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 – Introduction
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 herbivore-
carnivore 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. We need more information before
This study source was downloaded by 100000858936669 from CourseHero.com on 01-23-2023 01:32:56 GMT -06:00
https://www.coursehero.com/file/47912940/The-Wolf-the-Moose-and-the-Fir-Tree-A-Case-Study-of-Trophic-Interactions/
, we can confirm or reject these assumptions.
Part II – Trophic System Data
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
This study source was downloaded by 100000858936669 from CourseHero.com on 01-23-2023 01:32:56 GMT -06:00
https://www.coursehero.com/file/47912940/The-Wolf-the-Moose-and-the-Fir-Tree-A-Case-Study-of-Trophic-Interactions/
“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 – Introduction
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 herbivore-
carnivore 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. We need more information before
This study source was downloaded by 100000858936669 from CourseHero.com on 01-23-2023 01:32:56 GMT -06:00
https://www.coursehero.com/file/47912940/The-Wolf-the-Moose-and-the-Fir-Tree-A-Case-Study-of-Trophic-Interactions/
, we can confirm or reject these assumptions.
Part II – Trophic System Data
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
This study source was downloaded by 100000858936669 from CourseHero.com on 01-23-2023 01:32:56 GMT -06:00
https://www.coursehero.com/file/47912940/The-Wolf-the-Moose-and-the-Fir-Tree-A-Case-Study-of-Trophic-Interactions/
