| Summary: | 1. Syrphidae were chosen as indicators of diversity in samples from five Malaise traps chosen to represent the transition from deciduous to coniferous woodland. 20, 234 syrphids of 115 species were captured between 1 April and 30 September 1980 - 1982. 2. Seasonal distribution is bimodal, with abundance peaks in May and late July through August. This contrasts with unimodal distribution in other habitats. Seasonal variation in aphid quality is proposed as the explanation. 3. Hoverfly species show enormous variation in abundance over the years, but, with one exception, the number of individuals and species at each site gives a consistent ranking each year. More species were caught in the first half of the sampling period whereas more individuals were captured in the second half. 4. Variation in species' abundances each year inhibits the identification of indicator species along the coniferization gradient. 5. Species diversity indices are used to estimate site quality and community structure. The former indices give site rankings in agreement with the number of species present and confirm the relative diversities of the two sampling periods. The latter indices demonstrate community changes over the three years, which is ascribed to the fluctuating nature of the species populations involved. 6. Principal components analysis isolates the catches from all sites according to season. High-diversity sites show more seasonal variation than low-diversity ones. 7. 75% of the syrphids captured have aphidophagous larvae but all five trophic categories are represented. Each site has a distinct trophic composition and this apparent trophic stability questions the over-reliance on the species as the fundamental ecological unit. 8. Chaotic phasing of species abundances, mediated through climatic control, is proposed as a non-equilibrium theory of population control which maximizes niche exploitation whilst minimizing intra-guild competition, thus maintaining species diversity.
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