Competition between two estuarine snails: Implications for invasions of exotic species

An introduced snail, Batillaria attramentaria, has successfully invaded sev eral salt marshes and mud flats in northern California. In these areas popu lations of the native mud snail, Cerithidea californica, have declined prec ipitously. Since both species feed on epipelic diatoms, I hypothesized that the decline in Cerithidea was a result of exploitative competition with Ba tillaria. To test this hypothesis I manipulated snail densities in Bolinas Lagoon, California, to estimate their effects on, and responses to, food re sources. For two size classes of each snail species I quantified (1) the ef fect of intraspecific density on food availability, and (2) the growth resp onse of each size class to different food levels. These relationships were used to generate predictions of the exploitative competitive effect of each snail on the other species. These predictions were tested against direct m easurements of the effect of interspecific competition on snail tissue grow th using field experiments. The observed values of snail growth matched wel l the predicted relationships, suggesting that exploitative competition doe s occur. Although the two snails did not differ in their effect on resource levels at any experimental snail density, the introduced snail was always more efficient at converting limited resources to tissue growth. Similar re sults were obtained in three different experimental periods. Batillaria's e nhanced resource conversion efficiency provides a sufficient explanation fo r its successful invasion and subsequent exclusion of Cerithidea. Theoretic ally, conversion efficiency of resources should be just as important as res ource suppression and uptake to exploitative competitive ability, but it is rarely examined experimentally. By separating exploitative competition int o its component parts, the protocol used here allowed identification of a s eldom implicated mechanism that can affect invasion success.