Of Wolves and Pike

This doesn’t fit any of the subjects of this blog. Still, I thought people might find it interesting.This is an article that I wrote for an on-line magazine, back in 1999, about some observations I made while working at Isle Royale National Park back in 1995-6.

No Wolves = Skinny Pike
By Tim Roettiger

Those attracted to this brief essay by the title are no doubt asking themselves: "What have wolves to do with northern pike?" As I found out for myself, under proper conditions, quite a bit. I observed the end result of the indirect interaction of these two species’ populations while conducting lake surveys at Isle Royale National Park. It is a classic example of cascading trophic interactions that cross the land-water interface.

This phenomenon was first noticed at Sargent Lake. I was part of a crew conducting surveys of backcountry lakes. Gillnets, seines, and minnow traps were used for these surveys. During the course of the survey, only a few small pike were caught. The most notable thing about these pike was that their stomachs did not contain the usual pike table fare. Instead, their stomachs contained leeches, insects, and the occasional rock. This seemed to indicate a lack of suitable forage.

Seining proved this was not the case. In the evening, large schools of shiners, prime forage for pike, could be seen cruising the shallows. It was no problem to seine in hundreds at a time. So the forage was there, but the pike would not, or could not, make use of it.

So the question was: Why aren’t the pike making use of an apparently substantial forage base? This is where moose enter the picture. The pike is an ambush predator. They are most efficient at a medium density of aquatic vegetation. Too much vegetation interferes with their foraging. Too little vegetation and they do not have effective ambush cover. The problem in Sargent Lake was a near total lack of aquatic vegetation. The culprit: moose.

Moose love aquatic vegetation, an important source of sodium for them, and can forage effectively to a depth of about 9 feet, most of a lake’s littoral zone. Parvovirus had killed many wolves, reducing predatory pressure on the moose. At the time of the Sargent Lake survey, the wolves numbered only about 14 and the moose population was the most dense in the world. This put the standing crop of aquatic vegetation, which had remained about the same, under intense browsing pressure. That is how Sargent Lake came to be devoid of aquatic vegetation and, subsequently, the local pike became leech-eaters.

A sequence of reactions such as this, parvovirus-to-wolf-to-moose-to-aquatic vegetation-to-pike, is known as a "cascading trophic interaction". What this means is that a disturbance at one trophic level (in this case a top predator, wolves) will pass (cascade) from level to level (organism to organism) causing an often seemingly unrelated end result. The pike of Sargent Lake are a good example of a cascading trophic interaction that crossed the land-water interface. After all, who would have expected that wolves dying of a viral infection would result in poorly conditioned pike?

The Sargent Lake situation arose in large part due to Isle Royale’s unique island ecosystem. No more wolves could come or moose leave, it is a closed system. What happened at Sargent Lake is unlikely to occur elsewhere. However, it does stand as a powerful example of the inter-connectedness of all parts of our ecosystem. It is food for thought. Next time you find yourself complaining about bullheads, or crows, or even mosquitoes, think what might happen if they were gone.