Sunday, May 06, 2007 10:14 AM
We present another lecture of our series from the recent Forests to Combat Desertification Conference, summarizing the effects of livestock grazing
The first talk was presented by Dr. Yagil Osem of Israel’s Volcani Center, about the effects of grazing on ecosystem structure in semiarid rangelands in the Negev. The areas under research were the Lehavim region, near Beersheba and KKL-JNF Lahav Forest. This area is grazed by Bedouin sheep from February through August. Soil productivity depends on location, so wadi shoulders are more fertile and can be compared to sub-humid regions, unlike the hill slopes, which have a sparser herbaceous cover. Soil productivity also depends on seasonal fluctuations such as whether the year is rainy or dry. Annual precipitation averages 300 millimeters of rain, with a dry year of less than 75 mms occurring once every five years, roughly.
For research purposes, grazed and un-grazed plots (enclosures) were compared on different topographical sites (wadi shoulders, slopes and hill tops). In general, a correlation was found between soil productivity and the effects of grazing. The less productive the soil, the more affected it is by grazing. There was less difference between the grazed and un-grazed plots in the wadi, where the soil is more productive. The opposite held true of the slopes and hilltops, where the difference between grazed and un-grazed land was substantially greater. This information is essential for decision-making on management policy about location and quantity of permissible grazing.
Furthermore, it was discovered that recovery after overgrazing was surprisingly swift, as evidenced in the plots in which grazing was prevented. The ecosystem is apparently highly resilient and able to regenerate its natural biodiversity.
Photo:KKL-JNF Photo Archive
Aspects of Overgrazing around the Globe
Dr. Monica Betiller spoke about vegetation dynamics in ecosystems of Patagonia, Argentina, where sheep have been the source of livelihood for many generations. There too, sheep grazing, trampling and wind erosion create gaps in plant cover- the more grazing, the greater the gaps. The addition of (1) organic matter, (2) nitrogen and (3) seeds to soil or (4) planting target species in rainy seasons may help to restore the more degraded vegetation states. It is important to intervene before soil degradation is too severe.
Professor M. Keith Owens spoke about the effects of livestock grazing on rangelands in southern Texas. This area was formerly grasslands and is now mixed woodland. Excessive grazing over a two hundred year period has led to sparse herbaceous cover, resulting in what is called a “leaking ecosystem,” since it is leaking soil and water. It was found that cattle, unlike deer, consume and redistribute seeds. Professor Owens recommended (1) creating patches large enough to sustain diversity; and (2) moving livestock from place to place, dependant on the degree of patch disturbance. (3) Supplemental feeding and (4) land treatment, allows us to direct livestock to different sites, which may moderate damage caused by overgrazing on a long-term basis.
Dr. Concepcion L. Alados showed how in Spain, the recent abandonment of grazing is having a negative effect on rangeland ecosystems and is reducing seed dispersal. Ecosystems adapt to change, but sudden changes could cause them to collapse. It is therefore important to analyze how these systems self-organize and develop so that we can manage lands that have been subjected to both human-induced and natural changes.
Peru is the place where Dr. Avi Perevolotsky chose to look at goats, trees and the overgrazing-desertification controversy. Northern Peru was originally covered by tropical forest, but climatic change turned it into arid land. The locals are goat-herders, with flocks of 50-200 goats per family. These animals, which supply soft and hard cheeses and meat, provide for most of the families’ needs. Dr. Perevolotsky arrived in Peru during a drought year, but the local residents assured him that during a rainy year, he could ride a horse through the forage without being noticed. In fact, one year after the drought, there was tremendous productivity and the land was unrecognizable. There seems to be a natural balance between grazing needs and local ecosystems. If forests are planted in a region such as this, they would be used for timber and ultimately take more from nature than they would give. It is true that overgrazing can lead to desertification, but we should learn to distinguish between overgrazing and over-utilization. This issue can become more of a socio-cultural question than a scientific one. Human systems are very close to natural systems, and we must take human needs into consideration when defining the goals of ecosystem management.
The discussion on livestock grazing and desertification was summed up by Dr. Marcelo Sternberg of Tel Aviv University. He pointed out that the livestock industry accounts for 40% of agricultural produce and employs approximately 1.3 billion people. Grazing areas take up about 30% of the earth’s surface in a world that is becoming increasingly crowded (there is a daily population increase of 203,000). When land is converted to pasture, there is a loss of biodiversity and soils unprotected by vegetation become subjected to erosion by water and wind, trampling by the grazing animals and salination by evaporation. As much as 7% of the Amazon tropical rain forest, for example, has been converted from forest to pasture and feed crops. Livestock are also responsible for 18% of greenhouse gas emissions. On the optimistic side, processes of desertification caused by overgrazing are reversible. The key word is management, that is, managing soil and livestock and ensuring that vegetation maintains its biodiversity. We need to adopt a policy of “adaptive management,” or in other words, "learning by doing."