Persistence in a Saltmarsh Insect Community: Experiments with Local Extinctions, Dispersal, and Colonization

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Local populations of the gall midge Asphondylia borrichiae Rossi and Strong apparently become extinct and are re-established by immigrants frequently, as indicated by our work with this new species of fly. Patches of the sole host plant, sea oxeye daisy (Borrichia frutescens) are all small and discrete. There are no vast "mainland" patches, and gall-midge populations contributing colonists to locally extinct populations are about the size of those that have become extinct. The investigator posit that this is a true "metapopulation" system (sensu Levins 1970), in which persistence is due to recolonization after local extintion among roughly even-sized patches. Parasitoids are a major threat but are spotty in their effects. In excess of 95% parasitism occurs in some sea oxeye patches, while other patches have much lower rates. Patches with temporarily low parasitism rates are likely to provide immigrant to extinct local midge populations. Three sets of experiments will assess metapopulation dynamics. (1) Forced extinction of midges in natural sea Oxeye patches and (2) creation of new patches in salt marsh will indicate the role of natural immigration in persistence. (3) New patches of sea oxeye daisy, created in bayou areas far inland from natural midge population, will permit comparisons of persistence, with and without parasitoids, in the absemce of midge immigration. This investigation will yield critical information regarding scales of interactions important to many biological system //