The Herbivorous Insect Communities of Open and Woodland Bracken: Observations, Experiments and Habitat Manipulations

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Abstract
In this paper we examine how the insect herbivores that feed on bracken fern, Pteridium aquilinum (L.) Kuhn., are influenced by open and woodland habitats. Most, but not all herbivore species in Britain are commoner on open bracken. We examined three possible causes of these distribution: differences in host plant phenology between the two habitats, the effect of shade per se, and differences in host plant quality. Woodland bracken emerges earlier, but unshaded bracken grows more rapidly, so that by mid-season both have attained a similar stage of development. Nevertheless, we found that the abundances of some herbivore species were correlated with the data of emergence, regardless of habitat, although others were not. Thus plant phenology was one of several factors that might explain differences between open and woodland herbivore assemblages. The effects of shade per se were examined by removing woodland trees and artificially shading open areas for comparison with unmanipulated controls. Again some species responded by changing their distributions according to their known preferences for light or shade, whilst others did not. Finally we examined feeding preferences, growth and mortality rates of a sawfly caterpillar, Strongylogaster lineata, on open bracken, where it is common, and on woodland bracken where it seldom occurs. However, the type of habitat proved to have no effect on the performance of the caterpillar. We conclude that the degree of shading of the host plant has pronounced, but variable effects on the distribution of bracken herbivores. While it was possible to identify some of the factors influencing some species along the light-shade gradient, it is apparent that processes determining the distributions and abundances of each species are both complex and idiosyncratic.