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 contends 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?

Traditionally, the major approaches to the biological control of insect pests have consisted of classical (importation) biological control, augmentation and conservation. Although the use of the first two approaches has been widespread and relatively successful, the use of biological control-enhancing genetically modified crops (BCEGMC) has been minimal. However, before such a strategy is implemented much more information is needed on between and within cultivar differences in attraction of natural enemies and, in particular, the aspects of plants that most affect natural enemy survival and effectiveness.

• Field Plot Evaluations
  
  Field plots of five major crops which may serve as model systems for the proposed research. The five systems evaluated have been soybeans, cole crops, tomatoes, peppers, and corn. Fifteen lines of corn also have been evaluated and are continuing to be evaluated under field conditions. Seven varieties of peppers are under greenhouse evaluation and we hope to do field evaluation in the near future. The names of the cultivars tested of each the latter crops are available on request. These evaluation have given some insights into the variable pest load on several different crops and the levels of activities by predators and parasitoids in several crops.



• Evaluations of Tri-Trophic Level Interactions
  
  The research in this domain has involved developing an understanding of the role of specific plant traits on natural enemies.
  
  Research has been conducted on the effects of plant morphological complexity on a predator of the pea aphid Coccinella septempunctata L.. This has been accomplished using isolines of peas (normal, tl, and aftl). Differences in morphology are such that morphological complexity of the near-isogenic lines can be ranked from low to high complexity. The near-isogenic lines exhibit two mutant genes af (afila) and tl (acacia) which alter the normal pea leaf considerably without exhibiting any other differences.
  
  Plant type and associated differences in leaf morphology were found to have no significant influence on aphid fecundity, intrinsic rate of increase, or pattern of distribution of aphids in the absence of predators. However, differences in the leaf morphology of the near-isogenic lines had a significant effect on the median residence time of the predator, when on leaves without aphids or with aphids. Beetles on the aftl leaf (the most complex leaf) had the longest residence time, whereas beetles on the Normal and tl leaves had the intermediate and the shortest residence times, respectively.
  
  Foraging times followed a pattern that was similar to that found for residence times. In the absence of aphids on leaves, leaf complexity had significant affects on foraging times. Beetles on the more complex leaf, aftl, had the longest foraging time compared to the Normal and tl. The frequency of falls from a leaf was not significantly associated with leaf morphological complexity. This was the case in both leaves with and without aphids. Leaf complexity had a significant effect on the search efficiency of the beetles with or without aphids. Search efficiency was significantly reduced on the more complex aftl leaves as compared to the Normal and tl leaves. Coccinellid area search efficiency was significantly increased on the tl vs the Normal leaves when aphids were present. The morphological complexity of the plant did not have a significant effect on the proportion of aphids consumed by C. septempunctata. While the predation efficacy of C. septempunctata was not affected by leaf morphology, plant complexity had a significant effect on the degree of prey disturbance by the predator. The location of surviving aphids was significantly different on each of the isolines.

Prey become aware of potential predator encounters by using information about the predators The responses of prey to predator signals allow prey the flexibility to react to varying levels of predation risk. Responses to predation risk by prey, such as predator avoidance and escape behaviors, have been viewed by behavioral ecologists as the result of decision making by prey. Decision making by prey is dependent on 1. the ability of prey to assess the presence of a predator, 2. the ability of prey to distinguish among predators and/or the degree of risk presented by a predator, 3. the availability of one or more defensive behaviors for prey to choose from and their relative effectiveness against a particular predator, and 4. the existence of trade-offs (predator avoidance or escape versus resource use, e.g. feeding or mating) that make decision-making necessary.

Several studies have shown that prey appear to assess the degree of risk, which implies the ability to assess the likelihood of predator success. Prey also may vary their responses to a given predator depending on the degree of risk as reflected in the predator's proximity to the prey and the abundance of the predator. However, very few studies have experimentally shown how prey respond to the degree of risk presented by the predator, probably because only a few studies have addressed which signals determine the prey's behavior under different risk situations, and such signals have rarely been related to the prey's fitness. The costs of anti-predator behavior have generally been measured by comparing the effect of predators against a no predator control using, for example, food eaten by the prey as the currency of costs. However, relatively few studies have related this effect to the prey's fitness. Costs of anti-predator behavior can also arise when prey move to a new microhabitat as a way of escaping predators if the new microhabitat is occupied by other predators.

Thus, this research project examines decision making by prey by determining whether prey can specifically recognize predators, whether prey assess the degree of predation risk, how prey choose from different anti-predator behaviors and their relative effectiveness against different predators, and the trade-offs between anti-predator behaviors and lost opportunity for feeding. By examining these aspects of prey decision the hypothesis that anti-predator behavioral defenses represent a predation risk assessment which produces a favorable cost:benefit relationship to the prey will be tested. To test if the use of anti-predator behavioral defenses represents a predation risk assessment which produces a favorable cost:benefit relationship to the prey, the following two objectives will be address: 1. Determine if prey are capable of a qualitative and quantitative evaluation of predation risk. 2. Determine the costs of anti-predator behavioral responses by the prey.

To address if prey are capable of a qualitative and quantitative evaluation of predation risk, experiments will be conducted to determine whether a. prey distinguish between predators and abiotic factors, b. prey distinguish between predators and non-predators (herbivores), c. prey distinguish between predator species, and d. there is a relationship between predation risk and predator signals. To determine the costs of anti-predator behavioral responses by the prey experiments will conducted to a. evaluate if and how often caterpillars leave their host plant in the presence of predators, and b. evaluate the costs of dropping behavior and of hanging from a silk thread behavior when encountering a predator.


Influence of Plant Species and Habitat Fragmentation on the Dynamics of Inter-patch Movement of Insect Parasitoids

Many pest-management recommendations are based on assumptions about the movement and dispersal of invertebrate natural enemies. However, since very few studies have attempted to quantify natural enemies dispersal capabilities, these assumptions may be wrong. With the development of molecular an genetic techniques, studies on insect movement are becoming more feasible to perform and are providing additional information on population structure.

This study will focus on the movement of Diolcogaster facetosa ( Weed) (Hymenoptera: Braconidae), an important parasitoid of the agricultural pest green cloverworm ( Plathypena scabra (F.)) (Lepidoptera: Noctuidae). In this study the influence of interpatch distance, habitat fragmentation (the presence of a physical barrier, i.e., a forested area), and patch plant species composition on D. facetosa movement will be assessed. Microsatellite markers will be used to quantify population sub-structure and to give an overall interpretation of the extent of movement of D. facetosa .