In southeastern Oregon, a natural experiment has been quietly taking place over nearly a quarter of a century. In 1991, cattle grazing became prohibited in the Hart Mountain National Antelope Refuge. Now, twenty-four years later, researchers are attempting to understand how the cessation of grazing has allowed the wetland ecosystems to recover.

Livestock grazing is permitted on a million square kilometers of public land in the western US, which includes nearly 80% of land managed by the Bureau of Land Management and 60% of US Forest Service lands. But that land isn’t uniformly grazed; cattle tend to prefer riparian areas, thanks to the easy access to water and the abundance of plants to eat. While grazing is a good way to manage land – that’s the rationale for allowing them there in the first place – overgrazing by cattle can lead to changes in hydrology, plant and animal community composition, and soil characteristics.

As cattle remove shrubs, bird populations, which rely on those plants for habitat, decline. As water depth and bank stability are altered, fish populations can also decrease, a trend that’s been especially apparent for salmon species and their relatives. As riverbanks get trampled, streams become more highly sedimented, water becomes muddier, and thanks to feces, it also becomes more polluted. There’s also some evidence that ecosystems that are stressed by intensive grazing are less able to adapt to the temperature and moisture challenges imposed by climate change.

Given the myriad ways in which grazing can degrade an ecosystem, the US Fish and Wildlife Service banished all cattle from the Hart Mountain National Antelope Refuge. By contrast with active restoration (which involves plantings or explicitly adjusting stream channels), this is a sort of passive restoration attempt. Twelve years after the cattle were sent packing, the plant communities in the refuge’s riparian areas changed dramatically, reverting to a state closer to what was present prior to grazing. Bird abundance increased by 33%. Now, after nearly 25 years, Oregon State University Department of Forest Ecosystems and Society researcher Jonathan L. Batchelor and colleagues are continuing to monitor the recovery of the wetland habitat. And they’re doing it with photographs.

Photos of riparian areas taken between 1984 and 1991 within the 1,095 square kilometer refuge, which was initially designated to protect pronghorn antelope, were collected from the refuge’s archive. Sites for 110 of those photos were relocated and photographed again in 2013 and 2014. When possible, the photos were re-taken in the same season as the historical photo. Ultimately, they were left with sixty-four photo pairs, one historical (during grazing) and one present-day (post-grazing). Instead of relying solely on qualitative descriptions of features of the landscape, the researchers also extracted quantitative measurements, such as stream width or the height of the tallest five willow trees.

By comparing the new photos with the historical ones, the researchers determined that following 23 years of passive recovery after cattle were excluded from the Hart Mountain National Antelope Refuge, stream channels narrowed, woody vegetarian increased, and there was a noticeable reduction in eroding stream banks. Nearly all sites displayed a decrease in bare soil, resulting in an overall 90% increase in plant cover, mainly thanks to grasses, sedges, forbs, and willow. Willow and rush cover increased fourfold. Batchelor verified that the changes could not be attributed to climate-related changes more generally, especially since the landscape has been more stressed by drought recently compared with when the historical photos were taken.

They also discovered that in some sites, passive restoration was sufficient for full recovery; active practices such as prescribed burns and the planting of willow were not necessary after the removal of cattle. “The extent to which passive restoration has facilitated vegetation recovery at Hart Mountain speaks to the capacity for riparian systems to recover without further intervention,” writes Batchelor. However, the absence of willows at many sites suggests that the ecosystem is still in the very early stages of recovery; continued vigilance over long time scales will be critical to ensure the continued recovery of the area’s landscapes.

Assuming that the recovery continues unimpeded, the researchers expect increasing biodiversity across all taxa: birds, mammals, reptiles, amphibians, fish, and invertebrates. The conclusion is clear: “Simply removing cattle from areas may be all that is required to restore many degraded riparian areas in the American West.” – Jason G. Goldman | 04 March 2015

Source: Batchelor J.L., William J. Ripple, Todd M. Wilson & Luke E. Painter (2015). Restoration of Riparian Areas Following the Removal of Cattle in the Northwestern Great Basin, Environmental Management, DOI: 10.1007/s00267-014-0436-2

Header image: Pronghorn antelope at Hart Mountain National Antelope Refuge via Flickr/wild trees.

