Exclude wild vertebrates: brackish/salt marshes

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, of physically excluding wild vertebrates from brackish/salt marshes. Five studies were in the USA. The other studies were in France and Sweden. In five studies, the problematic vertebrates were mammals. In the other two studies, they were birds. Two of the studies were conducted in the same area, but with different experimental set-ups.

VEGETATION COMMUNITY

  • Overall richness/diversity (3 studies): Two replicated, paired, controlled studies in brackish marshes in the USA found that fencing to exclude nutria Myocastor coypus had no significant effect on total plant species richness: fenced and open plots contained a similar number of plant species after 1–2 growing seasons. One replicated, randomized, paired, controlled, before-and-after study in brackish marshes in the USA reported that excluding mammals typically had no significant effect on changes in plant species richness over two years.

VEGETATION ABUNDANCE

  • Overall abundance (5 studies): Five replicated, paired, controlled studies involving brackish marshes in France and the USA found that fencing to exclude medium-large vertebrates maintained or increased overall vegetation abundance. Vegetation cover or biomass were compared between fenced and open plots, after 1–2 growing seasons or over the winter after fencing.
  • Individual species abundance (6 studies): Six studies quantified the effect of this action on the abundance of individual plant species. The six replicated, controlled studies in brackish and salt marshes in France, Sweden and the USA reported that fencing to exclude medium-large mammals typically maintained or increased the abundance of the dominant herb species over 1–4 growing seasons. Four of the studies found that fenced and open plots contained a similar abundance (biomass, cover or density) of cordgrasses Spartina spp. Three of the studies found that bulrushes Schoenoplectus spp./Scirpus spp. were more abundant in fenced than open plots. However, one study reported no clear difference in bulrush abundance between treatments and one study reported mixed effects depending on moisture levels and which mammals were excluded.

VEGETATION STRUCTURE

  • Height (3 studies): One replicated, paired, controlled study in a brackish marsh in France found that overall vegetation height increased over two years in plots fenced to exclude medium-large mammals, compared to a decline in plots left open. Two replicated, controlled studies in brackish and salt marshes in Sweden and the USA found that vertebrate exclusion did not reduce (i.e. maintained or increased) the height of dominant herb species over 2–4 growing seasons.

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 replicated, paired, controlled study in 1975–1978 in an ephemeral brackish marsh in southern France (Bassett 1980) reported that the effects of excluding large mammalian herbivores on the dominant vegetation depended on the marsh type and which animals were excluded (horses Equus caballus and nutria Myocastor coypus, or horses only). The drier part of the marsh initially had 96–98% total vegetation cover, dominated by the grass Aeluropus littoralis (in 100% of quadrats) and alkali bulrush Scirpus maritimus (in 48–59% of quadrats). Two years later, total vegetation cover remained high in exclusion plots (full: 100%; horses: 100%) but had declined in grazed plots (83%). A. littoralis frequency had not changed in exclusion plots (full: 100%; horses: 100%) but had declined in grazed plots (79%). Alkali bulrush frequency had increased in all plots (full exclusion: 97%; horse exclusion: 88%; grazed: 96%). The wetter part of the marsh was initially dominated by alkali bulrush (in 85–93% of quadrats) and common reed Phragmites communis (in 11–27% of quadrats). Vegetation was 40–69 cm tall. After two more years, common reed frequency had increased in exclusion plots (full: 38%; horses: 64%) but decreased in grazed plots (2%). The same was true for vegetation height (full exclusion: 169 cm; horse exclusion: 116 cm; grazed: 18 cm). Alkali bulrush frequency had increased in full exclusion plots (100%), but decreased in horse exclusion plots (49%) and grazed plots (76%). Methods: In winter 1975/1976, eighteen 7 x 7 m plots were established in a brackish marsh (nine in the drier margins and nine in the wetter centre). In each part of the marsh, three plots received each treatment: full exclusion (of horses and nutria; wire fence with 3 cm mesh), partial exclusion (of horses only; fence with two barbed wire strands) and no fence (continued grazing, including <0.15 horses/ha). Vegetation was surveyed in summer 1976–1978 (frequency of each species and height of the tallest plant in fifty 15 x 15 cm quadrats/plot/year; bare ground at 100 points/plot/year).

