Use grazing to maintain or restore disturbance: freshwater marshes
Overall effectiveness category Awaiting assessment
Number of studies: 5
Background information and definitions
Disturbance can clear dominant plants, maintain light availability and control nutrient levels – and may maintain vegetation in a desirable and/or species-rich state (Hall et al. 2008; Middleton 2013). Therefore, conservationists sometimes want to actively restore disturbance where it has ceased, or maintain disturbance at a site where it would otherwise be lost. Grazing with animals such as sheep, horses, cattle, goats or water buffalo (Gulickx et al. 2007) might be one way to do this. Grazing can also give some economic value to wetlands, strengthening arguments against conversion to other land uses. Grazing itself may be the disturbance that has been reduced. Some wetlands have been abandoned following historical low-intensity grazing (e.g. Plassman et al. 2010).
This section includes studies evaluating the effects of grazing implemented for conservation (e.g. with species and intensity aligned with vegetation conservation goals). Studies of the impact of intense commercial grazing, for example, are not included. Bear in mind that the effects of grazing might be highly dependent on how it is carried out (e.g. species, intensity, timing, frequency and duration) and site conditions (e.g. nutrient availability, water levels, presence/density of wild herbivores) (Rinella & Hileman 2009).
Gulickx M.M.C., Beecroft R.C. & Green A.C. (2007) Introduction of water buffalo Bubalus bubalis to recently created wetlands at Kingfishers Bridge, Cambridgeshire, England. Conservation Evidence, 4, 43–44.
Hall S.J., Lindig-Cisneros R. & Zedler J.B. (2008) Does harvesting sustain plant diversity in Central Mexican wetlands? Wetlands, 28, 776–792.
Middleton B.A. (2013) Rediscovering traditional vegetation management in preserves: trading experiences between cultures and continents. Biological Conservation, 158, 750–760.
Plassmann K., Jones M.L.M. & Edwards-Jones G. (2010) Effects of long-term grazing management on sand dune vegetation of high conservation interest. Applied Vegetation Science, 13, 100–112.
Rinella M.J. & Hileman B.J. (2009) Efficacy of prescribed grazing depends on timing intensity and frequency. Journal of Applied Ecology, 46, 796–803.
Supporting evidence from individual studies
A replicated, randomized, paired, controlled, before-and-after study in 1988–1997 in wet grassland and flush vegetation in Scotland, UK (Humphrey & Patterson 2000) found that restarting cattle grazing had no clear effect on plant community composition, and typically had no significant effect on plant species richness, cover or height. After nine years and within each vegetation type, the overall plant community composition was similar in grazed and exclusion plots (data reported as a graphical analysis; statistical significance of differences not assessed). Total plant species richness declined within each vegetation type, but by a statistically similar amount in grazed plots (1–5 fewer species/2 m2 after nine years of grazing than before) and exclusion plots (6–8 fewer species/2 m2). Changes in cover of key plant groups were also statistically similar in grazed and exclusion plots (forbs: 6–21% decline; grass-like plants: 34–63% decline; bryophytes: 9% increase or 6–11% decline). Over the first four years of study, maximum vegetation height was statistically similar in grazed and exclusion plots in five of six comparisons (for which grazed: 24–90 cm; exclusion: 35–92 cm). Methods: Four pairs of 100-m2 plots were established in each of two vegetation types: a rush-dominated wet grassland and a seepage flush. Annual summer cattle grazing (2.2–2.4 cattle/ha in the wetlands and surrounding grassland) was restarted in August 1988, after a 10-year hiatus. However, one random plot/pair was fenced to exclude cattle. Wild roe deer Capreolus capreolus could access the whole site, including fenced plots. In summer 1988 (before cattle reintroduction), 1991 and 1997, plant species and vegetation cover were recorded in 1–4 permanent 2-m2 quadrats/plot. In autumn 1988, 1989 and 1991, the tallest plant shoot was measured in eighty 400-cm2 quadrats/plot.Study and other actions tested
A replicated study in 1993–1998 of four dune slacks in the Netherlands (Grootjans et al. 2001) reported that slacks where grazing was reintroduced (after stopping groundwater extraction and removing topsoil) developed plant communities with habitat-characteristic species. Restored slacks developed plant communities, the overall composition of which changed through time (data reported as a graphical analysis; statistical significance of changes not assessed). After three years of grazing, restored slacks contained 84–108 plant species overall and 48–86 species/100 m2. This included species characteristic of dune slacks (5–11 species/100 m2) and nutrient-rich marshes (2–11 species/100 m2) alongside other wetland and upland species. In each slack, total vegetation cover was always <50% and only two individual species – creeping willow Salix repens and bushgrass Calamagrostis epigejos – ever had cover >1%. Methods: In 1995, traditional grazing (by a “small herd” of cattle and ponies) was resumed in four degraded dune slacks (stabilized and covered with undesirable, mature vegetation). Dune slacks are wetter, low-lying areas between dune ridges. In 1993, groundwater extraction had been stopped. Vegetation and topsoil were also stripped, completely or partially, from each slack. The study does not distinguish between the effects of these interventions. Each spring or summer between 1994 and 1998, seed-plants were surveyed: species across the whole of each slack; species and cover in five comparable 100-m2 plots/slack.Study and other actions tested
