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 Lake Superior 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 moose more 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 moose more 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 moose more moose should lead to fewer fir trees

Population parameters of the Isle Royale ecosystem from 1958-1994. 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. Do these data support a top-down or bottom-up model?

Population parameters of the Isle Royale ecosystem from 1958-1994. 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. Do these data support a top-down or bottom-up model? Supports a top-down model (does not prove!)

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.