Overgrazing of livestock leads to many adverse effects on the ecosystem—this is a well-established fact. Just by staying in the loop of education, science, and the news, citizens of the world have heard and believed this harsh reality, one of many not-so-positive human footprints on planet Earth. But think about it: has this ever been proved in a concrete, experimental manner? Just like millions of other Americans, the answer is no, and there are no precise data-backed examples we can whip out to prove this stark grain of truth. This is not to say that this fact is not valid—it is—but it is quite striking that more studies to focus on experimental analysis of the effects of livestock grazing on already established ecosystems have not been done.
Realizing that such a void exists in the scientific field, Susan Earnst of the U.S. Geological Survey and her colleagues David Dobkin, of the High Desert Ecological Research Institute, and Jennifer Ballard, of the Hart Mountain National Antelope Refuge, set out to fill it. Earnst decided to focus on the Northwestern Great Basin, an area of the United States that has a long past dealing with livestock grazing, shedding light on a tangible long-term trend of the relations between livestock removal and the subsequent recovery of the plant and avian species.
The Northwestern Great Basin of southeastern Oregon is what ecologists call riparian woodland, a forest that is generally abundant in deciduous trees, specifically aspen, and is found in only small parts of the United States, yet is home to a miraculous diversity of bird and plant life not found in neighboring habitats. Such riparian woodland has been drastically altered due to human land-use as well as due to changes in fire regimes, drought, disease, insect infestations, and the rise in dominance of certain plant species.
What constitutes land-use in Earnst’s study? It is cattle grazing on the majority of the federal area. Cattle cause, in addition to direct effects, many indirect effects such as soil compaction, bank instability, greater depth of ground water, and a decrease in stream invertebrate species. Specifically, the effects of the cattle do not allow proper regeneration of aspen, leading to two main consequences: the decrease of ground- and shrub-nesting birds and the decrease of plant species.
In 1990, cattle and other livestock were removed from this area which had been a graze land for more than 120 years. The area, divided systematically into 26 plots, was monitored by taking measurements of ground cover, shrub cover, and aspen stem densities, and all bird species of both the nesting and foraging types found by sight or sound in 25 minutes at the same time in the center of each plot at similar periods of the year. This was done in two phases, years 1 to 3 (phase one) and years 10 to 12 (phase 2).
What did Earnst find? She found overwhelming evidence showing significant regeneration of aspen and that the riparian forest plots had twice the bird species compared to neighboring similar snow-pocket aspen forests. After the 12 years of cattle removal, there was a 33% increase in the amount of birds in the riparian areas—with ground-nesting birds increasing the most. Earnst was thus successful in establish one of first studies that quantifies recovery of a habitat after removal of livestock, and provided staunch proof to support the fact that livestock grazing can cause long-term harm to areas. She found that the increase of birds in the area was significant and widespread, while the increase in shrub cover around the dense aspen shoots and smaller regenerated trees created a stout under-story of previously-found native species. The intense density of the shoots is consistent with the idea that this was an immediate response of the species following the lack of cattle in the area. The canopies of the forests were also able to become more layered over the 12-year period. There were some limitations to the study, however. Earnst also found that some species, such as bark-gleaning birds, had no change in their numbers after cattle removal.
Overall, Earnst was able to show that there were a large movement towards the restoration of biodiversity following livestock removal—a positive impact on the whole ecosystem, at the plant, herbivore, and carnivore level. Such studies can provide stimulus for actions by federal agencies to control livestock management as to reduce the negative ecological impacts and improve the resilience of trees to natural processes such as drought. This way, a mutual balance can be created between man and environment.
By: Shachi Daru
Source article: EARNST, S. L., DOBKIN, D. S. and BALLARD, J. A. (2012), Changes in Avian and Plant Communities of Aspen Woodlands over 12 Years after Livestock Removal in the Northwestern Great Basin. Conservation Biology, 26: 862–872.