Intricate combination

An ecosystem is the intricate combination of a community of organisms and their environment. My dictionary (the American Heritage Dictionary, Fourth Edition) defines a ‘system’ as “a group of interactive elements forming a complex whole,” and for an ecosystem, sometimes it’s not immediately obvious how the discrete elements interact.

Scientists have long studied the importance of trees and other vegetation growing on riverbanks, known collectively as riparian vegetation. (A quick Google Scholar search for that term returns more than 57,000 articles and books, the oldest of which were published in the late 1700s.)

“Such vegetation [has] a crucial role in stabilizing streambanks, moderating microclimates and shading streams, delivering litter and large wood to the river, providing habitat and food-web support for a wide range of terrestrial and aquatic animals, and supplying other ecosystem services,” write two scientists from Oregon State University who recently published a paper in the journal Ecohydrology.

Riparian vegetation is a fundamental part of many ecosystems; these researchers were interested in investigating the link between riparian cottonwood trees and the reintroduction of wolves to Yellowstone National Park. Wolves, historically part of the Yellowstone ecosystem, were eliminated from the park in the mid-1920s and reintroduced in the mid-1990s. During their absence, elk were released from the pressure of wolf predation and proliferated, and the consequences ricocheted throughout the ecosystem – the sprouts and seedlings of the plants that herbivores like elk eat, including cottonwoods, were unable to grow into mature trees.

Meandering stream and wide floodplain in Yellowstone National Park with almost no riparian trees. 

(Image by Emily Benson)

The scientists found that riparian cottonwood trees were rebounding in the part of a river valley where elk density had declined following wolf reintroduction, but not in a similar, nearby area with a large bison population; whereas cottonwoods more than five feet tall were absent from both locations prior to the reintroduction of wolves, by 2009 the researchers found about 380 cottonwoods over five feet tall in the section of the river valley with fewer elk, and only seven in the area teeming with bison. At the site where bison were plentiful, they write, “[t]he initial pattern of browsing suppression of young cottonwoods . . . by elk, which began several decades earlier when wolves were absent, is being continued by bison even though wolves are again present.”

Wolves, elk, bison, cottonwood trees, and streambanks are all part of the same ecosystem, their fortunes intertwined in an elaborate web; circumstances that affect one reverberate throughout that web to influence them all.

Bison in Yellowstone National Park.

(Image by Emily Benson)