Jan G. Sevenster

AN EVOLUTIONARY ARGUMENT FOR TIME LIMITATION

Shine et al. (1996) argue that the Shine and Schwarzkopf (1992) model demonstrates that energy allocation measures will provide little insight into evolution of RE in squamate reptiles. Our analysis shows why that interpretation is premature if not incorrect. But the claim that energy allocation measures should be abandoned also stems from confusion about the meaning of CR and RE. Contrary to what Shine and Schwarzkopf (1992) and Shine et al. (1996) state, CR is not the same as RE. In fact, CR is a function of RE. CR is measured in units of fitness and RE is measured in units of energy. The relationship between CR and RE can be derived from Williams (1966): E (reproductive effort), is the ratio of current reproduction to expected future reproduction (Fig. 5). A quantitative expression for CR involves an assumption and a mathematical expression, neither of which were provided by Williams. In the context of Williamss construct, an expression for CR must specify the relationship between the expected future reproduction and current reproduction. For example, we could specify an expression for CR as the magnitude of reduced future expectation of reproduction, Z, attendant upon changing current reproduction (Fo) from F0 1 to F02 (Fig. 5). From this formalism, it is clear that CR and RE are not the same, and that the assertion that the Shine and Schwarzkopf (1992) model demonstrates that energy allocation measures will provide little insight into the evolution of RE is unfounded.

Explaining Local Species Diversity

In this paper we examine the explanations for local species diversity. Using six extensive data-sets for drosophilid flies (which include both temperate and tropical species) we compare three major categories of explanation (Cornell & Lawton 1992): niche heterogeneity (resource partitioning), spatial heterogeneity (intraspecific aggregation), and the fullness of the niche space (saturation level). We conclude that these Drosophila communities are dominated by intraspecific aggregation, not by resource partitioning, and they are not fully saturated.

A life history trade-off in Drosophila species and community structure in variable environments.

Within taxa at the class or family level, the developmental period is often proportional to adult life span. In Drosophila species, a short developmental period increases larval competitive ability. Species with a long adult life, however, may have a better chance to reach new breeding sites in time and space. In another paper (Sevenster & Van Alphen 1993), we presented a model incorporating this trade-off. It shows that fast larval developers («fast species») are dominant when breeding opportunities are frequent, and that good adult survivors («slow species») are dominant when breeding opportunities are scarce. Moreover, the model demonstrates that a fast and a slow species may coexist in intermediate environments

The Shape of the Trade-Off Function Between Egg Production and Life Span in the Parasitoid Asobara Tabida

Despite a large number of studies on the cost of reproduction, the shape of the trade-off between reproduction and survival is still unresolved. In this paper, two prominent alternatives with respect to this problem are compared in the parasitoid Asobara tabida (Nees): a linear relationship between egg production and physiological life span, and a linear relationship between egg production and intrinsic mortality rate. This is the first study that directly assesses the shape of the trade-off function between reproduction and survival in an insect parasitoid. We manipulated egg production to be low, medium or high and measured the associated life span under laboratory conditions. The results showed a decrease in mean life span with increasing egg production. The data differed significantly from a trade-off function between egg production and mortality rate, while a linear trade-off function between egg production and physiological life span produced an accurate description of the data.

Coexistence in Stochastic Environments through a Life History Trade Off in Drosophila

The coexistence of competing species may be mediated by various mechanisms including resource partitioning and various kinds of environmental heterogeneity. In this paper we show how differences in life history enhance coexistence in stochastic environments. In Drosophila species, as in many other taxa, the developmental period is proportional to adult survival. A short developmental period, i.e. a high developmental rate, enhances the competitive ability of larvae. High adult survival, on the other hand, must increase the probability of reaching new breeding sites in space and time. We present a model of two competing species to investigate the consequences of the trade off. The model features density dependent mortality (due to competition) in the larval stage, and age dependent mortality in the adult stage. Breeding opportunities occur with a certain probability per time step. This is the only stochastic component of the model. The model demonstrates that fast growing, short lived species are superior when breeding opportunities are frequent. Slower growing, long lived species are superior when breeding opportunities are rare in time. A sensitivity analysis indicates that this conclusion is qualitatively robust. The mutual invasibility criterion reveals that stable coexistence will occur for certain feeding probabilities.

THE STATISTICAL SIGNIFICANCE OF DIETS AND OTHER RESOURCE UTILIZATION PATTERNS

The assessment of the statistical significance of differences in diets is surprisingly rare. In this note we discuss a powerful randomization test, that can be applied to diets as well as to other patterns of resource utilization and, for example, to patterns of species composition. We argue that ecological questions often require that both between-group overlaps and within-group overlaps are compared with their expected distribution under the null-hypothesis of no differences between groups.