Snail-tadpole Interactions in Streamside Pools

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We studied competitive interactions and grazing effects of the snail Physella integra and the American toad Bufo americanus in ephemeral streamside pools in W-central Kentucky. Snails and tadpoles reared alone and combined at ambient densities in artificial pools revealed that costs of coexisting differed between taxa. Snails invested resources into growth and survival at the expense of reproduction when with anuran competitors, whereas tadpoles exhibited slowed development and decreased biomass per pool in combined-species treatments. These negative effects were likely a result of food limitation, considering that herbivory by both taxa significantly reduced algal abundance relative to ungrazed controls. Herbivory by both species also affected the assemblage of benthic algae. Diatoms, particularly Nitzschia spp. were the predominant algae in both single-species treatments, whereas the green algae Eudorina and Oedogonium comprised most of the community biovolume in com- bined-species treatments. The filamentous green algae Cladophora was dominant in ungrazed controls. A survey of 16 isolated pools along three third-order streams revealed patterns of snail egg production and benthic algal assemblage similar to treatment effects in the artificial pool experiment. The survey also indicated that snails and tadpoles seldom co-occur in these pools; thus competition between them may be infrequent. Linkages between phosphate avail- ability, predation and habitat persistence seem particularly important in mediating compet- itive interactions between these taxa.