Top-down control and its effect on the biomass and composition of three grasses at high and low soil fertility in outdoor microcosms

We used outdoor microcosms in order to freely manipulate three trophic levels (ladybird/aphid/grass) at two soil fertility levels (low and high). Two hypotheses were tested: (1) that top-down control is only a mechanistic factor at high soil fertility, and (2) that herbivory increases secondary plant succession by preferentially feeding on the fast-growing early-successional grasses. Plant biomass responded dramatically to the high soil fertility treatment, as did aphid numbers in the absence of ladybirds, and ladybird activity (ladybirds feeding on aphids). At low soil fertility, plant biomass was low, aphid numbers were small, and ladybird activity was minimal. Only at high soil fertility did top-down control cause a significant response to plant biomass and species composition. The two fast-growing, early-successional grasses (Poa annua and Arrhenatherum elatius) had a greater biomass in the presence of the ladybirds compared to when the ladybirds were absent, while the slow-growing, late-successional grass (Festuca ovina) suffered. The opposite was found when ladybirds were absent but aphids present. These results suggest that herbivory may increase the rate of secondary succession, but that top-down control of herbivory by carnivores may reduce the impact of herbivory in high productivity communities. Received: 2 May 1997 / Accepted: 25 July 1997