Setting sustainable grazing levels for heather moorland: a multi-site analysis.
Many habitats of high conservation value are managed by grazing but could be damaged by poor grazing management. Approaches and methods to set appropriate grazing regimes must be developed that can be applied with confidence under different situations to ensure that deleterious habitat changes are unlikely to occur. Heather moorland is an important habitat for conservation, but is a cultural plagio-climax with low productivity and is under threat from high grazing pressures. Too high a pressure converts the dwarf-shrub dominance to grass dominance. Thus, it is imperative to be able to assess a sustainable grazing level for moorland: the no-effect level. Data from ten grazing experiments on heather moorland, each carried out and monitored in a similar manner, were analysed together to estimate the impact of heather utilization on the balance between heather and grasses, sedges and rushes. The analysis indicated a no-effect level of 31.6% utilization of current year's growth to maintain the balance between heather and monocotyledonous plants. However, the 95% confidence intervals for the fitted line crossed zero change at 22.5% and 41.4% utilization; a considerable degree of uncertainty that indicates a precautionary approach would be appropriate. Synthesis and applications. The current assumed utilization level for sustainable grazing of heather (40% of current year's growth) appears too high. A conservative utilization figure to reduce the risk of heather loss should be set nearer 20%. Developing models based on utilization is more efficient than basing models on stocking rate information due to the wide range of factorial combinations of stocking rate and vegetation composition that would need investigating. This approach could be extended to other vegetation types where monitoring of key indicator species could be more efficient than developing experimentally based grazing prescriptions.