Wolves, Moose, and Fir Trees:Who Controls Whom on Isle Royale?

A case study of trophic interactions

Based on research published in Science 266:1555, December 1994

Isle Royale

LakeSuperior

approximately 35 miles by 5 miles

Balsam fir trees are common on the islands of Lake Superior

Moose first came to Isle Royale around 1900, crossing the frozen lake (~12 miles)

population boomed to about 2,000 by 1950s (exponential growth!)

Balsam fir trees are common on the islands of Lake Superior

Moose first came to Isle Royale around 1900, crossing the frozen lake (~12 miles)

population boomed to about 2,000 by 1950s (exponential growth!)

Wolves first came to Isle Royale around 1950

Balsam fir trees are common on the islands of Lake Superior

Moose first came to Isle Royale around 1900

population boomed to about 2,000 by 1950s

Wolves first came to Isle Royale around 1950

Fairly simple trophic web

Top-Down or Bottom-Up Control?

Lots of data are available for Isle Royale

• Balsam fir growth over time (tree rings)

Lots of data are available for Isle Royale

• Balsam fir growth over time (tree rings)

• Moose population density over time (census data)

• Wolf population density over time (census data)

If this community shows bottom-up control, what correlations should we see between population densities and growth rates?

If this community shows top-down control, what correlations should we see between population densities and growth rates?

If this community shows bottom-up control, what correlations should we see between population densities and growth rates?

Positive correlations: more fir trees should lead to more moosemore moose should lead to more wolves

If this community shows top-down control, what correlations should we see between population densities and growth rates?

If this community shows bottom-up control, what correlations should we see between population densities and growth rates?

Positive correlations: more fir trees should lead to more moosemore moose should lead to more wolves

If this community shows top-down control, what correlations should we see between population densities and growth rates?

Negative (inverse) correlations:more wolves should lead to fewer moosemore moose should lead to fewer fir trees

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

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

Tree ring-widths, N=8.

Population parameters of the Isle Royale ecosystem from 1958-1994.

Do these data support a top-down or bottom-up model?

Population parameters of the Isle Royale ecosystem from 1958-1994.

Do these data support a top-down or bottom-up model?

Supports a top-down model (does not prove!)

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

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

Tree ring-widths, N=8.

Historic Data

Moose only arrived on Isle Royale around 1900.

How did that arrival affect Balsam Fir population density?

1848: Balsam firs were the dominant species (~46% of trees)1978: Balsam firs were ~13%1994: Balsam firs were ~5%

On nearby islands, balsam fir is still the dominant species.

The data are most consistent with top-down control of this

community on Isle Royale.