Grazing systems can be intrinsically unstable. This means that excessive harvesting can lead to sustained productivity losses. In many unfertilized grazing systems in temparate zones, productivity is sustained through nitrogen-fixing legumes, such as white clover. But the legume content of pastures commonly varies from year to year, sometimes dramatically.

Some pasture models suggest that mixed populations of grasses and legumes oscillate for much the same reason as predator-prey systems, i.e. the grass population uses the legumes' ability to fix nitrogen for accelerating growth, while the legume population gains no benefit from the presence of grasses.

Models have also shown that spatial variation in pastures can help dampen population oscillations and associated yield uncertainties. Spatial variation in pasture quality can also help minimize the deleterious effects of overgrazing. While traditional management practices have emphasized pasture homogeneity to maximize productivity, these new models suggest conversely that pasture heterogeneity can help minimize year-to-year yield variation.

Further Reading (download list as pdf file or Endnote file)

Chapman, D.F., Parsons, A.J. & Schwinning, S. 1996. Management of clover in grazed pastures: expectations, limitations and opportunities. In: White Clover: New Zealand's Competitive Edge. Symposium of the New Zealand Grassland Association, Lincoln, N.Z., pp. 55-64.

Parsons, A.J., Carrere, P. & Schwinning, S. 1999. Dynamics of heterogeneity in a grazed sward. Proceedings of the International Symposium on Grassland Ecophysiology and Grazing Ecology, deMoraes, A., Nabinger, C. de Faccio, P.C., Alves, S.J. & Campos Lustosa, S.B. (eds), Curitiba, Parana, Brazil: pp. 187-214.

Schwinning, S. & Parsons, A.J. 1996a. Analysis of the coexistence mechanisms for grasses and legumes in grazing systems.Journal of Ecology 84,799-813.pdf file

Schwinning, S. & Parsons, A.J. 1996b. A spatially explicit population model of stoloniferous N-fixing legumes in mixed pasture with grass.Journal of Ecology 84, 815-826.pdf file

Schwinning, S. & Parsons, A.J. 1996c. Interactions between grasses and legumes: understanding variability in species composition. In: Legumes in Sustainable Farming Systems. Proceedings of the Sustainable Farming Systems/ British Grassland Society Joint Conference, pp. 153-163.pdf file

Schwinning, S. & Parsons, A.J. 1999. The stability of grazing systems revisited: spatial modls and the role of heterogeneity, Functional Ecology 13: 737-747.pdf file

Thornley, J.H.M., Bergelson, J. & Parsons, A.J. 1995. Complex dynamics in a carbon-nitrogen model of a grass-legume pasture. Annals of Botany 75: 79-94.