    Study and other actions tested
  2. A replicated, controlled, before-and-after study in 1980–1983 on a coastal salt marsh in Sweden (Pehrsson 1988) found that excluding geese increased the height of saltmarsh grass Puccinellia maritima. In 1981 and 1982, saltmarsh grass was taller in plots from which geese had been excluded (16–24 cm) than plots left open to geese (13–16 cm). In 1980, before intervention, the opposite was true: saltmarsh grass was shorter in plots destined for goose exclusion (10 cm) than plots destined to remain open to geese (11 cm). The study also noted broadly similar changes in vegetation cover under both treatments, between 1980 and 1983. Cover of saltmarsh grass consistently declined, whilst cover of creeping bentgrass Agrostis stolonifera and saltmarsh bulrush Scirpus maritimus consistently increased (statistical significance not assessed; data not clearly reported). Methods: In late 1979, four 25-m2 plots were established in grassy areas of a salt marsh and fenced to exclude cattle. In 1981, geese were also excluded from two of the plots using circular chicken nets. Vegetation was surveyed using point quadrats each autumn before goose exclusion (1980) and after goose exclusion (1981–1983). Height was measured for 3–186 plants/species/treatment/year.

    Study and other actions tested
  3. A replicated, paired, controlled study in 1991 in a brackish marsh in Louisiana, USA (Taylor et al. 1994) found that plots fenced to exclude nutria Myocastor coypus contained more vegetation biomass than plots that remained open to grazing, but had similar plant species richness. After six months, fenced plots contained more above-ground vegetation biomass (974 g/m2) than open plots (538 g/m2). Individual species showed mixed responses. For example, fenced plots contained more biomass of chairmaker’s bulrush Scirpus olneyi (fenced: 244 g/m2; open: 27 g/m2), but statistically similar biomass of the other dominant species, saltmeadow cordgrass Spartina patens (fenced: 557 g/m2; open: 381 g/m2) and less biomass of Cyperus sedges (fenced: 1–2 g/m2; open: 11–46 g/m2). Fenced and open plots contained a statistically similar number of plant species (data not reported). Methods: In March 1991, twenty 1-m2 plots were established (in five sets of four) in a coastal brackish marsh. Ten plots (two plots/set) were fenced (2.5 x 5.0 cm plastic-coated mesh) to exclude nutria (and other large mammals). The other plots were not fenced. Half of the plots under each treatment were burned, in June. In September 1991, all vegetation was cut from each plot then identified, dried and weighed.

    Study and other actions tested
  4. A replicated, randomized, paired, controlled study in 1991 in two brackish marshes in Louisiana, USA (Taylor & Grace 1995) reported that plots fenced to exclude nutria Myocastor coypus contained more overall vegetation biomass than plots that remained open to grazing, but had similar plant species richness. Statistical significance was not assessed. After two growing seasons, above-ground vegetation biomass was 990–1,150 g/m2 in fenced plots, compared to 720–910 g/m2 in open plots. However, fenced and open plots contained a statistically similar biomass of the dominant plant species: saltmeadow cordgrass Spartina patens (fenced: 501; open: 290 g/m2) and smooth cordgrass Spartina alterniflora (fenced: 993; open: 713 g/m2). Fenced and open plots had similar plant species richness: 10.3–10.4 species/m2 in one marsh and 1.5–2.0 species/m2 in the other. Methods: In March 1990, twenty-four 4-m2 plots were established (in six sets of four) across two brackish marshes. Twelve of the plots (two random plots/set) were fenced (2.5 cm plastic-coated mesh) to exclude nutria (and other large mammals). The other 12 plots were left open. In September 1991, all vegetation was cut from one 1-m2 quadrat/plot. Plant species were identified, then the vegetation was dried and weighed. This study was in the same area as (5), but used a different experimental set-up.

