Determinants of Species Abundance Distribution in Insect Communities and Assemblages

Using a theoretical model to describe species abundance distribution is an effective tool for both characterizing and comparing the structure of plant and animal assemblages. The logseries and the lognormal distributions are among the most common theoretical distributions used in studies of communities and assemblages. Although the logseries distribution is considered by some to be the best model for depicting species abundance distribution and thus relatively widespread, others have questioned whether it is an accurate and widespread depiction of species abundance, compared to the lognormal distribution. Some contend that often the logseries is observed because of sampling biases produced by small sample size and underrepresentation of scarce species. Previous research in our lab has characterized and compared two macrolepidopteran assemblages, on two riparian tree species, black willow Salix nigra (Marsh) and box elder Acer negundo L., that are comprised of scarce species. The structure of these assemblages parallels that of an assemblage described by a logseries distribution. However, since the characterization of the assemblages was based on relative sampling of larval abundance rather than absolute sampling, it was still subject to the criticism that the fit to the logseries was a consequence of sampling bias. Such unbiased (absolute sampling) data on the abundance of larvae of species on box elder and black willow were provided by fogging tree canopies (Barbosa et al., submitted) and demonstrated that the logseries best described species abundance distribution.

Theoretically, a lognormal distribution is assumed to arise when multiple factors determine species abundance distribution in natural communities or assemblages, even though the factors may be, and usually are, unknown. In contrast, a logseries distribution presumes that single factors determine abundance distribution. The two that are most obvious broad sense factors are the influence of natural enemies (top-down forces) or plant quality (bottom-up forces). Thus, the critical question in this research project is, Do top-down, bottom-up, or both factors influence species abundance distribution? Specifically, we focus on the box elder macrolepidopteran assemblage in order to test three alternative hypotheses, i.e., that (1) The presence and impact of natural enemies on an array of species in the macrolepidopteran assemblage on Acer negundo (box elder) determines survival and abundance and thus species abundance distribution (i.e., the structure) of the assemblage, (2) Resource quality (in the form of host plant quality) determines growth and survival and thus species abundance distribution (i.e., the structure) of the assemblage on box elder, or (3) Both natural enemies and host plant quality jointly determine the levels of survival and abundance and, thus, the species abundance distribution (i.e., the structure) of the assemblage on box elder.

The genetic structure within and between demes represents the distribution of genetic variance at sequential hierarchical scales. That variance and its distribution clearly is influenced by numerous factors but among the most important factors, particularly within the context of assemblages, populations, or communities are the dispersal of species and associated levels of gene flow. Genetic variance and its distribution within and between demes (subpopulations) is critical to maintaining genetic diversity; upon which selection acts. The genetic structure within and between subpopulations establishes the distribution and to some extent the abundance of similar and dissimilar, behaviorally and ecologically interacting individuals. If the variance in the outcome of host-parasitoid interactions is in large part a function of the spatial distribution of genotypes that vary in their susceptibility to parasitoids then the genetic structure of host demes and populations (i.e., inter- and intra-demic genetic structure) is essential for the understanding of host-parasitoid dynamics. Thus, we propose to undertake a hierarchical, genetic characterization using molecular markers of individuals in populations, in which the smallest sampling unit will be parasitoids and macrolepidopteran larvae (collected from individual trees of several species, across several sites). We will determine the degree and scale of relatedness which ultimately is a reflection of patterns of progeny distribution of individual females of herbivore hosts and their parasitoids. Thus, we will be able to address questions such as, Are there distinct subpopulations (i.e., demes) of herbivore host species and their braconid parasitoids? If not, is there significant intrademic genetic variation across sites? That is, Are the distributions of the progeny of herbivores and that of parasitoids clumped or random, and at what scale? Does the pattern of distribution of the progeny of herbivore species represent a bet hedging strategy which minimizes vulnerability to parasitoids? If there are distinct demes, What are the differences in inter and intra demic genetic structure of herbivore species and their parasitoids? Are differences in inter- and intra-demic genetic structure associated with differences in gene flow and dispersal capacity of herbivores and parasitoids?

Geographic structure of a species is defined as the distribution and abundance of genotypes within and among populations. Important evolutionary questions in population biology can be answered through analysis of geographic structure. In the case of the dead-leaf butterfly Anaea ryphea (Nymphalidae), a widely distributed species, populations display a wide array of variation, similar throughout its whole distribution range except for Panama. The relatively small amount of variation in Panama might be the result of (1) relatively constant physical factors, (2) biological interactions exerting selection on wing patterns, or (3) demographic processes. Because variation in annual precipitation and elevation in Panama mirrors that in Costa Rica and other Central and Latin American countries, relatively constant physical factors would not appear to be an explanation for the small variation in wing pattern. There is also no reason to believe that predation patterns in Panama would be different from those in Costa Rica. That leaves a genetic event as a probable evolutionary cause for the different levels of variation in this species.

This study analyses of population structure is used to answer the question: Is there a genetic explanation for the loss of variation in Panama, or has differentiation occurred in the populations of Panama and Costa Rica? Sampling of Panamanian and Costa Rican specimens will address this question.

Several factors interact and influence the distribution, abundance, and diversity of organisms, including environmental conditions and the availability of resources. Among important environmental factors are disturbances, natural or otherwise that cause habitat degradation. It is important to monitor these changes in conservation-oriented studies because habitat degradation can adversely affect diversity (so that only a few common species remain) or abundance (so that species have markedly reduced abundances). Butterflies are possibly the best group for assessing and monitoring patterns of terrestrial arthropod diversity. Butterfly biology and taxonomy are well known, and an estimated 90% of species are described. Deforestation and human activity directly affect butterfly population structures because they usually reduce oviposition sites and change habitat characteristics necessary for reproductive processes, such as sunlight level, temperature, and relative humidity. It is known that some butterflies are characteristic of disturbed areas, whereas others can only be found in relatively undisturbed sites. Therefore, the analysis of variation in the composition and structure of butterfly communities in protected areas is a powerful conservation biology tool for monitoring habitat changes. Conservation and management plans are usually based on vertebrate and plant data, but patterns of insect diversity are quite different from those of other groups and should also be used when management decisions are made.

This project aims to estimate butterfly diversity in Amazonian sites subjected to different levels of deforestation (high-impact and low-impact logging). These diversity data will show if there are significant differences between areas, and seasonally within one area, to be used to infer levels of disturbance. The data can then be used in conservation and restoration of these and other areas.