The Wolf, the Moose, and the Fir Tree:
Who controls whom on Isle Royale?
A case study of trophic interactions

HANDOUT #3

Ring Width Indices
The local topographies for the two samples depicted below are substantially different. The chronologies in figure A are from an east-end subsample of trees designated RH (Rock Harbor). This 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). These firs are in a closed-canopy hardwood forest that has been heavily browsed by moose for some time.

Fig. 2. Ring-widths of balsam firs 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.
  1. Location RH (N=10), firs from this area were 26-48 years old and exceeded 3m in height during the late 1970s.
  2. Location SS (N=9), firs from this area were 48-60 years old and were less than 2m in height.

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

Questions
  1. Are there any confusing aspects to the figures or caption above?
  2. The 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 difference in canopy cover affect growth rates? How will the height of the trees at each site affect their response to changes in primary productivity? The authors suggest that primary productivity was increasing during the late 1970s and most of the 1980s--does either ring-width index appear to reflect that change?
  4. Which hypothesis do you feel is best supported by the ring-width chronologies above?
  5. What final 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?

GO TO Handout #1: The moose, the wolf and the fir tree
GO TO Handout #2: The moose, the wolf and the fir tree