Reduce intensity of livestock grazing: brackish/salt marshes

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

Study locations

Key messages

  • Nine studies evaluated the effects, on vegetation, of reducing livestock grazing intensity in brackish/salt marshes (without stopping grazing entirely). Five studies were in Germany. Four studies were in the Netherlands. Livestock were cattle, sheep or horses. There was overlap in the sites used in two of the German studies and three of the Dutch studies.

VEGETATION COMMUNITY

  • Overall extent (1 study): One controlled study of a salt marsh in Germany reported that the total vegetated area was slightly larger in plots grazed at a lower intensity, for eight years, than plots grazed at a higher intensity.
  • Community types (4 studies): Two controlled studies of salt marshes in Germany and the Netherlands reported similar coverage, or similar change in coverage, of plant community types under different grazing intensities. Two studies of brackish and salt marshes in the Netherlands and Germany reported that reducing grazing intensity (along with other interventions) affected coverage of plant community types. In one study, the precise effect varied with environmental conditions.
  • Community composition (1 study): One replicated, randomized, paired, before-and-after study on a salt marsh in the Netherlands found that plots grazed under different grazing intensities experienced a similar turnover of plant species over six years, and had a similar overall plant community composition after six years.
  • Overall richness/diversity (5 studies): Three replicated, paired, controlled studies on salt marshes in Germany and the Netherlands found that plots grazed at lower intensities never had greater plant species richness, after 1–22 years, than plots grazed at higher intensities. One controlled study on a salt marsh in Germany found that paddocks grazed at low intensity had greater plant species richness, after 16–18 years, than paddocks grazed at higher intensities. Two studies of salt marshes in the Netherlands found that plant species richness increased over 6–14 years of reduced grazing intensity (sometimes along with other interventions).

VEGETATION ABUNDANCE

  • Overall abundance (1 study): One controlled study on a salt marsh in Germany reported that overall vegetation cover was greater in lightly and moderately grazed paddocks than in a heavily grazed paddock – with the highest cover of all in the moderately grazed paddock.
  • Individual species abundance (6 studies): Six studies quantified the effect of this action on the abundance of individual plant species. For example, three studies (including two controlled) on salt marshes in Germany and the Netherlands reported that plots under different grazing intensities supported a similar abundance (frequency or cover) of saltmarsh grass Puccinellia maritima – but with a tendency for greater abundance under lower intensities.

VEGETATION STRUCTURE

  • Height (6 studies): Six controlled studies (three also replicated and paired) in salt marshes in Germany and the Netherlands reported that vegetation was taller on average (or contained taller vegetation patches) in areas that had been grazed at lower intensities. However, in one of the studies, this was only true for canopy height: understory grasses were a similar height under all grazing intensities. One of the replicated, paired, controlled studies found that, after two summers, variation in vegetation height between patches was similar under all grazing intensities.

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 controlled study in 1980–1988 of a salt marsh in northern Germany (Andresen et al. 1990) reported that reported that less intensely grazed plots had a taller vegetation canopy than more intensely grazed plots, but that reducing grazing intensity little effect on understory height or vegetation cover. Statistical significance was not assessed. After eight years, the vegetation canopy was 26–89 cm tall in plots grazed at low or medium intensity (vs 0–55 cm tall in a plot grazed at high intensity). In contrast, understory grasses reached a similar height under all grazing intensities (low: 6–22 cm; medium: 5–10 cm; high: 3–14 cm). The distribution of different vegetation types was broadly similar across all grazing intensities, with similar communities at similar elevations. However, the plots grazed at low/medium intensity supported a slightly larger total vegetated area than the plot grazed at high intensity (data reported as maps), and a slightly greater frequency of the dominant saltmarsh grass Puccinellia maritima (data reported as frequency classes). Methods: Three plots were established in a coastal salt marsh, historically grazed by 2.0 cattle/ha. From 1980, grazing intensity was reduced in two of the three plots: one to low intensity (0.5 cattle/ha) and one to medium intensity (1.0 cattle/ha). The third plot remained grazed by 2.0 cattle/ha. In 1988, vegetation was surveyed along one 750-m transect/plot, at 50 m intervals. Cover was assessed using a point quadrat (50 points/interval). Vegetation stands were mapped.

