ANALYSIS OF THE COEXISTENCE MECHANISMS FOR GRASSES AND LEGUMES IN GRAZING SYSTEMS

1 It is widely assumed that grass-legume associations offer a way to s ustainable, low input land use, with reduced environmental impact. How ever, a combination of both ecological and physiological principles ma y be needed to understand the sustainability of species balances. 2 To increase understanding of grass-legume dynamics, we developed a model that extends a recently proposed pasture model (Thornley, Bergelson & Parsons: Annals of Botany 1995, 75, 79-94) by including selective gra zing and spatial considerations, Population oscillations were shown to stem from the way grasses can exploit leguminous N fixation. If the l egume is a relatively good competitor for light, populations do not os cillate near equilibrium, but in the converse case, populations do osc illate. 3 Large amplitude oscillations can arise when there are suffic iently long time delays in the plant populations' responses to changes in the competitive environment. In the present model, these stem from variable internal substrate pools (of C and N), which uncouple biosyn thesis from resource uptake, but other time delay mechanisms are easil y envisaged. 4 Urine deposits prevent the establishment of equilibrium within patches, but spatially random urine deposition stabilizes popu lation fluctuations at the field scale. This is because perturbations to local N cycles desynchronize patches with regard to the grass-legum e population cycle. 5 Differences in the soil N environment (fertilize r input, leaching rate) determine whether the species can coexist, but where coexistence is possible, species composition regulates soil min eral N. 6 Selective grazing (herbivory) does not essentially alter the grass-legume interaction, but complex foraging trade-offs lead to her bivory effects that may seem counterintuitive. The model has important implications for attempts to control the legume content of mixed spec ies communities.