To eat or Be Eaten

This column was published in the Cranbrook Daily Townsman Tuesday September 7, 2010.

It’s that time of year again: men wear camo, wives are lonely and elk are nervous. Yes, elk season is open, this year with a twist. Anterless elk are in the sights in some parts of the Trench, a move to try to reduce their numbers in agricultural areas and convince them to hike for the high country come spring.

Ungulate migration has long been viewed as an evolutionary strategy to avoid predators and access good forage. For most animals, life is a trade-off: eat or be eaten. You can hide as long as you want, but eventually you’ll get hungry enough to venture out to that really tasty patch of grass. Biologists have tried to answer the question “when does predation risk outweigh foraging opportunities?” to identify a trigger point for migration. The answer isn’t easy.

Mark Hebblewhite is a Canadian biologist at the University of Montana who studiesthe movement of ungulates, especially elk in the Rocky Mountains. Recent work looked for differences between overlapping migratory and non-migratory elk populations in Banff National Park and the adjacent Ya Ha Tinda winter range.

In this system, wolves (elk’s main predator) are more numerous at low elevation, so the urge to migrate was driven by an understandable desire to avoid being eaten (especially strong for soon-to-be-moms). This strategy was effective: migration lowered predation risk by 70% compared to elk who chose to stay in the valley bottom. Freed from the threat of wolf predation, migratory elk’s movement decisions were based on finding quality feed. They were successful at that too, averaging 6% better forage than their non-migratory cousins. Better food was available to resident elk, but the high predation risk kept them from accessing it.

The question then is why would any elk in the Bow Valley choose not to migrate? Why stay and run a wolf gauntlet to access poorer food? Dr. Hebblewhite and his colleagues found that human activity “effectively decouples” the forage – predation risk trade-off. By selecting areas of high human activity where wolves fear to tread, resident elk lower their predation risk from 70% to 15% higher than migratory elk. The food around human activity may not be the best, but that’s more than compensated by volume: there’s lots of it.

Are there applications for this research to the resident vs migratory elk issue here in the East Kootenay? The Banff-Ya Ha Tinda situation is different. Here, elk predators (especially wolves) are in greater numbers at higher elevations. Lower predation rates in the Trench has likely helped drive the shift over the past 20 to 30 years into separate migratory and resident or homesteader elk populations, with the latter stubbornly refusing to leave the Trench in the summer.

Add in wide swaths of tasty alfalfa forage and one wonders why any elk would leave the Trench come spring. With elk now fenced out from many of those hay fields, if the predation risk side of the equation can be shifted to again favour the high country, perhaps elk will return to their migrations.

That predation is not likely to come from four-legged animals, unless we Trench-dwelling humans suddenly decide to co-exist with wolves and other predators. Increased hunting pressure is hoped to be a predation surrogate to help encourage elk to leave. This is part of the decision by the BC government to open an anterless elk season in the Trench. The hunt is based on sound science like Dr. Hebblewhite’s that increased predation risk may convince elk that the Trench is not such a safe place after all.