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 particularly acute problem due to the difficulties of quantifying their effects on lifetime reproductive success and survival. 2 Our purpose here is to extend our previous short-term empirical results that two contrasting sheep grazing regimes have a significant effect on seed output but no effect on the establishment and survival of seedlings in a natural population of Anthyllis vulneraria by combining them with stochastic long-term stage-transition models in which the life history stages were classified according to age and developmental state. Our objectives 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 sheep grazing either as 'one-off' instantaneous effects of high-intensity grazing or via effects due to repeated low intensity grazing. 3 The results of the simulations predict that periods of decline in population growth can be followed by an increase in the rate of population growth with the pattern depending on the sequence of poor and favourable years since these determine annual variation in seed output. The occurrence of consecutive poor years caused a shift in age structure towards a population biased towards older aged plants. As a result, seed output was greatly improved by a subsequent favourable year due to the high reproductive capacity of older plants. 4 The limit that variation in seed output may impose on population growth, which is more severe under repeated free grazing than under a single episode of controlled high intensity grazing, will thus depend on how the population is regulated 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 environmental conditions and herbivory.