Crassula helmsii: Use grazing to control plants

Overall effectiveness category Unlikely to be beneficial
Number of studies: 2
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How is the evidence assessed?

Effectiveness

23%
Certainty

43%
Harms

not assessed

Calculating overall effectiveness category

Background information and definitions

Grazing by livestock is often used as a conservation measure to increase plant species diversity in grassland. Depending on the dietary preferences of the animals, grazing could potentially increase or reduce the competitive advantage and coverage of C. helmsii on the margins of waterbodies or marshy areas. Increased grazing pressure was observed to be related to a decrease in C. helmsii cover at waterbodies in the New Forest, but several potentially confounding factors were present (Ewald 2014).

Ewald N.C. (2014) Crassula helmsii in the New Forest. Final report on the status, spread and impact of this non-native invasive plant, and the efficacy of control techniques following a 3 year trial. Prepared on behalf of the New Forest Non-Native Plants Project, Freshwater Habitats Trust, Oxford, UK.

Study locations

Key messages

One of two replicated, controlled studies in the UK found that excluding grazing reduced the abundance and coverage of Crassula helmsii. The other study found no difference in cover of C. helmsii between ungrazed and grazed plots.

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

A replicated, controlled study in 2012–2013 on the margins of a lake in Cambridgeshire, UK (Dean et al. 2015) found that excluding grazing reduced the abundance of C. helmsii compared to grazed plots. Cover of C. helmsii in ungrazed plots decreased from approximately 95% to 60% between July 2012 and October 2013, but remained above 90% in grazed plots. The abundance and diversity of other plants was higher in ungrazed compared to grazed plots (average abundance: 97% vs 38% cover respectively; mean species diversity (Shannon-Weiner): 1.1 vs 0.88). C. helmsii also had lower proportional abundance in ungrazed compared to grazed plots (approximately 47% of total vegetation abundance vs 74%). Six 4 m² ungrazed fenced exclosures, interspersed with six 2 m²grazed plots, were set up in February 2012. The area was grazed by sheep in January-March 2012 and August 2012-October 2013, and by buffalo in July-December 2012. Percentage cover of C. helmsii and other plants was estimated eight times between July 2012 and October 2013.
Study and other actions tested
Referenced paper
Dean C.E., Day J., Gozlan R.E. & Diaz A. (2015) Grazing vertebrates promote invasive swamp stonecrop (Crassula helmsii) abundance. Invasive Plant Science and Management, 8, 131-138.
A small, replicated, controlled study in 2009 at four ponds in the New Forest, UK (Ewald 2014) found that excluding grazing did not reduce the cover of C. helmsii. There was no significant difference between average cover of C. helmsii between ungrazed areas (42%) compared to grazed exclosures (26%). There was no difference in cover of plant species of conservation importance in ungrazed areas compared to grazed areas (7% vs 10%). Exclosure fences were erected in March 2009 to create ungrazed areas in four ponds with at least 75% C. helmsii cover. Grazing was mainly by ponies and cattle, but the area was also used by deer, pigs and donkeys. Exclosures included plants under the water and on the bank. Cover of plants in five random quadrats was surveyed in each pond in autumn 2009.
Study and other actions tested
Referenced paper
Ewald N.C. (2014) Crassula helmsii in the New Forest. Final report on the status, spread and impact of this non-native invasive plant, and the efficacy of control techniques following a 3 year trial. Freshwater Habitats Trust (Oxford) report.

Please cite as:

Aldridge, D., Ockendon, N., Rocha, R., Smith, R.K. & Sutherland, W.J. (2020) Some Aspects of Control of Freshwater Invasive Species. Pages 555-87 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Control of Freshwater Invasive Species

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