Non-Equilibrium `Island' Communities: Diptera Breeding in Dead Snails

Abstract

units or ecological 'islands' scattered through other habitats. Communities develop within them that are characterized by the absence of green plants and the presence of a diverse fauna of arthropods as well as multitudinous microorganisms. These communities have at their disposal a limited amount of energy, which is gradually used up by the activities of the community members. The successional changes that take place in these 'islands' are usually rapid, occurring on a scale of days (or even hours) rather than years. The changes are largely the results of the activities of the organisms themselves. The physical environment often has relatively' little direct effect. Because of the rapidity of the successional changes, there is usually only a single generation of any species (excluding microorganisms) before the habitat unit has become either unsuitable or exhausted. Dispersal is then necessary to find other suitable units for colonization. These ecological 'islands' are not self-sustaining, and the communities in them can never reach an equilibrium at least on the scale of the individual 'island'. Thus they differ in several respects from the geographical islands considered by MacArthur & Wilson (1967), the habitat islands on the mainland (Vuilleumier 1970; MacArthur 1972), and the host plants considered as islands by Janzen (1968, 1973). Dead snails form small habitat units of the type considered above.; They are usually soon exploited by a variety of Diptera. The fly larvae feed on and complete their development within a single snail. Each isnail is essentially an isolated unit. Interchanges with the external environment are usually limited to the import of Dipteran eggs (or larvae in the case of the ovoviviparous Sarcophagidae), and the export of mature larvae, ready to pupate in the soil, or of adult flies. The community that develops in the snail is shortlived and each species normally has only a single generation before the food supply is exhausted. The number of species and individuals is limited, compared to the communities of larger carrion, and it is possible- to make a complete census of all those attaining the pupal or puparial stage. It is also easy to set up many replicate habitats and study the variation among the communities in different snails in a small area and at different times of the year. This paper is concerned with the community in the dead snails from the aspect both of the single snail and the set of snails exposed to attack at one time. It examines the patterns of abundance of species and individuals among the snails, and successional and seasonal changes in these patterns. It then looks at the effects of predation, parasitism and competition on the communities. The life history strategies used by the different species and