Part I – Introduction Isle Royale National Park, the largest island in Lake Superior, provides biologists with a fairly unique system for studying the interactions between diff erent trophic levels. Isle Royale has a rather simple food chain consisting of producers and a single large herbivore that in turn has only a single predator, the gray wolf (Canis lupus). Th e island had a rather large abundance of balsam fi r (Abies balsamea) until the park was colonized by moose (Alces alces) that swam to the island in the early 1900s. After the establishment of this large herbivore, the balsam fi r declined from 46% of the overstory in the 19th century to about 5% today. Nearby islands that are inaccessible to moose continue to have a large fi r component in their forests; thus the decline of the fi r on Isle Royale has been attributed to moose herbivory. Balsam fi r is not considered optimal forage for moose but it can comprise up to 59% of their winter diet. Over the last several decades, signifi cant temporal fl uctuations have been observed in the densities of the wolf and moose populations and the growth rates of balsam fi rs. Two hypotheses have been suggested to account for these fl uctuations. Th e primary productivity or “bottom up” hypothesis suggests that plant growth is limited by the energy available to plants, which is determined in turn by temperature and precipitation. Additional plant growth means more forage is available—thus herbivores, and ultimately carnivores, should increase in abundance. Alternatively, the trophic cascade or “top down” model predicts that changes in one trophic level are caused by opposite changes in the trophic level immediately above it. For example, a decrease in moose abundance should produce increased plant growth if moose herbivory limits plant growth. Changes in primary productivity would only have a discernible eff ect on vegetation if higher level interactions had been removed. Th e Isle Royale ecosystem provides us with a good opportunity to test the predictions of these alternative hypotheses. Longitudinal data are available for each of the key variables, including annual plant growth, NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE “Th e Wolf, the Moose, and the Fir Tree” by Gary M. Fortier Page 2 herbivore density, and carnivore density. Th e historical growth rates of balsam fi r have been determined through tree-ring analysis. When herbivores remove large quantities of the foliar biomass, annual wood accrual decreases and ring widths are reduced. Th us tree ring data allow us to estimate the intensity of herbivory over time. Moose and wolf populations have been censused for decades on Isle Royale, providing us with annual estimates of herbivore and carnivore densities. Long-term records are available for each trophic level in the Isle Royale ecosystem, providing the necessary data to evaluate both hypotheses. Questions 1. What type of correlation (positive or negative) would you expect to see between the population densities or growth rates of each trophic level in this system (fi r/moose/wolves) under the primary productivity hypothesis? 2. What type of correlations would you predict under the trophic cascade hypothesis? 3. What would you predict as the eff ect of wolf removal on plant growth under each hypothesis? 4. What assumptions are made regarding the measurement of growth rates in balsam fi r? Regarding the long-term impact of moose herbivory on balsam fi r? Do these assumptions seem warranted?

 

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