Plant-animal interactions in a continuously grazed mixture. II. The role of differences in the physiology of plant growth and of selective grazing on the performance and stability of species in a mixture.

Published online
16 Nov 1991
Content type
Journal article
Journal title
Journal of Applied Ecology
DOI
10.2307/2404573

Author(s)
Parsons, A. J. & Harvey, A. & Johnson, I. R.

Publication language
English
Location
UK

Abstract

A physiologically based mechanistic model of the dynamic interaction between plants and animals is used to consider the role of differences in growth rate between plant species, and of selective grazing by animals, in the stability of plant species in a continuously grazed grass/legume mixture, and so the long-term consequences of different grazing intensities on sward and diet composition. The model provides a means to apply a detailed account of plant physiology in controlling the availability of food resources (prey) in analyses of predator-prey interactions in grassland. The model confirms that mixtures are intrinsically unstable. In the absence of a mechanism in either species to escape grazing, one which possesses a physiological advantage is shown progressively to dominate the mixture. Where the advantage is a simple one, the mixture tends to a monoculture of that species at all intensities of grazing but where the advantaged species is also the preferred species, the intensity of grazing determines which species dominates the mixture. The model suggests that stable mixtures may arise when either species can escape grazing to some extent. Under these circumstances, the otherwise advantaged species takes the brunt of variations in the intensity of grazing. The model is used to consider the sustainability of differences in composition between the diet and sward. At high grazing intensities diet composition is more a reflection of differences in growth rate between plant species than of the preferences of grazing animals. Because of its adverse impact on vegetation dynamics, the long-term consequence of a preference for one species in the diet is that there may be less of that species in the diet (both as a proportion and as an amount) than if there had been less preference for it or a preference against it. The implications of these analyses for the biological interpretation of field data are discussed.

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