Stream Geomorphology and Benthic Habitat Predictability as Determinants of the Population Dynamics and Life History of the SnailJuga pllclfera

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Abstract
Populations of J. plicifara were examined for a second, third and sixth order stream in western Oregon over all seasons. I hypothesized that traits indicative of r-selection should be more apparent as benthic habitat predictability or stream size decrease. Mean individual biomass and apparent survivorship were directly correlated with stream width and benthic habitat predictability while mean fecundity and population density were inversely related to the above factors. Fecundity and population density were regulated primarily by geomorphological factors through the abundance of breeding sites and the number of trematode parasites causing snail gonadal castration. Mortality and survivorship were directly influenced by the severity of the winter freshets and the predictability of the benthic habitat at that time. Although a trade-off between fecundity and survivorship was apparent, consistent with predictions from r and K-selection theory, this pattern was in fact the result of more complex interactions. I conclude from these results that: (1) benthic habitat certainty may be an important factor determining life history and population dynamics patterns in stream benthic invertebrates and (2) local stream geomorphological and biotic conditions, indirectly correlated with the degree of habitat certainty, can be critical in explaining overall life history patterns in stream benthic invertebrates.