Coexistence and Local Extinction in Two Pierid Butterflies

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Abstract
Interactions of 2 sympatric, closely related Pieris spp., indigenous P. oleracea and naturalized P. rapae, are described. The spatial and temporal distribution of adults and juveniles indicates broad overlap but each species occupies a habitat or utilizes a foodplant not successfully exploited by the other. P. oleracea utilizes indigenous Dentaria diphylla, a woodland crucifer, and flies in wooded areas rarely occupied by P. rapae; P. rapae utilizes naturalized Barbarea vulgaris, which is usually lethal to P. oleracea larvae. Although potential for interspecific competition is substantial, there is no evidence for ecological displacement. Crop rotation and undisturbed woodland promote growth of crucifers that support large open populations of both species. No evidence of reproductive interference was observed. These observations suggest that composition and abundance of the crucifer flora are more important than interspecific interactions in determining Pieris abundance and ecological distribution. Circumstances surrounding the presumed competitive exclusion of P. oleracea from southern New England [USA] by P. rapae were reconsidered. Shifts in land use and concomitant changes in the crucifer flora are probably sufficient to explain local extinction without inferring competition.

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