The role of livestock in habitat management
J.A. Milne and K. Osoro¹

Macaulay land Use Research Institute, Craigiebuckler, Aberdeen
¹ Centro de Investigación Aplicada Y Tecnología Agroalimentaria, 33300 Villaviciosa, Asturias, Spain

Introduction

Many of the ecosystems found in disadvantaged areas of Europe are managed through the presence of large herbivores, mainly domestic ruminants. The impacts of domestic ruminants fall directly on the vegetation and soils but they also influence birds, mammals and invertebrates and affect landscape value and water quality. Grazing impacts can be found at the level of the individual plant, the patch, the plant community and the landscape (Senft et al., 1987; see Figure 1). They are a function of the density of individual ruminant species, even when expressed on a livestock unit basis, their foraging behaviour, i.e. where animals graze, and their ingestive behaviour, ie what animals choose from a particular grazing site. The combination of these effects result in the major impacts of vegetation offtake, trampling damage and excretal return which have in addition a strong between-season and between-year temporal basis and, particularly, an important spatial dimension. These need to be understood if the role of livestock in habitat management is to be quantified.

Impacts of grazing biodiversity at the field scale

Defoliation of a single plant can influence plant species composition by reducing the ability of the plant grazed to compete with other plants. Species vary in the ability to withstand grazing depending, inter alia, upon the position of their apical meristem. Trees are less resistant to grazing than shrubs or forbs with grasses being the most resistant. Within each of these vegetation types, there are differences between species with, for example, annual forbs being less resistant to defoliation than perennials and tussock grasses less resistant than tillering grasses. Preference by grazing animals is also important; a plant which is highly preferred and with a low resistance to defoliation is likely to be unable to compete. This combination does not occur frequently in grazed plant communities and hence is one reason why rapid shifts in species in a plant community, which has been grazed for some time, do not occur.

Between the patch and community scale Grime (1979) postulated a now widely accepted bell-shaped relationship between plant species diversity and standing plant biomass or grazing pressure (see Figure 2). High and low grazing pressures produce few plant species whilst intermediate grazing intensities lead to maximum species diversity. In temperate areas of Europe the grazing pressures which maximise grass growth and animal output per hectare, and give acceptable levels of individual animal performance, require sward heights of 4 cm and 8 cm for sheep and cattle respectively (Hodgson, 1990). Such sward heights, especially the 4 cm sward heights, are equivalent to high grazing pressures in Figure 2. In a long term study at 3 sites Marriott et al. (1997) conducted an experiment whereby sheep grazed swards at 4 or 8 cm for a period of 6 years and where impacts of fertiliser had also been removed. The grazing intensity on the 8 cm sward was reduced by between 40 and 70% compared with a sward maintained at 4 cm. Over 6 years little appreciable change in species diversity occurred. It was only when grazing was removed altogether that rapid change occurred. At the field scale large changes in plant diversity are unlikely to occur by simply lowering stocking density. Indeed there are some circumstances where a general reduction in grazing pressure can actually reduce species diversity. Smith et al. (1996) showed that the further that the grazing management moved away from a system of hay-making and specific high grazing pressures in the autumn and spring, the greater was the reduction in plant species diversity.

Grazing pressures which create maximum plant species diversity in grassland are also likely to lead to increases in the general species diversity of invertebrates, particularly of the phytophages and predators, although there will be a reduced diversity of soil decomposers, scavengers and parasitic insects because there is less dung produced and less animals (Curry, 1994). As for plant species diversity it is likely to be a combination of different management strategies involving, for example, grazing by different species at different intensities and at different times of the year that will maximise diversity (Dennis et al., 1995). For small mammals and birds within small territories a combination of grazed areas, often suitable for the provision of food, and ungrazed areas for shelter and nesting are often required to maintain high levels of diversity.

Milne and Fisher (1994) modelled the effect of increasing the patchiness of pastures, as represented by the proportion of tall grass in a sward, on individual animal performance. From Figure 3 it can be seen that, once patchiness of tall grasses reached proportionally about 0.40 of the sward, appreciable losses in individual animal performance occurred. It can thus be concluded that the types of grazing management systems that are required to maximise individual animal performance, and also output per unit area, are unlikely to maximise biodiversity. Specific management systems already exist which provide high levels of diversity in disadvantaged areas but the design of further grazing systems to achieve that objective is unlikely to be met by simply reducing stock numbers and hence grazing pressure.

