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Habitat fragmentation and the distribution of the risk of parasitism.
Cronin, James1, 1
ABSTRACT- Host-parasitoid stability is a central issue in ecology. Recent theoretical investigations have suggested a general criteria for the conditions necessary for stability: if the distribution of parasitism among host patches is sufficiently aggregated (the coefficient of variation in the risk of parasitism is greater than 1; i.e., the CV2 > 1 rule), host-parasitoid stability is possible. To date, there have been very few empirical investigations that have examined the influence of spatial factors on the distribution of the risk of parasitism. In the tallgrass prairies of the Great Plains, the planthopper Prokelisia crocea and its egg parasitoid Anagrus columbi are widely dispersed among host patches of prairie cordgrass. Across six generations and 12 prairie sites, natural distributions of parasitism are strongly aggregated: mean CV2 = 1.61 (95% CI: 1.31-2.08). In an experimental study in which host density was manipulated among cordgrass patches I found that the CV2 was negatively correlated with patch size (R = 0.39, P = 0.05), and strongly positively correlated with patch isolation (R = 0.59, P < 0.01). In addition, parasitism was more strongly aggregated at the patch edge than the interior. These results imply a strong linkage between habitat fragmentation and host-parasitoid stability. As cordgrass habitat becomes more fragmented, i.e. a decrease in patch size, an increased patch perimeter: area ratio, and greater patch isolation, the distribution of parasitism is expected to become more aggregated.
KEY WORDS: spatial ecology, aggregation, host-parasitoid stability, metapopulation