Require mitigation of impacts to marshes or swamps

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

Study locations

Key messages

  • Nine studies evaluated the overall effects – on vegetation or human behaviour – of requiring mitigation of impacts to marshes or swamps. All nine studies were in the USA.

VEGETATION COMMUNITY

  • Overall extent (6 studies): Four studies in the USA reported that requiring mitigation of impacts to wetlands did not prevent loss of wetland area: the total area restored/created was less than the area destroyed. One study in the USA reported that the total area of wetlands restored/created for mitigation was greater than the area destroyed. However, the area restored/created was smaller in most individual projects. Two of the studies reported that fewer individual wetlands were restored/created than destroyed. One before-and-after study in the USA found that wetland area declined after legislation to offset impacts came into force, but at a slower rate than before the legislation applied. Four of the studies reported discrepancies between the area of specific vegetation types restored/created vs destroyed.

VEGETATION ABUNDANCE

 

VEGETATION STRUCTURE

 

OTHER

  • Compliance (8 studies): Eight studies, all in the USA, provided information about compliance with required mitigation. Five of the studies reported that the total area of wetlands conserved was less than the area required in permits. Three of the studies reported that most mitigation projects failed to meet targets stipulated in permits. One of the studies reported that only one of seven vegetation targets was met in all mitigation sites. One of the studies reported that 64–74% of assessed mitigation areas met success criteria stipulated in permits.

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 study in the late 1980s/early 1990s in Ohio, USA (Wilson & Mitsch 1996) reported that five development permits that demanded compensation for impacts to marshes and swamps did not maintain their area, number or vegetation type. The permits required creation of 42 ha of marsh/swamp in five sites, compared to 24 ha lost to development in five sites. All five permits were followed through, but only restored/created 16 ha of marsh/swamp: a net loss of 8 ha. Further, compensation was not “in kind” in two of the five sites: one failed to establish woody vegetation that was present in a lost swamp, and one created a deep pond surrounded by trees on upland (rather than a marsh). The study also quantified the vegetation of created sites in more detail – see Action: Restore/create marshes or swamps (specific intervention unclear). Methods: This study analyzed data relating to five permits, issued in the late 1980s/early 1990s for projects involving filling of marshes or swamps. Permits were issued under Section 404 of the Clean Water Act. The assessment was based on available published reports and field surveys carried out 1–4 years after restoration/creation was completed.

    Study and other actions tested
  2. A study in 1983–1997 of 114 development projects in Massachusetts, USA (Brown & Veneman 2001) reported that permits requiring compensation for impacts to marshes and swamps did not prevent a loss in their area, and found that compensation was compliant with permit conditions in only 43% of completed projects. The study examined 114 development projects which encroached onto marshes or swamps and for which permits required compensatory mitigation. Compensatory sites required by the permits were 23% larger than impacted sites on average, but they were generally not the same type (e.g. impacted: 71% forested; designated: 61% shrubby). In practice, compensatory sites were 34% smaller than impacted sites. Furthermore, 39% were not even wetlands. In only 47 of 109 completed projects was compensation compliant with the permit (i.e. a wetland was created of similar size to the impacted wetland, with ≥75% cover of emergent vegetation). The study also compared vegetation in compensatory wetlands and remnant natural wetlands – see Action: Restore/create marshes or swamps (specific intervention unclear). Methods: The study examined 114 projects that were granted permits, between 1983 and 1994, to destroy marshes or swamps as long as replacements were created elsewhere. Data were extracted from permit records and collected in vegetation surveys in summer 1997.

    Study and other actions tested
  3. A study in 1998–1999 in Indiana, USA (Robb 2002) reported that permits requiring compensation for impacts to marshes and swamps did not prevent a loss in marsh/swamp area or maintain habitat types. The study examined 345 sites where marsh/swamp or aquatic habitats should have been restored or created to compensate for loss. Of these sites, 214 had been constructed as planned. In a sample of 31 constructed sites, permits required restoration/creation of 26.3–30.2 ha of marshes/swamps to compensate for impacts to 12.1–12.8 ha. The actual area of marsh/swamp created was 10.2 ha. So, there was a 16–20% net loss of marsh/swamp area in constructed sites. For three of four vegetation types (shrubby swamps, wet meadows, shallow marsh), the area restored/created (6.4 ha) was greater than the area lost (4.1 ha). For forested swamps, the area restored/created (3.8 ha) was less than the area lost (8.0 ha). Methods: The status of 345 compensatory mitigation sites, required by Section 404 permits issued before December 1996, was checked in 1998–1999. Status was classified as constructed, incomplete or no construction attempted. Vegetation of 31 sites was surveyed in more detail.

    Study and other actions tested
  4. A study in the 1990s of 50 development projects in Tennessee, USA (Morgan & Roberts 2003) reported that requiring mitigation of impacts to wetlands did not prevent loss of wetland area. Permits for 50 completed development projects required conservation of 104 ha of wetlands to mitigate impacts to 38 ha of wetlands. However, only 78 ha of wetlands were conserved in practice. Furthermore, only 35 ha of these were restored or created, leading to a net loss of 3 ha of wetlands. The other 43 ha of conservation involved preservation or enhancement of existing wetlands, which does not replace lost area. Finally, the study reports that in only 2 of the 50 projects did conservation meet all permit specifications (e.g. hydrology and vegetation establishment). Methods: This study analyzed data from 50 completed development projects across Tennessee, for which state-level Aquatic Resource Alteration Permits demanded mitigation of impacts to wetlands. Wetland areas were surveyed in the field in summer 1997 and 1998: 1–6 years after project completion.

