found that overall plant species richness and diversity were similar around excavated pools and natural pools, 1–3 years after excavation. Two studies involving freshwater marshes in Guam and the USA simply quantified plant species richness 12–18 months after excavation (along with other interventions).

VEGETATION ABUNDANCE

  • Overall abundance (1 study): One replicated, site comparison study in the USA found that excavated and natural pools had similar cover of emergent vegetation, seven years after excavation. The same was true for submerged vegetation.
  • Characteristic plant abundance (2 studies): Two replicated studies in the USA reported the abundance of native pool-characteristic species over 3–8 years after excavating pools. One of the studies was also a site comparison and reported that these species were less abundant in the excavated pools than nearby natural pools.
  • Shrub abundance (2 studies): One replicated, site comparison study in the USA found that excavated and natural pools had similar cover of shrubby vegetation after seven years. One replicated study in the USA simply quantified shrub abundance over five years after excavating pools/potholes (along with other interventions).
  • Algae/phytoplankton abundance (1 study): One replicated, site comparison study in the USA found that excavated and natural pools contained a similar biomass of surface-coating algae and phytoplankton, after seven years. The same was true for phytoplankton after eight years.
  • Individual species abundance (5 studies): Five studies quantified the effect of this action on the abundance of individual plant species. For example, one replicated, site comparison study in the USA found that excavated and natural pools had similar cover of loosestrife Lythrum sp. seven years after excavation, but that excavated pools had greater cover of duckweed Lemna sp., cattails Typha spp. and common reed Phragmites australis.

VEGETATION STRUCTURE

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 1992–1993 on a tourist resort in Guam (Ritter & Sweet 1993) reported that a freshwater pool created by excavation, lining with wetland soil and planting herb species contained two of the four planted species after one year, and four additional species. The two planted species present after one year were spikerush Eleocharis dulcis (60% cover) and rusty flatsedge Cyperus oderatus (<1% cover). Four additional species were present after one year: two rushes, one grass and one forb (<1–10% cover). Methods: In January 1992, a 600-m2 wetland was excavated on a natural valley slope, lined with wetland soil (30 cm deep) and planted with four herbaceous species (120 spikerush, an unclear number of rusty flatsedge, 20 taro, 5% cover of water lettuce). The study does not distinguish between the effects of these interventions on non-planted vegetation. The wetland was fed by ground and surface water, and had a stable 20–60 cm water depth. Final vegetation cover was estimated in January 1993.

    Study and other actions tested
  2. A study in 1991–1993 of an excavated and planted freshwater wetland in Ohio, USA (Niswander & Mitsch 1995) reported that it developed vegetation cover, including 13 of 17 planted herb species, after 18 months. Eighteen months after planting, 50 herbaceous plant species were recorded in the marsh and wet meadow zones (vs 35 after six months and 44 after 15 months). Of these, 13 were planted species (12 emergent marsh and wet meadow species, plus one cover crop). The other 37 species had colonized spontaneously. No submerged vegetation was recorded within pools in the wetland. Methods: In autumn 1991, two connected wetland basins (6.1 ha total area) were excavated from former farmland. In spring 1992, seventeen wetland herb species (including three intended as cover crops) were planted into flooded and saturated areas of the basins. In autumn 1992, summer 1993 and autumn 1993, herbaceous plant species were recorded along six transects spanning the wetland. The study does not distinguish between the effects of excavation and planting on non-planted vegetation.

    Study and other actions tested
  3. A replicated study in 1997–2002 of excavated ephemeral pools/potholes (within replanted uplands) in Maine, USA (Vasconcelos & Calhoun 2006) reported that they were colonized by vegetation – mostly common cattail Typha latifolia – within five years. These results were not tested for statistical significance. After five years, cattail dominated three of three pools (60–84% of their vegetation cover) and 21 of 50 surveyed potholes (percent cover not reported). Shrubs were present in 30 of 50 surveyed potholes and were the dominant vegetation in seven (percent cover not reported). The study also reported that vegetation cover and species richness increased between three and five years after excavation (data not reported and not statistically tested). Methods: In autumn 1997, three seasonal freshwater pools (350–900 m2) and 200 seasonal freshwater potholes (0.3–110 m2) were excavated in an abandoned commercial development. Fill material was removed to expose soil from the forested wetland that historically occupied the site. Upland grasses, shrubs and trees were planted around the pools/potholes to stabilize the soil. Emergent vegetation cover, up to the high water mark, was estimated in each pool and 50 potholes in May–September 1999–2002.

