Structure and functioning of the Lepidoptera community in Ravenelia macowaniana Pazschke fungus galls on Acacia karroo Hayne :prospects for environmental monitoring

Abstract

Community patterns and processes operate within a hierarchy, and multiscale approaches to community studies are essential for understanding the role of local and regional processes in the structure and functioning of communities. This information enables viable management decisions to be made for the conservation of communities and ecosystems. In addition, monitoring terrestrial biotic communities to determine the impact of global climate and land-use changes has been highlighted as a priority. Studies of herbivore insect communities are likely to contribute to the formulation of general principles of the structure and functioning of terrestrial communities, because insects and green plants together dominate most terrestrial ecosystems. Furthermore, communities living concealed in plant tissue structures, such as galls, are ideal for this purpose because they have quantifiable habitat boundaries, are multispecies and multitrophic level structures, and are easily sampled. A community of phytophagous Lepidoptera larvae inhabit galls induced by the fungus Ravenelia macowaniana Pazschke on Acacia karroo Hayne. The objectives of this study were to examine the biology, structure and functioning of the community across South Africa, to contribute to understanding the community ecology of herbivore insects in discrete plant structures and, to evaluate the potential of this association for use as a bioindicator. The community was sampled from nine localities (over a range of climates) in South Africa between 1991 and 1994. The galls were found to be seasonal, ephemeral, patchily distributed and of a high tissue quality. Twenty-two Lepidoptera species were recorded and parasitism of the larvae was low. A key to the larvae was compiled and diagnoses, illustrations, instar numbers and head-capsule sizes were determined for the dominant species. Gall occupancy and utilization was consistently high, and gall occupancy appears to provide the larvae with protective, nutritional and microenvironmental advantages. Positive aggregation, positive associations and abundance covariation and resource mediated competition occurred between individuals and species in the galls. The organization of this community had more in common with other communities associated with ephemeral, patchy resources, i.e. interactive and local processes were important, than with herbivore insect communities in general. The value of community properties (evenness, diversity, larval density, species composition) were indicative of habitat quality differences (patch size and isolation, physical disturbance and proximity from agroecosystems). Climate variables best correlated with species richness, community structure and seasonality were total annual rainfall, rainfall variability and winter climate conditions. The community was spatially concordant, although the abundance rankings of rare species were variable. Typical and discriminant species were identified from the extent and constancy of their contribution to community structure. These species are sensitive indicators and potential pivotal species. Community properties were differentially sensitive to particular organizational processes and were identified as suitable for monitoring aspects of environmental change. The community was found to be a sensitive and practicable bioindicator, responding to habitat quality and climate gradients. A baseline for it's use in monitoring has been established, but the full value of such monitoring lies in an ongoing, long-term, regional programme.