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Fretwell and Oksanen's theory of trophic dynamics was tested in two plant communities located in a North Derbyshire dale, including: (1) a low productivity calcareous grassland; and, (2) a highly productiveUrtica dioica (nettle) patch. Two methods (herbivore removal throughpesticide application, and transplanting established, intact turves (0.5 m2) between the two community types) were employed, and analysed in a two-way ANOVA, to test the hypothesis that highly productive communities are controlled by 'top-down' forces and low productivity communities are controlled by 'bottom- up' forces. The Fretwell-Oksanen theory proposes that herbivores limit growth in low productivity communities, not highly productive communities. Therefore, removal of herbivores will result in an increase in plant biomass only in the low productivity community. The results presented in this paper support the Fretwell-Oksanen hypothesis. Furthermore, when small turves were transplanted from the highly productive community to the low productivity community the removal of herbivores through pesticide application greatly increased the above-ground plant biomass. This result suggests, firstly that the vegetation grown in a highly productive environment is generally very palatable, and secondly, it strengthens the evidence that herbivores are limiting plant growth in low productivity communities but not highly productive communities. Individual plant species response to herbivore removal was related to known relative growth rate values using linear regression and was found to be significant in one case: nettle turves transplanted into the grassland. In this case, relative growth rate accounted for 68.3% of the variation in the response of the plants to herbivore removal. This suggests that fast-growing plants from a highly productive environment are most likely to respond to the release of a limiting factor, in thiscase herbivory, within a community. [ABSTRACT FROM AUTHOR]

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 along natural productivity gradients. The results supported the theories of Fretwell and Oksanen in which trophic dynamics are predicted to be dependent upon primary productivity. Furthermore, the results extend the Fretwell-Oksanen model by the inclusion of invertebrates, and the applicability of the model to the small, individual habitat scale. At very low productivity, the vegetation was dominated by slow-growing, unpalatable species and did not experience a detectable amountof herbivory. In circumstances of high productivity, 'top-down' control of herbivores by carnivores appeared to protect the resident fast-growing and relatively palatable perennials from herbivory. Vegetation of intermediate productivity responded strongly to the removal of herbivores; here we conclude that herbivore pressure is high because productivity is insufficient to sustain a high intensity of 'top-down' control from carnivores. [ABSTRACT FROM AUTHOR]