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.
New research from a team at the University of Oregon, Corvalis argues that the more than 4,000 bison that currently live in Yellowstone National Park are causing major environmental damage
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 team focused their research on the Lamar Valley in Yellowstone’s eastern regions
The bison were originally culled by park officials to keep their numbers to around 100, but after congress prohibited the practice in 1968, the animal’s population began surging, along with elk, both of which wear heavily on trees and smaller woody vegetation
The heavy trampling and grazing of bisons has helped disperse many of the valley’s shallow rivers, causing them to dry out and slowly killing off the surrounding vegetation
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.
After nearly going extinct in the 19th century, bison found refuge in Yellowstone, where in 1901 there were just 22.
By 1925, that figure had grown to 750, prompting park management to begin culling the animals to preserve a balance with other species in the park.
99 percent of the aspen cover in the valley was eliminated between 1954 and 2015, dropping from 7.5 hectares to just one-tenth of a hectare, due to the combined effects of elk and bison
In order to restore the region’s biological diversity, the team argue, the ‘ongoing environmental effects of bison would have to be significantly reduced’
Without culling, the bison population grew to over 1,000 in 1988, and is currently estimated to be over 4,000, an all-time high for the region.
‘The ongoing environmental effects of bison would have to be significantly reduced in order to restore biologically diverse communities dominated by willows, cottonwoods and aspen,’ Beschta said.
‘As park administrators make management decisions that affect ungulate densities and distributions in Yellowstone, as well as those in other parks and reserves with high ungulate densities, our findings indicate a need to take into account the often wide range of ecological effects that abundant large herbivores can have on terrestrial and aquatic ecosystems.’
