What do grizzly bears in Yellowstone National Park eat? Comedians are likely to answer: Whatever they want. But what they want—native Yellowstone cutthroat trout—are struggling, thanks to the illegal introduction of non-native lake trout in the 1980s by fishers. The bigger lake trout have been feasting on the cutthroat variety, and now there simply aren't enough of the latter for the bears, which dine on the cutthroats when they spawn in shallow waters. The lake trout don't provide an alternative food source, because they spawn at depths that are inaccessible to the bears. So, the grizzlies have increasingly turned to a different, but equally nutritious prey item: elk calves. These days, the bears spend far less time prowling the shores of Yellowstone Lake and streams for trout, and instead hunt the calves, as in the photo above. In a study this week in the Proceedings of the Royal Society B, researchers report that their simulations of elk demographics indicate that the grizzlies' dietary switch is a surprising factor in the park's elk population decline. Especially hard-hit are the migratory elk that spend summer around Yellowstone Lake and winter in valleys outside the park. The team modeled the bears' new diet, and they concluded that every year, the grizzlies are likely reducing the number of calves reaching maturity by 4% to 16% and the herds' overall population growth by 2% to 11%. Wolves, which were reintroduced to the park in 1995, have been blamed for the elks' dropping numbers by some scientists and ranchers around Yellowstone. But wolves, which also hunt adult elk, may play a smaller role than previously thought. The elks' troubles are more likely the fault of the bears, the lake trout, and ultimately the humans who introduced the fish to the ecosystem in the first place.
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