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Journal Article

Competition and Coexistence among Panorpa Scorpionflies (Mecoptera: Panorpidae)

Randy Thornhill
Ecological Monographs
Vol. 50, No. 2 (Jun., 1980), pp. 179-197
Published by: Wiley on behalf of the Ecological Society of America
DOI: 10.2307/1942478
Stable URL: http://www.jstor.org/stable/1942478
Page Count: 19
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Competition and Coexistence among Panorpa Scorpionflies (Mecoptera: Panorpidae)
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Abstract

The ecological and behavioral interactions of adults of nine species of Panorpa scorpionflies were studied during 1971-1974 at eight woodland sites in southeastern Michigan, USA. The species overlap considerably in habitat use, diurnal foraging time and location, and diet. All species inhabit primarily the herb stratum of moist forests. Soft-bodied dead arthropods comprise from 88.5 to 100% of the diets of the different species. Panorpa scorpionflies are very aggressive around food. The species can be ranked linearly in relation to their ability to dominate heterospecific individuals at food. The aggressive dominance ranking of the species is related to increasing size (r = .78, P = .04). The study sites vary in Panorpa species richness from three to eight. Panorpa species clearly interact. Mark-recapture data reveal that at the site where only three species occur adults of P. banksi live longer and move less than adults of the same species at a site where six species occur. The adult seasonal distributions of the three species at the site where only three species occur are extended in comparison to the seasonal distributions of the same species at other sites with greater Panorpa species richness. Species or species groups partition the season to some extent by successional emergence. Seasonal overlap, however, is sufficient to result in competitive interactions between some species. Analyses of the seasonalities of the species reveal that the numbers of individuals and species of Panorpa present during a species' adult seasonal period inversely influence the duration of that period. The durations of the adult seasonal period of the more aggressive species are primarily influenced by the number of conspecific individuals present, whereas the durations of the adult seasonal periods of the less aggressive species are primarily influenced by the number of individuals of heterospecific and more aggressive species present. Very aggressive species cause less aggressive species to feed above the herb stratum; this may result in increased exposure to predators and to reduced longevity of the less aggressive species. I suggest that the emergence times of the species allow coexistence and have been selected in the context of interspecific competition for food. The emergence times of the species in relation to each species' aggressive dominance appears consistent with this hypothesis. The variation in species richness between sites is probably not due to the extinction of certain species because of competition, but is the result of different amounts of Panorpa habitat destruction at the sites in the past.

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