TOP-DOWN VS BOTTOM-UP CONTROL INFLUENCED BY PRODUCTIVITY IN A NORTH DERBYSHIRE, UK, DALE

Fretwell and Oksanen's theory of trophic dynamics was tested in two pl ant communities located in a North Derbyshire dale, including: (1) a l ow productivity calcareous grassland; and, (2) a highly productive Urt ica dioica (nettle) patch. Two methods (herbivore removal through pest icide application, and transplanting established, intact turves (0.5 m (2)) between the two community types) were employed, and analysed in a two-way ANOVA, to test the hypothesis that highly productive communit ies are controlled by 'top-down' forces and low productivity communiti es are controlled by 'bottom-up' forces. The Fretwell-Oksanen theory p roposes that herbivores limit growth in low productivity communities, not highly productive communities. Therefore, removal of herbivores wi ll result in an increase in plant biomass only in the low productivity community. The results presented in this paper support the Fretwell-O ksanen hypothesis. Furthermore, when small turves were transplanted fr om the highly productive community to the low productivity community t he removal of herbivores through pesticide application greatly increas ed 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 herbivo res are limiting plant growth in low productivity communities but not highly productive communities. Individual plant species response to he rbivore 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 plan ts to herbivore removal. This suggests that fast-growing plants from a highly productive environment are most likely to respond to the relea se of a limiting factor, in this case herbivory, within a community.