Conservation grazing: the need for ecological monitoring
Why carry out conservation grazing?
Many ecosystems have developed historically under a moderate level of grazing - plants which can tolerate this have, over the centuries or millennia, colonised and become established. A moderate level of grazing therefore maintains habitats in the state to which they are accustomed. Competitive interactions between plants, where a vigorous plant outgrows its neighbours, are mediated or reduced under appropriate levels of grazing. Vigorous plants are kept in check, and less common plants can survive.
What are plant - herbivore interactions?
In a word, complicated! There are many different factors involved in grazing systems, and many different possible outcomes. For instance, different animal species select different plants preferentially, the ability of plants to withstand grazing varies between species, and the competitive interactions of species are altered under grazing. Not only this, all three above can be different under different levels of grazing!
In general, heavier grazing means shorter vegetation. As a very rough guide, grazing pressures can produce a range of different ecosystem types, as follows:
The numbers of grazing animals which constitute, for example light or moderate grazing cannot be generally prescribed - they vary depending on the species and size of grazing animal, the vegetation they are to eat, the plants which are desired, and the climate and fertility of the site. For instance, a stocking rate defined as moderate in North West Scotland might be only light in South West England.
Given low levels of grazing and other forms of disturbance, most bare sites would see a series of vegetation changes, termed ‘succession’. From bare ground, sites are colonised progressively by mosses, annual weeds, taller herbaceous plants, shrubs and eventually trees, with the whole process possibly taking tens or hundreds of years. However, disturbance, in the form of grazing animals (or fire, or land-slips….), can check succession and produce and maintain a different type of habitat.
The stage of succession that the grazing animals are introduced during is important. For example, if grazing animals are introduced into mature woodland, they will have little impact on mature trees but may prevent regeneration of young trees, by eating seedlings and saplings.
The species of animals graze in different ways, because they have different behaviours and physiologies. For instance, sheep, with small mouths, are selective in what they choose to eat, and bite plants off close to the ground - they can produce a smooth, cropped sward. Ponies also bite off plants, but leave a longer sward than sheep. Cattle are less selective, and eat by tearing up clumps of grass with their tongues. Cattle will browse on trees overhanging their fields, producing a ‘browse-line’, while goats target shrubby material, biting off leaves and young twigs. Pigs forage for food in a totally different way, by rooting around in leaf litter and the top layer of soil, searching for underground plant parts, insects and worms.
These behaviours mean that different animals are appropriate for different conservation grazing situations.
It is very important in conservation, as in any management activity, to have clearly defined aims. Several questions need to be considered:
Where conservation grazing is the most sensible option for a management situation, do it! And carry out ecological monitoring to ensure the aims are being achieved - this is important not just as the system is being set up, but also in the longer term.
Ideally, know which ecosystem type is desired, monitor the effects of grazing, and adjust the management accordingly.