Predators influence community composition and diversity through direct and indirect interactions with other species in their food webs. The specific interactions of predators within food webs often are difficult to trace because most of them are generalist feeders on prey of more than one trophic level. Stable isotope analysis (SIA) offers a more direct measure of what a predator is likely to be eating than simple control-treatment differences in field experiments. We compared SIA for enrichment of isotopes of carbon and nitrogen (δ13C; δ15N) in lab-reared praying mantids (Tenodera aridifolia sinensis Saussure) that were fed known diets of arthropods from the most important feeding guilds (leaf chewers, phloem feeders, and carnivores) to field-caught mantids over a growing season. Lab animals exhibited distinctly different isotopic signatures for prey of different feeding guilds, with both δ13C and δ15N generally increasing with successively higher trophic levels of prey. Field-caught mantids, by contrast, showed high variability in this regard, which is expected for this extreme generalist predator. However, δ15N showed a very definite pattern during growth and development of mantids, with eggs and adults having the highest values, decreasing from first to third instar as nymphs fed on lower trophic levels and increasing steadily thereafter, indicating a tendency to incorporate more carnivores (mainly spiders) in the diet as nymphs grew larger and matured.
1 Dept of Biology, Washington and Lee University, Lexington, VA; 2 Dept of Biology, Virginia Military Institute, Lexington, VA
Although red imported fire ants (Solanopsis invicta, Formicidae) have been shown to impact seed dispersal relationships between native ants and native myrmecochorous plants in complex ways, their effect on myrmecochorous invasive plants has not been studied. A possible mutualism between Solanopsis invicta and the myrmecochorous henbit (Lamium amplexicaule, Lamiaceae) in their newly shared ranges is presented. The mutualism may involve not only seed dispersal but also increased success of henbitnear fire antmound edges. Results from the following experiments are presented: seed preference trials, digitally imaged plots measuring density of mature plants in relation to the center of fire ant mounds, and a seed bank analysis comparing the number of Lamium seeds found at mound edges and away from mound edges.
63 • Clifton B. Ruehl1, David R. Chalcraft2, Heather Vance-Chalcraft2
A Test of Ratio-Dependent Predation
Ratio-dependent predation theory proposes that predator-prey interactions are determined by the ratio of prey to predator density rather than prey or predator density alone. We tested this theory with a factorial experiment that included four prey densities and two predator densities in outdoor mesocosms constructed to resemble freshwater ponds. Twenty, 50, 80, or 110 squirrel tree frogs (Hyla squirella) served as prey and one, or two giant water bugs (Belostoma flumineum) served as the predator. We found evidence of density dependent mortality for both prey and predator, but no evidence of an interaction that would suggest ratio dependent predation. Tadpole mortality was 68% when initial density was 20 compared to 47% and 48% when initial density was 80 and 110 tadpoles, respectively, resulting in roughly 36% greater mortality at lower tadpole density. A single Belostoma resulted in 50% tadpole mortality, while two Belostoma increased tadpole mortality to nearly 60%. These results do not support ratio-dependent predation theory. Rather, our findings suggest that multiple predators have non-additive effects on prey and independent of predator density prey experience reduced predation risk at increasing prey density. We propose that predators function independently of prey to produce community structure.
1 Dept of Biology, Columbus State University, Columbus, GA; 2 Dept of Biology, East Carolina University, Greenville, NC
64 • Amber Kincaid, James E. Russell
Population Genetic Analysis of Trichogramma kaykai Using Molecular Markers
Population structure and genetic diversity are interrelated factors that play a significant role in the evolution and ecology of all species. We investigated the population genetics of Trichogramma kaykai, an endoparasitoid wasp which parasitizes eggs of the butterfly Apodemia mormo in the Mojave Desert. Trichogrammatid wasps are known to exhibit phoretic transportation. This poses the question of whether T. kaykai breeding is limited to local geographical areas or the larger host range. To answer this, A. mormo eggs were collected, T. kaykai were cultured and DNA extracted for population genetic analysis using the mitochondrial cytochrome oxidase subunit one (CO1) gene region. If T. kaykai breeding is limited to a local geographical area, then a subdivided population genetic structure is expected. Additionally, the relationship between Wolbachia-infected and uninfected T. kaykai is of concern due to the potential for Wolbachia-induced mitochondrial selective sweep, and reduced mitochondrial genetic diversity. Mitochondrial selective sweep, associated with strict coinheritance of Wolbachia and mitochondrial genomes, would be suggested by the presence of a dominant maternal line among infected sequences. We hypothesized that the T. kaykai population structure is not subdivided. Additionally, we hypothesized that the characteristic reduction in genetic diversity observed during mitochondrial selective sweeps has not started in the T. kaykai population. Phylogenetic analysis supported these hypotheses, with a population structure characterized by mutations divided amongst geographical locations, and Wolbachia-infected sequences with diverse mitotypes. An unexpected observation was multiple Wolbachia-associated mitochondria descended from uninfected lineages, suggestive of Wolbachia horizontal transfer.
School of Science and Technology, Georgia Gwinnett College