    Study and other actions tested
  2. A controlled study in 1988–1992 in a salt marsh in northern Germany (Kiehl et al. 1996) reported that plots under lower grazing intensities developed different cover of some key plant species to a plot that remained heavily grazed, and had taller vegetation. Statistical significance was not assessed. After four years, moderately grazed plots had 19–27% cover of seablite Suaeda maritima (vs heavily grazed: 5%) but only 0–4% cover of glasswort Salicornia europaea (vs heavily grazed: 24%). Cover of the dominant saltmarsh grass Puccinellia maritima was similar under all grazing intensities (moderate: 70–74%; heavy: 71%). The perennial herbs sea aster Aster tripolium and sea purslane Halimione portulacoides had low cover under both moderate and heavy grazing (<2%), but occurred in a greater proportion of quadrats across moderately than heavily grazed plots, especially closer to the sea (see original paper). Finally, the moderately grazed plots had taller vegetation on average (10–20 cm) than the heavily grazed plot (8 cm), with the difference most pronounced closer to the sea. Methods: In 1988, sheep grazing intensity was reduced in two sections of a historically grazed coastal salt marsh (1.5–3.0 sheep/ha, April–October). A high grazing intensity was maintained in a third section (10 sheep/ha, April–October). The marsh was intertidal (flooded 80–200 times/year) but had a dense artificial drainage system. The cover of each plant species was surveyed annually between 1989 and 1992 in permanent quadrats. Perennial herb frequency and overall vegetation height were recorded in 1992, in quadrats or along transects respectively.

    Study and other actions tested
  3. A study in 1981–1997 of a salt marsh in the Netherlands (3 Esselink et al. 2002) reported that following a reduction in grazing intensity from 1981 (along with legal protection and abandonment of drainage systems), there were changes in the area of plant community types and the abundance of some dominant species, and an increase in plant species richness. Between 1981 and 1995, the area covered by pioneer succulents increased (from 0% to 19% of the marsh) and the area covered by short-grass communities decreased (from 76% to 56%). Statistical significance of these cover results was not assessed. Between 1983 and 1997, the frequency of two of the most abundant plant species did not significantly change: saltmarsh grass Puccinellia maritima (1983: present in 81% of plots; 1997: present in 84% of plots) and sea aster Aster tripolium (1983: 80%; 1997: 97%). Species showing significant changes in frequency included saltbush Atriplex prostrata (increase from 86% to 98%), seablite Suaeda maritima (increase from 38% to 70%), creeping bentgrass Agrostis stolonifera (decrease from 78% to 69%) and common cordgrass Spartina anglica (decrease from 52% to 31%). Between 1983 and 1997, plant species richness significantly increased: from 8 species/100 m2 to 10 species/100 m2. Methods: A degraded coastal salt marsh became part of a nature reserve in 1981. Cattle grazing intensity was gradually reduced (reaching 40–80 animal days/ha/season by the 1990s), and the drainage system was abandoned by 1984 (making the soils wetter and less aerated). Note that this study evaluates the combined effect of these interventions. Coverage of vegetation types was calculated from maps of the marsh made in 1981 and 1995. Plant species presence and cover were surveyed in 64 permanent 100-m2 plots, spread across four parts of the marsh and at a range of elevations, in 1983, 1991 and 1997.

