Robert M. Eisenberg

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 snails 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.

Arthropod Community Responses to Manipulation of a Bitrophic Predator Guild

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.

Cannibalism reverses male-biased sex ratio in adult mantids: female strategy against food limitation?

Adult populations of the mantid, Tenodera sinensis (Saussure) initially were malebiased, but females outnumbered males by the end of the life cycle because mortality was higher among males than among females. Male mantids were the most frequent items in the diet of females during oogenesis, when food limitation generally is greatest. Males had an 83% chance of escaping cannibalism during any given encounter with a female; however, females continued to attract males after first mating, raising the cumulative probability of male death with increasing number of intersexual encounters. We suggest female mantids continue to attract and cannibalize males beyond their need for sperm as a strategy to alleviate food limitation during oogenesis. This is more parsimonious than the adaptive suicide hypothesis, in which male fitness is enhanced by investment of his biomass in this offspring, since our hypothesis does not require that the victim share parenthood with his cannibal

Experimental density manipulations of the predator, Tenodera sinensis (Orthoptera:Mantidae), in an old-field community.I.Mortality, development and dispersal of juvenile mantids

(1) Field and laboratory experiments were carried out to examine the effects of density on early life-history characteristics of the mantid, Tenodera sinensis. (2) For three replicated field treatments of threefold differences in initial mantid density, relative mortality was not density-dependent. Mortality was >90% for all treatments. Most mortality occurred among first-instar nymphs. (3) Laboratory experiments indicated that nymphs could survive crowded conditions if provided with sufficient food. Cannibalism was negligible among well-fed nymphs at all densities. Starved nymphs exhibited substantial cannibalism, but this was not related to density. (4) Rate of development, measured as changes in proportion of nymphs among instars over time, was negatively related to density in the field. Laboratory tests indicated that this was probably due to food limitation. (5) Relative dispersal, measured by monitoring nymphs caught in Tangletrap barriers surrounding experimental field plots, increased with increasing density. This contributed to overall convergence in density to similar levels among treatments and may help to regulate natural populations of this species. Most dispersal occurred among second-instar nymphs.

DIVERGENT SELECTION FOR GEOTACTIC RESPONSE AND EVOLUTION OF REPRODUCTIVE ISOLATION IN SYMPATRIC AND ALLOPATRIC POPULATIONS OF HOUSEFLIES

Experimental populations of houseflies subjected to 95% selective pressure for geotactic preference under conditions of 50% potential gene flow and of allopatry evolved reproductive isolation after only 16 generations. No significant difference was found between sympatric and allopatric populations. Our study does not support the hypothesis that incipient reproductive isolation occurs more rapidly under conditions of sympatry than allopatry.

Two‐Dimensional Microdistribution or Cellular Slime Molds in Forest Soil

Four sample series consisting of 101 small samples (x wet wt = 0.2 g) of forest soil were examined for the presence of cellular slime molds. Although each sample series contained 5 cellular slime mold species, the distribution of species among the samples indicates that on a microlevel, 5, 4, or even 3 species seem seldom to co-occur. The results also demonstrate the extreme patchiness of cellular slime mold distribution in nature.

Experimentally synchronized phenology and interspecific competition in mantids

-TWO sympatric mantids, Tenodera sinensis (Saussure) and Mantis religiosa (Linnaeus), which normally exhibit different egg hatch phenologies, were experimentally synchronized in replicated field enclosures to test the hypothesis that interspecific competition occurs when body sizes are most nearly similar. Each species was enclosed both alone and with the other. Tenodera sinensis, which normally hatches earlier, was unaffected by M. religiosa, but exhibited intraspecific density-dependent mortality. In contrast, survival and body size of M. religiosa declined in the presence of T. sinensis. As a result, body size ratios (T sinensis: M. religiosa), were greater in enclosures containing both species than in monospecific enclosures.

Clonal Diversity in Populations of Polysphondylium Pallidum, A Cellular Slime Mold

Populations of the cellular slime mold Polysphondylium pallidum contain clones that differ in their abilities to use bacterial resources. We found abundant diversity among isolates obtained from plots of several square metres and from single soil cores 1.1 cm in diameter. The variation we were measuring was not due to genetic changes induced by our manipulation. Previously, isolation of different species of cellular slime molds from sample areas, metres to hectares in extent, has prompted studies of mechanisms of species coexistence in the face of assumed competition for bacterial resources. We conclude that competition has not only failed to eliminate different species from the community but has failed to eliminate clonal diversity from within species. Previous explanations of species coexistence need reevaluation.

Do interactions of cellular slime mold species regulate their densities in soil

Studies of the ecology of macroorganisms have produced a body of theory about the nature of biological interactions and their effects on species in the field. This body of theory describes the ways that species affect each other and also predicts the outcome of specific interactions such as competition or predation. It is reasonable to ask whether the same body of theory applies to species of microorganisms. We performed a simple field experiment to test the hypothesis that biological inter- actions influence species of cellular slime molds living in forest soils. Members of the guild of cellular slime mold species co-occur on both microhabitat and geographic scales and, in laboratory cultures, the species all consume the same food resources. We experimentally elevated the density of one cellular slime mold species and detected significant population responses in the remaining species in the cellular slime mold guild. Our results clearly show that biological interactions are important to cellular slime molds: (1) we observed resource limitation in the species that we added, and (2) we observed that the densities of cellular slime mold species are interrelated.

The cellular slime mold guild and its bacterial prey: growth rate variation at the inter- and intraspecific levels

SummaryA guild of cellular slime molds (CSM) consisting of two isolates from each of five species, representing two genera, and obtained from the same square meter of forest soil exhibited extensive growth rate variation when tested on a suite of 18 bacteria isolated from the same soil. Significant growth rate differences were found at each taxonomic level examined: among species of different genera, between genera, among species within genera, and between isolates (=clones) within species. The type of bacteria used as prey determined the relative rank of the growth rates in different CSM isolates, as well as the taxonomic level at which significant differences were found. We suggest a possible reconciliation between a previous hypothesis, based on competition, and contradictory experimental work on resource partitioning in this guild of bacterial predators. Our results raise a question about the efficacy of using single genotypes to represent a species when ecological ideas are developed through laboratory investigations.