Discovering Life Under the River
Next time you walk or drive by one of the many gravel bed rivers that snake their way along valley bottoms throughout the Yellowstone to Yukon region, just know there’s far more there than meets the eye.
You’re not just seeing a river, but an “under river” too, and a biodiversity hotspot that links land- and water-based ecosystems throughout mountain regions everywhere.
At our Telephone Town Hall last month, ecologist Mark Hebblewhite summarized the ground-breaking work of Dr. Ric Hauer and others, explaining how gravel bed river ecosystems have as much going on under the surface as they do above, and how seemingly unrelated species, like grizzly bears and bull trout, are intimately connected here.
“River floodplain systems are the most dynamic ecosystems in the Rocky Mountains,” explains Hebblewhite. “What we see as the river is only the tip of the iceberg—it’s only 10 percent of the water that flows through the floodplain region.” Most of the water, he explains, is actually percolating underground, passing through the porous gravel layer that was deposited there and is shifting continuously over countless spring melts.
Hebblewhite calls it the “under river” (or Hyporheic zone in technical terms), and says these types of floodplains, found throughout the Yellowstone to Yukon region, act almost like a wetland, providing prime habitat above and below the ground up to a kilometer away from the river edges.
“All those spaces between the rocky gravel deposits underneath the river—that’s actually habitat for hundreds of species of aquatic insects,” says Hebblewhite. “When people think about biodiversity, they usually think about grizzly bears and wolves. But really, biodiversity is mostly found in places like these.”
As water passes through the gravel, sometimes staying underground for up to 20 days, the system acts like a gigantic thermostat, moderating the water temperature by keeping it cooler in summer and warmer in winter.
Those temperature differences are important for all kinds of species, he says. It keeps water cool enough in summer for cold-water fish like bull trout, and provides an upwelling of nutrients and a constant food source for the fish. Conversely, warmer water flowing beneath the surface allows plants to shoot up in spring along the water’s edge.
“That early growth is a critical food source for elk and other grazers, who feed on the green band of vegetation that stretches along floodplains,” says Hebblewhite. And with so many animals drawn to the region, carnivores like wolves and bears are sure to follow. Wolves, for example, stalk these regions on the lookout for prey in the form of foraging ungulates, but they also use the floodplain’s shifting sands to build their dens.
Hebblewhite refers to a satellite photo that shows densely packed wildlife movements along the floodplains (see figure below). “These areas are like arenas for all the kinds of predator-prey interactions,” says Hebblewhite. “Predation happens all over these landscapes, and it’s an important instance where nutrients from those kills flow back into aquatic ecosystems.”
Looking at these ecosystems, says Hebblewhite, it’s easy to see why grizzly bears are considered an umbrella species. “Grizzlies eat much of the same things that elk and other ungulates do, such as berries and various types of vegetation,” he explains, “but they also prey on ungulates and fish. There’s just an amazing convergence going on in these places and grizzlies are a major part of it.”
To hear Mark's full Telephone Town Hall lecture, you can download both the Town Hall presentation (pdf) and audio recording (mp3) below:
If you would like to hear more about this topic, listen to Mark Hebblewhite's radio interview on CBC Alberta.