Crassula helmsii: Use grazing to control plants

How is the evidence assessed?
  • Effectiveness
    23%
  • Certainty
    43%
  • Harms
    not assessed

Source countries

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

  1. 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 m2 grazed 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
  2. 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
Please cite as:

Aldridge, D., Ockendon, N., Rocha, R., Smith, R.K. & Sutherland, W.J. (2019) Some aspects of control of freshwater invasive species. Pages 569-602 in: W.J. Sutherland, L.V. Dicks, N. Ockendon, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2019. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Control of Freshwater Invasive Species

This Action forms part of the Action Synopsis:

Control of Freshwater Invasive Species
Control of Freshwater Invasive Species

Control of Freshwater Invasive Species - Published 2017

Control of Freshwater Invasive Species Synopsis

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What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, terrestrial mammals, forests, peatland and control of freshwater invasive species. More are in progress.

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