    Study and other actions tested
  4. A replicated, site comparison study in 1998–2008 of 64 ephemeral pools on an air force base in California, USA (Collinge & Ray 2009) reported that excavated pools were colonized by five native, pool-characteristic plant species, but that these were less abundant than in nearby natural pools. Abundance of the five species peaked eight years after excavation, with a total frequency (summed across all species) of 5%. Over the eight years, individual species had a frequency of 0–21% in excavated pools, compared to 5–48% in nearby natural pools. Methods: In December 1999, sixty-four ephemeral pools were excavated in recently farmed grassland. The pools were 25–100 m2 and <150 m from natural pools. These pools were not sown with any seeds, but the surface was lightly raked. The frequency of five focal species (native species characteristic of Californian ephemeral pools) was recorded using grids of one hundred 2.5-cm2 cells. One grid was surveyed in some (number not specified) natural pools on the base in 1998 and 1999, and in each excavated pool in spring 2002–2008. This study was based on the same pools as (5).

    Study and other actions tested
  5. A replicated study in 1999–2008 of 64 excavated ephemeral pools on an air force base in California, USA (Collinge et al. 2011) reported that they were colonized by vegetation, but became dominated by non-native species after eight years. After 3–6 years, the excavated pools contained a mixture of native Californian pool-characteristic plants, and non-native plants. The abundance of each group was similar (native pool-characteristic abundance 1.1–1.5 times greater than non-natives). However after 8–9 years, and following a period of flooding then drought, the pools were dominated by non-native plants (non-native abundance 5–10 times greater than native pool-characteristic plants). Absolute abundance was reported as the sum of frequencies of species in each group (see original paper for data). Methods: In December 1999, sixty-four ephemeral pools were excavated in recently farmed grassland. The pools were 25–100 m2 and <150 m from natural pools. These pools were not sown with any seeds, but the surface was lightly raked. Each spring between 2002 and 2008, the frequency of every plant species was recorded in each pool, using a grid of one hundred 2.5-cm2 cells. Frequencies were added together to give the overall abundance for native, pool-characteristic plants and non-native plants (data for native, generalist plants were not reported). This study was based on the same pools as (4).

    Study and other actions tested
  6. A replicated, paired, site comparison study in 2011 in a freshwater marsh in Ontario, Canada (Schummer et al. 2012) found that the margins of excavated pools had a richer and more diverse plant community, but were less dominated by wetland-characteristic plants, than the margins of natural pools and reed/cattail stands. After 1–3 years, plant species richness was significantly higher on the shores of excavated pools (11 species/60 sampling points) than on the shores of natural pools (7 species/60 points) or in areas of the marsh dominated by common reed Phragmites australis or cattails Typha spp. (7 species/60 points). The same was true for plant diversity (data reported as a diversity index). Only 93% of individual plants recorded on the shores of excavated pools were wetland-characteristic species, compared to 99% on natural shorelines and 98% in reed/cattail stands (statistical significance not assessed). The study also reported data on the abundance of individual plant species (see appendix to original paper). Methods: In summer 2011, vegetation was surveyed in 11 areas of a freshwater marsh on the shores of Lake Erie. Each area contained three sites: one excavated pool (≤4 ha; ≤1.5 m deep; dug in reed/cattail stands 1–3 years previously, with dredge spoil deposited around pool margins), one “natural” pool (substrate not disturbed for >10 years) and one site still containing reed/cattail stands. Plant species were recorded at 60 points/site. At pool sites, points were in the surrounding marsh but ≤3 m from the open water.

    Study and other actions tested
  7. A replicated, site comparison study in 2013–2014 of 13 pools within forests in the northeast USA (Kolozsvary & Holgerson 2016) found that excavated pools and natural pools had similar abundance of some – but not all – vegetation types and taxa. Seven-year-old excavated pools and natural pools supported statistically similar cover of submerged vegetation, overall emergent vegetation, shrubs and loosestrife Lythrum sp. (all vegetation cover data reported as categories). Excavated and natural pools also contained a statistically similar biomass of surface-coating algae and phytoplankton after seven years (also true for phytoplankton after eight years; data not reported). However, 7-year-old excavated pools had greater cover than natural pools of duckweed Lemna sp., cattail Typha spp. and common reed Phragmites australis. The study also reported differences in some physical characteristics of the pools. For example, excavated pools were smaller, warmer, less acidic and received more light than natural pools (see original paper). Methods: In 2013–2014, plants, algae and phytoplankton were surveyed in seven excavated pools (326 m2 on average; created in 2006) and six natural pools (588 m2 on average). Most of the pools were seasonally flooded, but two excavated pools were permanently flooded. The excavated pools were in New York and the natural pools in Connecticut, but all were within similar mature forests. Algal and phytoplankton biomass were estimated from chlorophyll on glass slides or in the water column, respectively.

    Study and other actions tested
Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

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Marsh and Swamp Conservation

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Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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