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 pot...

No abstract

We used replicated field enclosures to manipulate population densities of two species of bitrophic predators in a terrestrial old—field community: a mantid (Tenodera sinensis), and a wolf spider (Lycosa rabida). The treatments consisted of adding mantids alone (8 individuals/enclosure), lycosids alone (10 individuals/enclosure), and lycosids and mantids together (8 + 10 individuals). A control consisted of enclosures to which no predators were added. The impact of these predators on numbers and biomass of other arthropods in the community was examined on several levels: overall community, different size (body length) categories, and major taxa. We asked whether the impact of these predators in combination could be predicted from their separate effects. Mantids depressed total numbers (10—15%) and biomass (50%), in the arthropod community over the course of 10 d. This effect was confined to the largest size categories in the community. Lycosids had no measurable effect at this level of resolution. Mantids depressed abundance of acridids in both mantid and mantid/lycosid enclosures, but again lycosids had no impact. Lycosids alone enhanced abundance of gryllids, but not in the presence of mantids. Both mantids and lycosids depressed numbers of small spiders (also members of this guild), but this effect was not additive. Interactions among members of bitrophic generalist predator guilds may contribute to the commonness of nonadditive and higher order effects in manipulative experiments. Depending upon the level of resolution, it may be impractical to predict the impact of the whole guild from summing the individual effects of single predator species on terrestrial arthropod communities.

No abstract

Dictyostelium discoideum is a eukaryotic microbe feeding on soil bacteria. A first step towards describing the genetic structure of populations of this species was made by examining multiple isolates from a single locale. The isolates were grown clonally and their RFLP patterns compared, using a probe specific for a family of tRNA genes. Thirty-nine types were distinguished in 54 isolates. To determine if genetic exchange occurs among members of the population, an analysis of linkage disequilibrium was performed on the RFLP data. Little disequilibrium was found, implying gene flow in the population. In conflict with this result is the finding that no recombinant progeny were recovered from many attempted crosses between pairs of isolates. The tentative conclusion is that genetic exchange does not in fact occur, and that the observed shuffling of RFLP bands is caused by insertion and excision of transposons known to be associated with the tRNA genes of Dictyostelium.