Logistic Interaction of Natural Populations of Two Species of Waterboatmen

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Abstract
The zero isoclines for the Volterra interspecies competition model are calculated and graphed using data from enclosure experiments with 2 cooccurring species of waterboatmen [Hesperocorixia-lobata Sigara-macropala] (Corixidae). The isoclines predict coexistence. Independent field samples are used to plot whole-summer 2-spp. paths along with the model. There is a striking agreement. The model may even predict the proportional growth of the 2 spp. in other years and habitats to some extent. The 2 spp. are not correlated positively or negatively in space. There is evidence of only weak niche differentiation. The species do show a seasonal switching of dominance. The observed fit to the Volterra model probably depends principally upon the sufficiency of the models limiting term in characterizing the late summer ecology of the 2 spp. Part of the explanation of their coexistence may arise from imperfect resource tracking by the dominant species.