Refugia and habitat partitioning among midges (Diptera: Chironomidae) in rain-pools

ABSTRACT.

• 1 Rain-pools on granite exposures are common in tropical Africa and they are inhabited by the larvae of two midge species, Chironomus imicola Kieffer and Chironomus pulcher Wiedemann.
• 2 These pools differ from other ephemeral island habitats such as carrion, fallen fruit, sap flows, mushrooms and dung in their high degree of spatial predictability. Most pools are predictably inhabited by either C.imicola or C.pulcher. This finding is considered in the light of contemporary competition theory.
• 3 An arrangement whereby the same species repeatedly invades the same temporary habitat indicates that there is something about the habitat itself causing the discrimination. I suggest that degree of exposure to sunlight may provide a cue allowing females to discriminate between pools as oviposition sites.
• 4 Both species survive the long dry season by retreating to pools of river water left by receding rivers. Dispersal to rain-pools by emerging adults occurs during the following season with imicola occupying rain-pools remote from shaded rivers. Appropriately, measured against pulcher, the biology of imicola is that of a colonizing species.
• 5 The possibility is discussed that the chironomids in rock-pools do not survive to breed. Instead, these populations may be maintained externally, for example by invasions from breeding populations in the Great Lakes of Africa, where larval densities may be too low to be detected by sampling. The evolutionary implications of such a situation are considered.