Migration used to be a good strategy for animals to find the best food, but a recent study appears to show that climate change and human interference favor animals that stay put.
A scientific study, published today in the journal Ecology, tracked the Clarks Fork elk herd for more than two decades as it migrated between central Wyoming and Yellowstone National Park, finding that the herd’s numbers have dwindled since 1989.
The cause appears to be lowered reproduction caused by climate change and human and predator pressures.
Biologists have hypothesized that migration was advantageous in natural systems, allowing herbivores to follow the spring green-up as it works its way into higher elevations. Thus animals can maximize the amount of high-nutrient grass they eat, especially after a long winter.
But that’s been hard to prove because, in recent decades, manmade barriers such as fences or roads have hindered migrating herds. In the case of bison, hazing hampers migration.
The Clarks Fork elk herd is one of the few that still travels old routes across mostly open territory, wintering in central Wyoming and migrating to the park for summer. But even without barriers, migration doesn’t mean success.
Scientists from the Wyoming Game and Fish Department and the University of Wyoming compared the Clarks Fork herd to one that stays in central Wyoming and found the Clarks Fork herd produces fewer young. Between 1989 and 2009, the production of calves declined by 73 percent.
No historic population numbers were provided, so 1989 may have been an artificial peak.
The decrease is explained partly by the fact that only 70 percent of the Clarks Fork cows would become pregnant while almost 90 percent of cows in resident herds became pregnant.
Using satellite imagery, scientists measured the amount of vegetation in grazing areas over time and concluded that long-term drought has made it harder for migrating elk to find good forage, so females don’t have enough energy to produce calves.
Meanwhile resident elk take advantage of irrigated farm fields so they have more to eat.
They also found that the green-up has started occurring earlier, before the cows calve. So the cows aren’t able to maximize their nutrition intake while they’re nursing their calves.
Predators also play a part in the equation. The study didn’t measure predation rates but cited corresponding trends in predator populations.
Few wolves live in central Wyoming. The ones that end up there are often shot. So wolves don’t affect resident herds. The combination of irrigated lands and no predators has led to population increases in central Wyoming.
But the Clark Fork cows drop their calves in central Wyoming and then migrate to Yellowstone, where wolves, grizzly bears, black bears and mountain lions await.
During the two decades of the study, the grizzly bear population increased fourfold. Wolves didn’t start having an effect until around 2000, but then their numbers climbed four times higher also.
A similar situation exists for migratory and resident elk herds near Banff, Canada. There, scientists found that resident herds suffered lower predation because wolves avoided more urban habitat. Wolves killed far more elk that migrated into the wilderness.
The authors concluded that the combination of predation, human barriers and drought from climate change would challenge the conservation of migratory herds.