The Regulation of Density in a Natural Population of the Pond Snail, Lymnaea Elodes

The role and mechanisms of density regulation were studied in a natural population of the pond snail, Lymnaea elodes, in a small permanent pond in southern Michigan. A portion of the snail's habitat along the margin of the pond was subdivided into 28 artificial snail—proof enclosures (pens). In two groups of four pens each, adult densities were altered to about 1/5 and 5 time initial spring density estimates (roughly 1,000/pen). A third group of four pens was left unaltered as a control. Sampling in these pens verified that alterations in adult density were maintained. The percentage of dead adults did not vary significantly among the three treatments, indicating the absence of any regulation to the numbers of adults through differential survivorship. On three dates in July the pens were sampled. Each time an inverse relationship was found between the densities of adults and young. A later, more extensive set of samples was taken after the margin of the pond was dry and the snail population in estivation. These samples showed no significant difference in the number of young snails (roughly 5,000/pen) among the three groups of pens. While the population was reproducing, a time—limited search for eggs was made which showed no significant difference in the total number of eggs per pen among the three groups. Thus treatment effects (alterations in adult density) had entirely disappeared in the numbers of eggs and young snails. In other pens predators were added or excluded without their having any apparent effect on the numbers of young or adults. Additions of food in the form of frozen spinach to two pens at regular intervals resulted in a dramatic (25—fold) increase in adult fecundity and an increase (4— and 9—fold on two separate dates in July) in the numbers of young. The differences between fed and unfed pens were taken as strong evidence for the presence of food limitation in the snail population, but there were indications that the limitation was one of a relative and not an absolute nature. There was an abundance of vegetation and coarse debris, but a lack of high quality food necessary for maximum fecundity and growth. Concurrent with the pen experiments, the general dynamics of a portion of the snail population not under enclosure was followed. Generally 12 to 24 samples were taken weekly from transects outside the pens. When the field data on growth and reproduction are compared to similar data from animals raised in the laboratory with superabundant food, it is obvious that field animals are realizing only a portion of their potential for fecundity and growth. These differences are attributed to food limitation. Field estimates of young snail mortality ranged from about 93 to 98%, but there is no evidence that this mortality was regulatory. Within the framework of the 20—fold differences in adult density in the altered pens, there is no indication that juvenile mortality contributed to the final observed convergence in densities of young. There are indications from other sources that mortality may be a potential regulatory force. The regulation of density in the pond snail was far more complete and occurred more rapidly than was thought possible. Although the mechanisms of the regulation are not as clear as the demonstration of their existence, the data strongly suggest that it was mediated through food limitation as expressed in fecundity.