The goal of this dissertation was to investigate some of the factors that determine species diversity across trophic levels at both broad and local scales. The taxa used to address these questions were grass and forb dwelling arthropods, mainly insects, found at experimental sites. The overriding question guiding this research is; what are the processes that determine arthropod diversity at local and regional scales? At local scales, I investigated trophic level diversity using correlational studies, measuring the natural variation in two grassland ecosystems. Results from this study support hypotheses from other empirical studies demonstrating that changes in diversity of a single trophic level can cascade to effect diversity at other, nonadjacent trophic levels. These studies were supplemented with a long-term manipulative experiment using the same ecosystems, in which I manipulated plant resources and higher trophic levels. After three years, and one year after the last experimental manipulations were applied, the plant resources manipulation (addition of fertilizers) tripled the average number of herbivore individuals and decreased herbivore diversity. Also at the local scale of diversity interactions, I conducted additional experiments measuring trophic level diversity, using the forage crop alfalfa (Medicago sativa ) and the associated arthropods as a model system. In this study I again manipulated plant resources and higher trophic levels and found significant positive associations between plant resources, plant defense, and herbivore diversity. Additionally, predator diversity and abundance were consistently associated with higher levels of herbivore diversity. This corroborates previous studies in these agricultural ecosystems demonstrating complex indirect trophic level interactions To examine larger scale patterns of diversity, I investigated regional and latitudinal gradients in arthropod abundance and diversity. Regionally, I conducted a study using alfalfa agroecosystems again and investigated the effects of climatic variation on the diversity of arthropod trophic groups across a large geographical area, from Colorado to Southern Arizona. I found that parasitoids were most susceptible to variations in climate, while herbivores and other enemies were more resilient to local climate, also in agreement with other studies demonstrating the differential susceptibility of natural enemies to climatic variation Finally, I investigated arthropod diversity along a latitudinal gradient from northern Colorado (N 40°30') to Ecuador (S 0°42'), using identical methods along the gradient and controlling some aspects of the type of community sampled (open fields versus forest edges). Arthropod species richness increased from the temperate latitudes to tropical regions, this however was not reflected in arthropod family richness or abundance. In addition, I found that controlling for community type across latitudinal gradients explained little of the variation in arthropod diversity. These results suggest that abiotic diversity-promoting mechanisms have a stronger influence on arthropod diversity than do biotic differences between community types. Results from the combined research that comprised my dissertation are the most thorough demonstration of 'diversity cascades' (indirect effects of biotic interactions on biodiversity) demonstrated to date. This is relevant to understanding a major global challenge: loss of biodiversity. Similarly, the consistent effect of plant resources on plant and animal abundance and diversity is relevant to the ubiquitous anthropogenic input of elements such as nitrogen and phosphorus. The results here are consistent with the hypothesis that as overall diversity increases, natural enemies have stronger effects on biotic communities, while less diverse communities are dominated by the effects of plant resource. (Abstract shortened by UMI.) / acase@tulane.edu
| Identifer | oai:union.ndltd.org:TULANE/oai:http://digitallibrary.tulane.edu/:tulane_25410 |
| Date | January 2008 |
| Contributors | Pearson, Clark Vincent (Author), Dyer, Lee A (Thesis advisor) |
| Publisher | Tulane University |
| Source Sets | Tulane University |
| Language | English |
| Detected Language | English |
| Rights | Access requires a license to the Dissertations and Theses (ProQuest) database., Copyright is in accordance with U.S. Copyright law |
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