PRIMARY PRODUCTIVITY AND TROPHIC DYNAMICS INVESTIGATED IN A NORTH DERBYSHIRE, UK, DALE

Trophic interactions were investigated on herbaceous communities in a limestone dale in Northern England. Manipulative experiments involved the application of molluscicides and foliar and soil insecticides alon g natural productivity gradients. The results supported the theories o f Fretwell and Oksanen in which trophic dynamics are predicted to be d ependent upon primary productivity. Furthermore, the results extend th e Fretwell-Oksanen model by the inclusion of invertebrates, and the ap plicability of the model to the small, individual habitat scale. At ve ry low productivity, the vegetation was dominated by slow-growing, unp alatable species and did not experience a detectable amount of herbivo ry. In circumstances of high productivity, 'top-down' control of herbi vores by carnivores appeared to protect the resident fast-growing and relatively palatable perennials from herbivory. Vegetation of intermed iate productivity responded strongly to the removal of herbivores; her e we conclude that herbivore pressure is high because productivity is insufficient to sustain a high intensity of 'top-down' control from ca rnivores.