    Study and other actions tested
  5. A replicated, randomized, paired, controlled, before-and-after study in 1992–1994 in two brackish marshes in Louisiana, USA (Ford & Grace 1998) found that fencing to exclude wild mammals increased overall vegetation biomass, but had mixed effects on the cover of dominant plant species and plant species richness. After two years, above-ground vegetation biomass was higher in fenced plots (600–1,200 g/m2) than in plots that remained open to grazing (280–450 g/m2). Fenced and open plots had similar cover of the dominant plant species in 7 of 10 comparisons. In two of the other comparisons, fenced plots had greater cover of American bulrush Scirpus americanus (54–57%) than open plots (18–19%). Fencing had no significant effect on plant species richness in three of four comparisons: there were statistically similar changes over two years in fenced and open plots (see original paper for data). In the other comparison, in burned areas, plant species richness increased in fenced plots (by 3.8 species/m2) but did not significantly change in open plots (non-significant increase of 0.2 species/m2). Methods: In autumn 1992, twenty pairs of 4-m2 plots were established across two brackish marshes. Twenty plots (one random plot/pair) were fenced (5 cm wire mesh with hooks to prevent burrowing) to exclude nutria Myocastor coypus and wild boar Sus scrofa (and other large mammals). The other 20 plots were not fenced. Half of the plots under each treatment were also burned in autumn 1992 and 1993. Plant species and cover were recorded in autumn 1992 (before intervention) and 1994. Vegetation was cut from one 0.25-m2 quadrat/plot, then dried and weighed, in autumn 1994. This study was in the same area as (4), but used a different experimental set-up.

    Study and other actions tested
  6. A replicated, paired, controlled study in 1996–1997 in four brackish and freshwater marshes in Delaware, USA (Sherfy & Kirkpatrick 2003) reported that plots fenced to exclude snow geese Chen caerulescens had greater vegetation cover (15–57%) than plots grazed by geese (<1–11%). Statistical significance was not assessed. Methods: In September–October 1996, sixteen goose exclosures were established across four impounded marshes with “slightly” brackish or fresh water. The study does not separate results for each marsh type. There were four exclosures/marsh. Exclosures were 1.2 x 1.2 m, fenced with 1.5 x 1.5 cm plastic mesh and topped with bright plastic strips to prevent snow geese from landing. Over winter 1996/1997, total vegetation cover was estimated in the 16 exclosures and 16 adjacent plots open to, and grazed by, snow geese.

    Study and other actions tested
  7. A replicated, paired, controlled study in 1991–1994 in four brackish marshes in Louisiana, USA (Johnson Randall & Foote 2005) found that fencing to exclude nutria Myocastor coypus increased the height and density of American bulrush Schoenoplectus americanus but not saltmeadow cordgrass Spartina patens. Bulrush was taller in fenced plots than open plots in 14 of 14 comparisons over 42 months (data reported as height categories). There were more bulrush stems in fenced plots in 12 of 14 comparisons (for which fenced: 70–240 stems/m2; open: 10–60 stems/m2). Cordgrass was taller in fenced than open plots in only 4 of 14 comparisons, with no significant difference in the others (data reported as height categories). There were more cordgrass stems in fenced plots in only 1 of 14 comparisons, with no significant difference in the others (for which fenced: 360–1,350 stems/m2; open: 270–1,750 stems/m2). Methods: In April 1991, forty 9-m2 plots were established across four brackish marshes (10 plots/marsh). Twenty of the plots (five plots/marsh) were fenced (4 x 6 cm plastic-coated mesh) to exclude nutria (and other large mammals). The other 20 plots were left open. Every 3–6 months for three and a half years, stems of the dominant plant species were counted and measured in two 25 x 25 cm quadrats/plot.

    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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