The structure of an aphid–parasitoid community

Authors


H. C. J. Godfray, Department of Biology and NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot, Berkshire SL5 7PY, UK. Tel. & Fax 01344 294215 & 01344 294339. E-mail: c.godfray@ic.ac.uk

Abstract

1. A quantitative parasitoid web was constructed describing the trophic relationships between the community of aphids, parasitoids and secondary parasitoids in an abandoned field in southern England. Root aphids were omitted and secondary parasitoids were associated with aphids rather than primary parasitoids. All aphids, parasitoids and trophic links were expressed in the same units (m−2). Over a 2-year period, separate webs were constructed for every month that aphids and parasitoids were active in the field.

2. Twenty-six species of plants were attacked by 25 species of aphid which were parasitized by 18 species of primary parasitoids. The primary parasitoids were attacked by 28 species of secondary parasitoids, of which 18 directly attacked the still living aphid (hyperparasitoids) and 10 attacked the aphid after mummification (mummy parasitoids). The aphids were attended by three species of ants.

3. Eleven webs were constructed for the months May to September 1994 and May to October 1995. Aphids and primary parasitoids were most abundant and diverse in June, and secondary parasitoids one month later.

4. The ratios of the number of aphid species to the number of species of primary parasitoid and hyperparasitoid were relatively constant across webs, as was the ratio of the number of links involving hyperparasitoids and mummy parasitoids. The ratio of the total number of links to the total number of species increased with web size.

5. The relative abundance of the species in the different webs was well described by a Dirichlet distribution with a common parameter. This implies a gamma distribution of aphid abundances in the field with few common and many rare species.

6. Predator (i.e. parasitoid) overlap graphs were constructed to assess the potential for indirect interactions between aphids. Mummy parasitoids were the most important group linking different aphids. A quantitative overlap diagram was developed to illustrate the potential strengths of indirect linkages. Common aphid species shared few strong, indirect links via primary parasitoids or hyperparasitoids, but could be strongly linked by mummy parasitoids. Symmetrical links were uncommon, and rare species were potentially strongly influenced by the presence of common aphids with which they shared parasitoids.

7. Ant-attended aphids were attacked by fewer species of primary parasitoids and hyperparasitoids than those species not attended by ants. Species of mummy parasitoids attack, on average, approximately twice the number of host species than species of either primary parasitoids or hyperparasitoids.

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