The Effects of Herbivorous Snails on Submerged Macrophyte Communities in Minnesota Lakes

The influence of herbivorous snails on freshwater macrophytes was examined both in the laboratory and the field. In the laboratory, 14 macrophyte species grown in the absence of herbivores had growth rates ranging from 1-10%/d. When grown with four densities of herbivorous snails, species that grew fastest in the absence of herbivores were, in general, most negatively influenced by grazing. In food choice tests, snails typically preferred the plant species that grew fastest in the absence of herbivores. Snail densities were manipulated in a small mesotrophic lake, Christmas Lake. In areas where high snail densities were maintained, macrophyte species richness decreased, and the plants remaining were the species least preferred in laboratory choice tests. In surveys of eight additional lakes having a range of snail densities, the relationship between snail density and macrophyte species composition and diversity was consistent with the laboratory and Christmas Lake results. The levels of herbivore damage in the lake were measured for one macrophyte species. For the leaves examined, from 6 to 13% of the total leaf area was lost to herbivores. These results demonstrate that herbivorous snails can strongly influence the distribution, abundance, and diversity of freshwater macrophytes, and suggest that the role of herbivores in freshwater macrophyte communities may be similar to their role in terrestrial and marine communities.