    Study and other actions tested
  4. A controlled study in 1991–2009 on a salt marsh in northern Germany (Rickert et al. 2012) found that less intensely grazed paddocks generally had higher plant species richness, greater vegetation cover and taller vegetation than more intensely grazed plots. After 16–18 years, lightly grazed paddocks had higher plant species richness (12.2 species/m2) than a moderately grazed paddock (9.9 species/m2), which had higher plant species richness than a heavily grazed paddock (6.7 species/m2). Total vegetation cover was greater in the lightly and moderately grazed paddocks (86–89%) than in the heavily grazed paddock (82%). Likewise, the vegetation canopy was taller in the lightly and moderately grazed paddocks (13.7–14.0 cm) than in the heavily grazed paddock (6.3 cm). Vegetation cover (but not canopy height) was also lower in the lightly grazed paddock than in the moderately grazed paddock. The study also reported cover of the dominant species in each paddock (see original paper for data). Methods: The study used three paddocks on a coastal salt marsh (historically heavily grazed by sheep). From 1991, the paddocks were grazed each summer at different intensities: one lightly (1–2 sheep/ha), one moderately (3–4 sheep/ha) and one heavily (10 sheep/ha). Vegetation was surveyed every three weeks in summer 2007–2009, in a total of thirty 1-m2 quadrats/paddock/year. All quadrats were at a similar elevation (±20 cm). The paddocks in this study were also used in (5).

    Study and other actions tested
  5. A replicated, paired, controlled study in 1988–2010 on three salt marshes in northern Germany (van Klink et al. 2013) found that moderately grazed plots contained a mosaic of short (species-rich) and tall (species-poor) vegetation patches, whilst heavily grazed plots contained only short vegetation. After 19–22 years, heavily grazed paddocks contained uniformly short and species-rich vegetation (5.0 cm canopy height; 7.3 plant species/0.33 m2; dominated by red fescue Festuca rubra: 35% cover). Moderately grazed paddocks contained some short, species-rich vegetation patches that were statistically similar to heavily grazed paddocks (5.8 cm canopy height; 5.8 plant species/0.33 m2; dominated by red fescue: 49% cover). They also contained taller vegetation patches with lower species richness than heavily grazed paddocks (19.4 cm canopy height; 5.3 plant species/0.33 m2; dominated by sea couch Elytrigia atherica: 46% cover). Paddock-scale species richness, which included both short and tall patches in the moderately grazed plots, did not significantly differ between treatments (moderate: 13.3; heavy: 14 plant species/1.1 m2). Methods: The study used six 11–15 ha paddocks: one pair on each of three coastal salt marshes (historically heavily grazed by sheep). From 1988 or 1991, one paddock/pair remained heavily grazed (10 sheep/ha, May–October) and the other was moderately grazed (3–4 sheep/ha, May–October). Vegetation was surveyed in summer 2010, in sixteen 30-cm-diameter circular quadrats/paddock. In moderately grazed paddocks, quadrats were split evenly across short and tall patches. All quadrats were at a similar elevation (±10 cm). This study included the paddocks used in (4).

    Study and other actions tested
  6. A replicated, paired, controlled study in 2010–2011 in a salt marsh in the Netherlands (Nolte et al. 2014) found that a lower grazing intensity produced taller vegetation on average, but had no significant effect on patchiness or variation in vegetation height. After two summers of grazing, plots under low grazing intensity contained taller vegetation stands on average (16 cm) than plots with high grazing intensity (11 cm). However, both grazing intensities produced vegetation patches (i.e. areas of vegetation with uniform height) of similar size (low intensity: 118 cm; high intensity: 169 cm diameter). Under both grazing intensities, variation in height amongst patches was similar (data reported as statistical model results). Methods: In 2010, eight 11-ha plots were established (in two sets of four) on a coastal salt marsh. The marsh had been “intensively grazed” for the previous 20 years. In May–October 2010 and 2011, four plots (two plots/set) were grazed at each intensity: low (0.5 livestock units/ha) or high (1.0 livestock units/ha). Half of the plots were grazed by cattle and half by horses. In August 2011, vegetation height was measured along six 25-m transects/plot (100 points/transect). Some or all of the plots in this study were also used in (8) and (9).

