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

HANDOUT #2

Trophic System Data
The data below include census information for the moose and wolf populations, ring width indices from firs on each end of the island, and actual evapotranspiration rates (AET) from April to October. The east and west ends of the island differ substantially in terms of climate and flora. The west end consists of hardwood forests with a higher AET rate and warmer, earlier summers relative to the boreal forests in the east. The AET rate varies with temperature and rainfall and serves as an index of the amount of water available for plant growth. This rate is strongly tied to primary productivity.

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.
  1. Population size of wolves each winter (based on aerial counts).
  2. Population size of moose each winter (based on aerial counts and skeletal remains).
  3. Ring-widths from the west end of Isle Royale, N=8.
  4. Ring-widths from the east end of Isle Royale, N=8.
  5. 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.

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

Questions
  1. What is the purpose of each figure? Are there unclear terms or confusing aspects to any figure?
  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 firs from either end of the island (east/west) respond the same way to changes in moose density? How can you account for any observed differences?
  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?

GO TO Handout #1: The moose, the wolf and the fir tree
GO TO Handout #3: The moose, the wolf and the fir tree