PREDICTING DEMOGRAPHIC-CHANGE IN RESPONSE TO HERBIVORY - A MODEL OF THE EFFECTS OF GRAZING AND ANNUAL VARIATION ON THE POPULATION-DYNAMICS OF ANTHYLLIS-VULNERARIA

1 Documenting the impact of herbivores on perennial plants is a partic ularly acute problem due to the difficulties of quantifying their effe cts on lifetime reproductive success and survival. 2 Our purpose here is to extend our previous short-term empirical results that two contra sting sheep grazing regimes have a significant effect on seed output b ut no effect on the establishment and survival of seedlings in a natur al population of Anthyllis vulneraria by combining them with stochasti c long-term stage-transition models in which the life history stages w ere classified according to age and developmental state. Our objective s were to predict how: (a) variation in fecundity, rate of germination and seed survival, and (b) the frequency of years with either high or low seed production, may regulate population growth in relation to sh eep grazing either as 'one-off' instantaneous effects of high-intensit y grazing or via effects due to repeated low intensity grazing. 3 The results of the simulations predict that periods of decline in populati on growth can be followed by an increase in the rate of population gro wth with the pattern depending on the sequence of poor and favourable years since these determine annual variation in seed output. The occur rence of consecutive poor years caused a shift in age structure toward s a population biased towards older aged plants. As a result, seed out put was greatly improved by a subsequent favourable year due to the hi gh reproductive capacity of older plants. 4 The limit that variation i n seed output may impose on population growth, which is more severe un der repeated free grazing than under a single episode of controlled hi gh intensity grazing, will thus depend on how the population is regula ted by the succession of poor and favourable years for seed production . This indicates that the demographic characteristics of natural plant populations may be markedly influenced by an interaction between envi ronmental conditions and herbivory.