A site comparison study in 1996–2000 in a riparian wet meadow in southern Germany (Zahn et al. 2003) reported that in plots where summer grazing was reinstated (along with cutting woody vegetation), there were changes in the area of plant community types, an increase in plant species richness, a reduction in vegetation height and growth of some woody vegetation. Statistical significance was not assessed. Over the first four years of grazing, there were slight increases in the area of reedbed/marsh vegetation (from 10 to 14%) and herbs typical of disturbed areas (from 45 to 50%) and a slight decrease in the area of meadow and pasture vegetation (from 45 to 36%). Total plant species richness increased in seven of seven plots, from 5–45 species/plot to 11–57 species/plot (increase of 3–22 species/plot). After four years, the grazed/cut area contained shorter vegetation than adjacent unmanaged land, including patches <10 cm tall) not present in unmanaged land (data reported graphically). Finally, woody vegetation grew back despite grazing: up to 15 bushes/100 m2, reaching a height of >1 m after four years. Around 80% of 400 black alder (Alder glutinosa) trees that had been cut back died over the four years. Methods: The focal wetland had been abandoned for 20 years, becoming overgrown with tall herbs and, in places, woody plants. From 1996, annual grazing was reinstated on 6 ha (6–9 cattle, April–November). Woody vegetation was also cut back, near ground level, in 1996. The study does not distinguish between the effects of these interventions. Vegetation was surveyed each summer 1996–2000, in seven grazed 100-m2 plots. In 2000, vegetation height was measured along a 34-m-long transect spanning the grazed/cut and unmanaged areas.Study and other actions tested
A replicated, randomized, paired, controlled, before-and-after study in 1998–2000 of a range of freshwater marsh and wet meadow habitats around one lake in Idaho, USA (Austin et al. 2007) found that grazing typically had no clear effect on plant community composition, but that summer grazing affected vegetation biomass in some vegetation types. Over two years, the overall plant community composition within freshwater habitats remained similar in autumn-grazed, summer-grazed and ungrazed plots (data presented as graphical analyses; statistical significance of differences not assessed). In 12 of 16 comparisons, changes in live, above-ground plant biomass (from before to after grazing) were not significantly different in grazed and ungrazed plots. This was true for all eight comparisons involving autumn grazing and four of eight comparisons involving summer grazing (see original paper for data). In the other four comparisons, all in the wettest habitats, vegetation biomass declined in summer-grazed plots (by 200–350 g/m2) but did not significantly change in ungrazed plots (non-significant increases of 20–230 g/m2). Methods: Three sets of three fields with similar neighbouring vegetation were studied. Each field contained a range of freshwater habitats, from permanently flooded marshes to ephemeral wet meadows. All fields had been historically grazed and cut, but were undisturbed from 1996. Three fields (one random field/set) received each treatment: annual autumn grazing (September–October 1998 and 1999), one-off summer grazing (July–August 1998) or no grazing. Grazing intensity was 2.3–2.5 animal unit months/ha (one AUM is the amount of feed required to sustain a 1,000-lb cow and her calf for one month). Vegetation was surveyed in June–July before intervention (1998) and for two years after (1999, 2000).Study and other actions tested
A replicated, before-and-after study in 1987–2003 of dune slacks within one sand dune system in Wales, UK (Plassmann et al. 2010) found that following the reintroduction of grazers, plots retained the same overall plant community type but developed greater plant species richness and diversity. The overall plant community type was the same in each plot before and after grazers were introduced. Each plot started with a community characteristic of wetter marshy or drier shrubby slacks, and retained that community over six months to 16 years of grazing (data not reported). However, averaged across both wetter and drier community types, there were increases in total plant species richness (before grazers introduced: 20; after grazers introduced: 27 species/4 m2) and diversity (data reported as a diversity index). More specifically, there were increases in richness of grass-like plants (before: 5; after: 8 species/4 m2) and indicator species for the dune slack communities (18% higher after grazers were introduced). Grazing had no significant effect on richness of bryophytes (2 species/4 m2 before and after) or lichens (<1 species/4 m2 before and after). Methods: At 1–7 year intervals between 1987 and 2003, vegetation was surveyed in 21 permanent 4-m2 plots. The plots were all within dune slacks (low-lying areas between dune ridges; some wetter, some drier) that had been grazed until the 1950s but had since become overgrown. Livestock (cattle, sheep and/or ponies at “low densities”) were introduced to the land containing each plot at various points between late 1987 and 2001. Rabbits were also present in the dune system.Study and other actions tested