Provide general protection for marshes or swamps

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

Study locations

Key messages

  • Three studies evaluated the overall effects, on vegetation or human behaviour, of providing general protection for marshes or swamps. There was one study in each of Puerto Rico, China and Canada.


  • Overall extent (3 studies): Two studies in China and Canada reported that the area of wetlands (including habitats other than marshes or swamps) in their study regions declined over 10–29 years, despite general protection of wetlands. However, in China, the decline was slower than in a previous period without protection. One before-and-after study of mangrove forests in Puerto Rico reported that their area increased following legal protection.




  • Overall structure (1 study): One before-and-after study in China reported degradation in wetland landscape structure over 29 years when wetlands were generally protected. However, the decline was slower than in a previous period when wetlands were not protected.

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 1959–2002 of mangrove forests in Puerto Rico (Martinuzzi et al. 2009) reported that their area increased following legal protection of all mangroves on the island. Between 1959 and 1971, the area of mangroves in Puerto Rico declined from approximately 7,285 ha to 6,745 ha. The study attributes this to urban expansion. In 1972, legal protection was granted to all mangroves in Puerto Rico. Subsequently, the area of mangroves increased to 7,443 ha in 1977 and 8,323 ha in 2002. The study suggests that active restoration efforts and declining agricultural production contributed to this increase, alongside legal protection. The study also notes that lowland freshwater swamps, which were not granted the same protection as mangroves, were “almost none existent” by the early 2000s. Methods: The study was based on historical estimates of mangrove forest area across Puerto Rico, derived from aerial photos or satellite images. Estimates were corrected to include mangrove forests only, not associated wetland ecosystems.

    Study and other actions tested
  2. A before-and-after study in 1954–2005 in northeast China (Wang et al. 2011) reported that following legal protection of wetlands, the area of marshland on the plain decreased but at a slower rate than before protection, and that the same was generally true for landscape structural metrics. Statistical significance was not assessed. Between 1954 and 1986, when wetlands were not protected and government polices instead encouraged conversion of wetlands to farmland, the area of marshland on the plain had decreased by 668 km2/year (from 35,300 km2 to 13,900 km2). Between 1986 and 2005, when local and national governments prohibited wetland reclamation and established protected areas, the area of marshland on the plain decreased by 305 km2/year (from 13,900 km2 to 8,100 km2). Most landscape structural metrics declined more slowly after protection than before (i.e. largest patch size, variation in patch size, complexity of patch outlines; see original paper for data). In contrast, average patch size declined after protection (from 941 to 781 ha) compared to an increase before (from 735 to 941 ha). Throughout the study, there were increases in cropland area, human population and air temperature, but no significant change in precipitation. Methods: Digital maps of marshland on the Sanjiang Plain were created from paper maps (drawn around the 1954 growing season) and satellite images (taken in growing seasons between 1976 and 2005). The digital maps were verified in the field. “Marshland” was defined as all non-woody vegetated wetlands, so included some open bogs as well as true marshes.

    Study and other actions tested
  3. A study in 1999–2009 of wetlands in the Beaverhill Subwatershed, Alberta, Canada (Clare & Creed 2014) reported that legal protection did not prevent the loss of wetlands. From 1999, wetlands in Alberta were protected by a general policy to conserve their natural state and maintain their area (1993 Wetland Policy), and a legal requirement to obtain a permit for activities that would negatively impact them (1999 Water Act). Between 1999 and 2009, a total of 242 wetlands covering 71 ha were lost in the Beaverhill Subwatershed. Of the area lost, 82% occurred without a permit. The authors suggest they underestimated wetland loss, as they only included cases when the wetland basin was completely removed (and not cases where the basin was drained but its profile remained). Methods: The number and area of wetlands in the Beaverhill Subwatershed were estimated based on remotely-sensed elevation data collected in 1999 and 2009.

    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?

List of journals searched by synopsis

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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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