Impacts of grazing on biodiversity at the landscape scale

At the landscape scale the foraging behaviour of farmed ruminants becomes important in determining which plant communities are grazed and in what seasons they are grazed. A commonly used paradigm is that animals distribute themselves about a resource in relation to the digestible energy intake that they can derive from each of the components of the resource. Different species also have different abilities to harvest vegetation. For example, sheep graze closer to the ground and are more selective in their grazing than cattle while cattle and goats are more able to harvest tussocky grasses than sheep and goats often select a more fibrous diet than sheep. This is illustrated in Figure 4 where at high biomasses sheep and cattle eat predominantly the same diet but at low biomasses cattle eat more of the less preferred species. Another example of the difference between species in grazing behaviour is illustrated well in an experiment conducted in the north of Spain where sheep and goats grazed the same initial vegetation mix of shrubs and herbaceous species for a period of three years. As can be seen from Figure 5, after three years, goat grazing had reduced the proportion of shrubs relative to that of sheep grazing because of the different dietary preferences of the two species with the goats favouring a more shrub-based diet. The temporal basis of the pattern of species grazing also had an effect. Hence different animal species can have different spatial and temporal grazing patterns in relation to their stocking intensity and therefore can have different impacts on the vegetation. This has significance in relation to the balance between the density of different grazers and habitat management to achieve natural heritage objectives.

Impacts of grazing on the vegetation at a landscape scale in terms of changing a plant community type are generally relatively slow. Depending upon the grazing pressure, they can take from 10 to 50 years to occur. Such change is not necessarily linear but is usually gradual and the rapid state transitions that occur in non-temperate ecosystems do not often occur. Figure 6 illustrates the changes from one vegetation type to another for the Cairngorms area of Scotland, UK between 1947 and 1988 using aerial photographic interpretation and analysis by Hester et al. (1996). It shows that the largest change has been from heath, pine woodland and grassland communities into plantation forestry, which is not through the impact of grazing, but that there are also small changes from heath into grasses associated with grazing.

These changes can be encapsulated into simple predictive models based on experimental evidence and expert knowledge. In Figure 7 predictions of the length of time and the stocking rate of sheep needed to move from one community to another are described for some plant community types in the UK These depend upon soil properties and the plant community being grazed. Considerable research effort is currently being expended on developing more quantitative models, particularly in order to describe more adequately the structural changes that take place within a community as it changes from one community to another. This is of particular importance to habitat management to meet nature conservation objectives. It is also important in predicting the effects of reduced grazing pressure on plant community change as this is becoming a major issue in parts of Northern Spain, Italy, Southern France and Greece where reductions in grazing pressure are causing a change from pasture land to scrub and woodland resulting in an elevated fire risk.

It is unlikely that the impact of the density of large herbivores at the landscape scale will influence invertebrate populations because the geographical scale that is important to them is at the scale of the patch or the field. The impacts of large herbivore densities on bird and mammal populations can be various. The balance between grassland, scrub and woodland can be important for a whole range of species in providing appropriate feeding and nesting sites. Reduced domestic livestock numbers will reduce on the one hand the amount of prey and carrion from domestic animals but on the other hand will increase the number of wild prey because of the changed structure of the vegetation and may also provide improved habitats by virtue of increased cover. An example of this is the reduced livestock numbers and associated management by man of them in North Spain and the increase in woodland providing the opportunity for wolves to show considerable increases in numbers (Zorita & Osoro, 1995).

Conclusions

It has been demonstrated in this paper that farmed ruminants play an important role in habitat management to influence biodiversity objectives. This is not to underestimate the need to understand better the habitat requirements of particular taxa or species, particularly those which have a rarity value, to augment the general understanding of the impacts of ruminants. The particular role for farm livestock in habitat management will depend upon the ecosystem found in the different regions of Europe but the application of the general principles outlined in this paper, combined with appropriate site-specific knowledge, can lead to the development of grazing strategies for different farmed species to meet nature conservation and environmental needs. These strategies do not necessarily conflict with animal production objectives and the maintenance of rural human populations.

References

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