    Study and other actions tested
  5. A study in 1995–2004 in Ohio, USA (Kettlewell et al. 2008) reported that 23 development permits requiring mitigation of impacts to wetlands prevented the loss of wetland area, but not habitat types or wetland number. Mitigation was carried out for all 23 permits. For two permits, compensation involved preserving existing wetlands. The other 21 permits mandated 27.8 ha of wetland restoration/creation as compensation. In practice, 26.3 ha of wetlands were restored/created, compared to 15.0 ha lost due to development: a net gain of 11.3 ha. However, 8 of 12 projects examined in more detail failed to restore/create the area mandated in their permits. Further, compensation in these projects was rarely “in kind”. Restored/created wetlands were mostly marsh (83% by area), whilst the impacted wetlands were a mixture of shrubby or forested swamp (57%) and marsh (43%). The 12 projects also restored/created fewer wetlands (65) than were impacted (134). Methods: The 23 permits in this study were issued between 1995 and 2003 for activities impacting wetlands. The permits were issued under the Clean Water Act. Information was taken from permit documentation and from field surveys of restored/created wetlands in summer 2004. Estimates of wetland area included some aquatic habitats (open water/submerged vegetation) and some existing natural wetlands incorporated into restoration/creation sites.

    Study and other actions tested
  6. A study in 2008 of 11 compensatory wetlands in Michigan, USA (Kozich & Halvorsen 2012) reported that only 45% were compliant with the permit requirement of <10% invasive plant cover. In the five compliant wetlands, invasive plant cover was 4–7%. In the six non-compliant wetlands, invasive plant cover was 11–26%. Compliance was greater in wetlands restored directly adjacent to existing wetlands (five of six compliant) than in wetlands created far from existing wetlands (zero of five compliant). Methods: Vegetation was surveyed in 11 compensatory wetlands in summer 2008. The wetlands had been restored or created 2–5 years previously, to compensate for impacts from road construction. All contained some emergent vegetation. Permits for impact were issued between 2003 and 2006, under Section 404 of the Clean Water Act and Part 303 of Michigan’s Natural Resources Environmental Protection Act.

    Study and other actions tested
  7. A study in 2007–2009 of 205 wetland components within 82 development projects in North Carolina, USA (Hill et al. 2013) reported 64–74% compliance with success criteria outlined in permits. The study examined 205 individual wetland components: distinct areas of wetland, habitat or mitigation type (e.g. “4 ha of swamp restoration” and “10 ha of marsh preservation”). Of these, 70–74% met permit success criteria. In turn, 64–70% of the area of these wetland components met permit success criteria. The lower values are for all projects (mitigation by creation, restoration, enhancement or preservation); the higher values exclude preservation projects, which were more likely to meet success criteria. Methods: This study analyzed the success rate of 205 wetland components within 82 randomly selected development projects. These had been granted permits to impact wetlands between 1996 and 2006. Permits were issued under Section 401 of the Clean Water Act. Mitigation success was assessed through field surveys carried out between 2007 and 2009. Success was defined as compliance with vegetation, hydrological, soil and/or protection criteria outlined in each permit. Vegetation criteria usually involved a minimum abundance of emergent vegetation.

    Study and other actions tested
  8. A before-and-after study in 1972–2012 in Oregon, USA (Fickas et al. 2016) found that after legislation to offset impacts to wetlands came into force, the overall area of vegetated wetlands still decreased, but at a significantly slower rate than before the legislation came into force. Over 22 years with offsetting legislation, the area of vegetated wetlands in the study region decreased by 2.1 ha/year (vs 13.3 ha/year over the 17 years before legislation). More specifically, the area of shrubby and forested wetlands decreased by only 3.6 ha/year after legislation (vs 9.3 ha/year before legislation). Furthermore, the area of emergent herbaceous wetlands (marshes, wet meadows, bogs and fens) increased by 1.4 ha/year after legislation (vs 3.5 ha/year decrease before legislation). The study notes that wetland gains/losses in the study area might compensate for losses/be compensated by gains outside the study area. Methods: Section 404 of the Clean Water Act came into force in 1990 and required compensatory mitigation for some unavoidable impacts to wetlands. The area of vegetated wetlands along the Willamette River floodplain was estimated from satellite images, taken every summer between 1972 and 2012. Wetland classifications were checked using aerial photographs and field surveys.

    Study and other actions tested
  9. A study in 1996–2012 of 30 compensatory mitigation sites in Illinois, USA (van den Bosch & Matthews 2017) reported that that they did not consistently meet targets specified in permits after 4–21 years. Of the 30 mitigation sites, 29 were classified as wetlands based on vegetation, water levels and soils. However, of seven targets related to vegetation, only one (cover of wetland plants) was met in 100% of applicable sites at some point during monitoring. The other targets were met in 13–86% of applicable sites at some point during monitoring. The study does not report the precise target for each site. The study also compared vegetation in compensatory sites and remnant natural wetlands – see Action: Restore/create marshes or swamps (specific intervention unclear). Methods: The study involved 30 sites where wetland restoration was required by permits for road construction and maintenance. Target wetlands were marshes (15 sites), swamps (13 sites) or a mixture of both (2 sites). Restoration was carried out between 1992 and 2004. Vegetation was surveyed once/site in 1996–2009 and once/site in 2012.

    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.

Where has this evidence come from?

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Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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