The Wolf, the Moose, and the Fir Tree: A Case Study of Trophic Interactions

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ANSWER KEY “The Wolf, the Moose, and the Fir Tree: A Case Study of Trophic Interactions” by Gary M. Fortier Department of Small Animal Science Delaware Valley College Part I – Introductio... n Experimental manipulations can be quite difficult in systems containing large vertebrates. Consequently, long-term observations and correlational analysis are often used to assess interactions between different trophic levels. The limitations of this approach are well known; strong correlations may not be indicative of causation. The Isle Royale data constitute a widely recognized study of predator-prey interactions. The censusing of moose and wolf populations has been thoroughly documented, allowing the authors to infer causation based on the relative timing of events. The main goal of this study was to determine whether changes in primary productivity or herbivorecarnivore interactions are responsible for the suppression of plant growth in the Isle Royale National Park. Before reviewing any data, students should be able to predict the nature of any relationships between plants, herbivores, and predators. Questions 1–3 The primary productivity hypothesis suggests that there should be positive correlations between each trophic level; inverse correlations would be predicted under the trophic cascade model. Furthermore, the direction of control is different under each hypothesis. Plant growth may be controlled from the top by herbivory or from the bottom by the availability of resources. Thus the removal of a top predator, such as the gray wolf, would lead to increased moose density and decreased plant growth under the trophic cascade model. If plant growth is limited by primary productivity, wolf removal will have no effect on the growth of the firs. Question 4 If we use tree-ring analysis as an index of herbivore pressure, we are assuming that any reduction in ring width has been due to a loss of foliar biomass through predation. This will only be true if growth is not being limited by precipitation or climatic conditions. Regarding the historical impact of moose on the fir, the balsam fir exhibited a strong decline following the colonization of Isle Royale by moose early in this century. However, this only supports the inference that moose herbivory is a significant pressure on firs if no other major changes occurred on the island in the last eighty years. Other factors, in addition to predation pressure, might have accounted for the demise of balsam fir. We must assume that there were no major fires or storms, no significant new diseases or pests were introduced, no new competitors arrived, etc [Show More]

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