Beetles, Competition, and Populations

Cannibalism facilitates the use of a novel environment in the flour beetle, Tribolium castaneum

Cannibalism is well known to affect both the population dynamics and the competitive relationships of organisms. Cannibalistic behaviour commonly increases in stressful conditions, such as when density is high or food is scarce, and cannibals often obtain a nutritional benefit. Might cannibalism also increase in a novel environment to which a population is poorly adapted physiologically? Moreover, might cannibalistic behaviour provide enough of a nutritional advantage in a nutritionally stressful environment to rescue individuals from its adverse effects and thus permit colonization and range expansion? Previous work has shown that oat flour is a particularly stressful environment for Tribolium castaneum. In the study reported here, egg cannibalism by two strains of T. castaneum was significantly enhanced in oat flour, and egg eating rescued larvae from the adverse demographic effects of this poor environment. Development time of the cannibals was accelerated almost to the level seen for individuals reared in the nutritionally superior environment (wheat plus brewer's yeast). Their survival and fecundity also increased relative to individuals reared in oat flour without the opportunity to cannibalize. A sib analysis revealed that for larvae reared in the presence of victim eggs, the extent of cannibalism was genetically variable, so that this trait could evolve, given a selective benefit exceeding its cost. These results suggest that colonization of a marginal new environment could be facilitated by enhanced rates of cannibalism. The possible interplay between cannibalism and physiological adaptation to a new environment is discussed.

The use of microcosms as an experimental approach to understanding terrestrial ecosystem functioning

Since 1986, a series of microcosm experiments has been conducted at the Unit of Comparative Plant Ecology (UCPE) in an attempt to test our understanding of the principles controlling the structure and dynamics of plant communities and ecosystems. In each experiment microcosms have been seeded with a common pool of organisms, and systems have been allowed to assemble under replicated controlled conditions. Experiment variables have included mineral nutrient supply, temperature, moisture supply, soil depth, carbon dioxide concentration, mycorrhizas, rhizobia, herbivores and carnivores. Results from these experiments are presented to illustrate the value of synthesised ecosystems in ecological research.

An experimental analysis of competitive indeterminacy in Tribolium

This report reexamines experimentally the problem of competitive indeterminacy in mixed-species populations of the flour beetles, Tribolium confusum and T. castaneum. Indeterminacy takes the form of alternative competitive outcomes: in some replicate cultures one species exterminates the other with a probability, say p, whereas in others, the opposing species wins with a complementary probability, 1-p. The conventional explanation for this is the genetic founder effect hypothesis--an explanation based on genetic stochasticity. The experiment reported here partitioned indeterminacy into founder effect and nonfounder effect components. The results implicate demographic stochasticity, not classical genetic founder effect, as a factor influencing the identity of the winning species.

Interference competition and niche theory

A linear model of interspecific competition with separate parameters for exploitation and interference is deduced. Interference is assumed to have a cost and an effect. The interfering species realizes a "profit" if some resources, which the species interfered against would have utilized, are made available as a result of the interference. Interference is favored when its cost is small, its effect is high, and the resource overlap with the species interfered against is high. Interference is likely to be an alternative strategy to high exploitation efficiency. The incorporation of interference into niche theory clarifies the competitive phenomenon of unstable equilibrium points, excess density compensation on islands, competitive avoidance by escape in time and space, the persistence of the "prudent predator," and the magnitude of the difference between the size of a species' fundamental niche and its realized niche.