Surging bison populations in Yellowstone National Park have come with 'a major ecological cost,' researchers say, causing rivers to run dry and the park's tree cover to decline

A surging bison population in Yellowstone National Park is doing serious and steady damage to the park's biological diversity. A team of researchers from Oregon State University, Corvallis studied the animal's effects on the park's landscape, focusing primarily on the Lamar Valley, an area on the park's eastern edge. The team analyzed data spanning more than 100 years, including population figures for a range of species as well as historic documents and photos detailing the evolution of specific parts of the valley. The team concluded that the steady increase in bison numbers in the park had come 'at a major ecological cost,' according to a report in Eurekalert. 'Even to a casual observer there are clear indicators of highly altered ecological conditions across the Lamar Valley, including a high density of bison trails, wallows and scat,' study co-author Bob Beschta said. 'High bison numbers have been an effective agent for accelerating the biological and physical modification of the valley's seeps, wetlands, floodplains, riparian areas and channels, trends that had begun decades earlier by elk.' There are currently over 4,000 bison in Yellowstone, an all-time high for the park, the consequence of congress demanding park officials stop seasonal culling of the animal in 1968, a time when the bison population had been limited to around 100. Large bison populations have had a range of damaging effects on the environment, including heavy grazing and trampling that kills off woody vegetation--something that's intensified by the park's similarly surging elk populations, which nibble on young tree leaves and strip them of their bark. The valley has seen a 99 percent reduction in aspen tree cover, dropping from 7.5 hectares in 1954 to just one-tenth of a hectare in 2015. The heavy trampling has also steadily caused riverside, or riparian, vegetation to die off as the heavy hoof falls widen the shallow streams and rivulets across the valley floor and either disperse them or dry them out entirely...MORE