    Study and other actions tested
  7. A site comparison study in 1988–2006 of coastal brackish and salt marshes in northern Germany (Rupprecht et al. 2015) reported that reducing grazing intensity (or stopping grazing entirely) affected vegetation development, but that the effect depended on multiple other factors. Grazing intensity was included as an important predictor in six of six statistical models of observed vegetation development (i.e. transitions between vegetation types over defined time periods). However, the effect of grazing intensity depended on other environmental conditions such as initial vegetation type, elevation and latitude (proxy for salinity and flooding frequency). In two models, for example, grazing intensity affected vegetation development at low but not high elevations. It is not possible to separate out results for reducing grazing intensity vs stopping grazing entirely. For example, moderately grazed plots showed similar responses to intensely grazed plots in some cases, but similar responses to ungrazed plots in others. Methods: In 1988, 1996, 2001 and 2006, plant community types were mapped across approximately 7,000 ha of brackish and salt marsh. Over time, grazing intensity was reduced in some areas, from intense (>10 sheep/ha) to moderate (≤3 sheep/ha) or zero. Where grazing was stopped, drainage systems were also abandoned but this had little effect on water levels. Statistical analyses were used to determine the influence of different factors, including grazing intensity, on changes in plant community types between the survey years.

    Study and other actions tested
  8. A replicated, randomized, paired, controlled, before-and-after study in 2009–2013 on a salt marsh in the Netherlands (van Klink et al. 2016) found that lower grazing intensities increased vegetation height in two of two comparisons but never increased plant species richness, and had no significant effect on the area of vegetation dominated by sea couch grass Elytrigia atheria. Both far from and near to the sea, vegetation was taller in plots grazed at low intensity (14–22 cm average height) than in plots grazed at high intensity (9–12 cm average height). Far from the sea, plant species richness was lower in plots grazed at low intensity (12–13 species/16 m2) than in plots grazed at high intensity (14 species/16 m2). However, near to the sea, plant species richness did not significantly differ between grazing intensities (low: 8 species/16 m2; high: 9 species/16 m2). Finally, over four years of grazing and across the whole marsh, the area of couch-grass-dominated vegetation experienced a statistically similar change under both grazing intensities (although with a trend towards a greater reduction under a higher grazing intensity; data not reported). Methods: In 2009, twelve plots were established (in three sets of four) on a historically grazed coastal salt marsh. From 2010, six plots (two random plots/set) were grazed at each intensity: low (0.5 animals/ha) or high (1.0 animal/ha). Grazing occurred in summer (June–October) only. Half of the plots were grazed by cows and half by horses. Vegetation height and plant species were recorded in late August/early September 2010–2013, in eight 16-m2 quadrats/plot/year. The area of couch-grass-dominated vegetation was mapped using aerial photographs taken before (2009) and four years after (2013) grazing treatments were applied. Some of the plots in this study were also used in (6) and (9).

    Study and other actions tested
  9. A replicated, randomized, paired, controlled, before-and-after study in 2009–2013 on a salt marsh in the Netherlands (Lagendijk et al. 2017) found that low and high grazing intensities typically had statistically similar effects on plant community composition and cover of two focal herb species, but that lower grazing intensities inhibited species richness. After six years, plots grazed at low intensity contained fewer plant species (7.8–8.4 species/16 m2) and had experienced a smaller increase in plant species richness (gain of 0.5–3.4 species/16 m2) than plots grazed at high intensity (10.2 species/16 m2; gain of 1.2–5.9 species/16 m2). In contrast, plots under both grazing intensities contained a similar overall plant community composition after six years (data not reported), had experienced a similar turnover of plant species (data reported as a turnover index), and had experienced similar changes – in at least two of three comparisons – in cover of sea couch grass Elytrigia atheria and sea aster Aster tripolium (see original paper for data). Methods: In 2009, ten 11-ha plots were established (in two sets of five) on a coastal salt marsh. From 2010, six plots (three random plots/set) were grazed at low intensity (0.5 cattle/ha every summer, 0.5 horses/ha every summer, or 1 cattle/ha every other summer). The other four plots were grazed at higher intensity (1.0 cattle/ha every summer or 1.0 horses/ha every summer). Plant species and their cover were recorded in August/September 2009 (after a summer of intense grazing to standardize plots) and 2010–2015 (during grazing treatments). Surveys were carried out in eight 16-m2 quadrats/plot/year. Some or all of the plots in this study were also used in (6) and (8).

    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

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