The wolf the moose and the fir tree answers

Page 1“Th e Wolf, the Moose, and the Fir Tree” by Gary M. Fortier

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

by Gary M. Fortier Department of Small Animal Science Delaware Valley College

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

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

Page 2“Th e Wolf, the Moose, and the Fir Tree” by Gary M. Fortier

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?

Photo credits: Wolf (Canis lupus) by Gary Kramer, U.S. Fish and Wildlife Service, public domain. Swimming moose and hikers in Isle Royale National Park are from the National Park Service Digital Image Archives, public domain.

NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE

Page 3“Th e Wolf, the Moose, and the Fir Tree” by Gary M. Fortier

Part II – Trophic System Data Th e data in Figure 1 include census information for the moose and wolf populations, ring width indices from fi rs on each end of the island, and actual evapotranspiration rates (AET) from April to October. Th e east and west ends of the island diff er substantially in terms of climate and fl ora. Th e west end consists of hardwood forests with a higher AET rate and warmer, earlier summers relative to the boreal forests in the east. Th e AET rate varies with temperature and rainfall and serves as an index of the amount of water available for plant growth. Th is rate is strongly tied to primary productivity.

Questions 1. What is the purpose of each fi gure? Are there

unclear terms or confusing aspects to any fi gure? 2. How do the maxima and minima of the ring-width

indices correspond to changes in moose density? Does this support the primary productivity hypothesis, the trophic cascade hypothesis, or neither?

3. Do fi rs from either end of the island (east/west) respond the same way to changes in moose density? How can you account for any observed diff erences?

4. How do the maxima and minima of the wolves correspond to changes in moose density? How might you account for this relationship?

5. Which hypothesis is supported by the data on annual AET?

Fig. 1. Population parameters of the Isle Royale ecosystem from 1958– 1994. Shaded areas signify periods of forage suppression that may be connected to interactions between herbivores and carnivores.

A. Population size of wolves each winter (based on aerial counts).

B. Population size of moose each winter (based on aerial counts and skeletal remains).

C. Ring-widths from the west end of Isle Royale, N=8.

D. Ring-widths from the east end of Isle Royale, N=8.

E. Actual evapotranspiration rates (AET), annual calculations based on data from April–October at a weather station 20 km from Isle Royale. AET is an approximation of primary productivity, it represents water availability as a function of temperature and rainfall.

Credit: Regraphed from information published in Science 226 (December 2, 1994): 1557.

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Page 4“Th e Wolf, the Moose, and the Fir Tree” by Gary M. Fortier

•

Case copyright held by the National Center for Case Study Teaching in Science, University at Buff alo, State University of New York. Originally published October 6, 1999. Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.

Part III – Ring Width Indices Th e local topographies for the two samples depicted below are substantially diff erent. Th e chronologies in fi gure A are from an east-end subsample of trees designated RH (Rock Harbor). Th is area contained an open-canopy section of previously disturbed boreal forest; it exhibits an increase in growth rates after a period of high wolf predation in the late 1970s. Figure B depicts a west-end subsample designated SS (Siskiwit Swamp). Th ese fi rs are in a closed-canopy hardwood forest that has been heavily browsed by moose for some time.

Questions 1. Are there any confusing aspects to the fi gures or caption above? 2. Th e moose population peaked in the mid 1970s and then declined over the next decade. How did the trees at

each site respond in the years following the peak? Are the results for these samples surprising given the larger data sets for tree ring-width on the previous page?

3. How should the diff erence in canopy cover aff ect growth rates? How will the height of the trees at each site aff ect their response to changes in primary productivity? Th e authors suggest that primary productivity was increasing during the late 1970s and most of the 1980s—does either ring-width index appear to refl ect that change?

4. Which hypothesis do you feel is best supported by the ring-width chronologies above? 5. What fi nal conclusions can you draw about the interactions between each trophic level on Isle Royale? Is control

exerted from the top down, as suggested by the trophic cascade model, or are interactions between trophic levels ultimately controlled by primary productivity?

6. Design an experiment that would allow you to clarify any ambiguities from Figures 1 or 2. Why might an experimental approach prove advantageous in this situation?

Fig. 2. Ring-widths of balsam fi rs from Isle Royale. Each line represents data from an individual tree harvested in 1992. Note that moose are able to browse as high as 3m.

A. Location RH (N=10), fi rs from this area were 26–48 years old and exceeded 3m in height during the late 1970s.

B. Location SS (N=9), fi rs from this area were 48–60 years old and were less than 2m in height.

Credit: Regraphed from information published in Science 226 (December 2, 1994): 1557.

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