Friday, October 30, 2009
DOES GRAZING BENEFIT GRASSES?
One of the most persistent myths perpetuated by the livestock industry and its proponents is the assertion that grasslands benefit from herb ivory. Actually this is not a new issue since it has been contested for decades in the ecological literature and largely resolved with most botanists and other ecologists concluding that the purported “benefits” from herb ivory—must be placed within a contextual framework that sees cropping of plants by animals and plant adaptations as largely an antagonistic response. Nevertheless, this has not stopped livestock proponent (Knight et al 2001) from repeating the same argument that livestock grazing benefits grasslands. Some (Savory 1999) even go further suggesting that if ungrazed, grasslands may deteriorate or become “degraded.” Over-rest, not overgrazing, they warn, is the problem (Savory 1999, Knight et al. 2001) in much of the West. Such assertions, of course, conveniently legitimize the exploitation of rangelands by the livestock industry and deserve careful scrutiny from conservationists.
CONFUSING TOLERANCE AND NEED
Grasses and large herbivores have co-existed for millions of year. In many ecosystems large mammal herbivores consume between 20-50% of the annual net primary productivity (Detling 1988). Over that time plants evolved mechanisms that allow them to survive, and even flourish with grazing animals. Grazed plants tend to have higher nitrogen levels (because they have a larger percentage of rapidly growing tissue), that are more palatable and often more nutritious to herbivores. Nevertheless, claims that grass require, rather than merely tolerate herb ivory demonstrate confusion between “tolerance” and “need.” Many grassland plants have developed mechanisms that permit them to persevere in spite of harm from herbivory. The ability to tolerate adverse impacts, however, should not be interpreted as evidence of “need.”
The blanket application of assertions that grasslands need large mammal herbivory pressure can easily be refuted by a quick tour of the West. Even in the Great Plains where the alleged benefits of large mammal herbivory are said to be greatest, there are numerous sites that remained ungrazed by large herbivores of any kind—whether livestock or bison, yet do not “suffer” from ecosystem degradation or decline. That is not to say there are no differences between grazed and ungrazed sites. There are sometimes differences between ungrazed sties and grazed sites in plant species, shifts between plant groups favored by C3 to C4 pathways and genetic responses (Detling 1988). And studies have demonstrated that even grazing by small herbivores such as prairie dogs can causes striking changes in plant species composition. One study in South Dakota found that 25 years of prairie dog grazing had caused the replacement of grasses by forbs (Coppock et al. 1983)
MOST WESTERN PUBLIC LANDS WERE NEVER GRAZED BY LARGE HERVIVORES
Over most of the West, arguments about the potential benefits of large mammal herbivory are a moot point. The vast bulk of public lands that are grazed by domestic animals including the Great Basin, Southwest, California grasslands, Palouse Prairie of eastern Washington, Oregon and adjacent parts of Montana and Idaho, and alpine and subalpine basins throughout the Rockies, had few , if any large herds of grazing mammals like bison (Baker 1978, Mack and Thompson 1982). Plants in these regions poorly tolerate any but the lightest amounts of grazing pressure.
PLANT ADAPTATIONS TO HERBIVORY PRESSURE
East of the Rockies on the Great Plains, bison, antelope, elk, and other herbivores were locally abundant, and this has led some to speculate that such grasslands are dependent upon herbivory pressure for their ecological health. Some of the evidence used to support this contention are the presence of high concentrations of silicates (Brizuela et al 1986), ground-level meristems, vegetative reproduction (as opposed to seed), increased tillering and the ability to translocate resources from roots to leaf production referred to as “compensatory growth” (McNaughton 1986, McNaughton et al. 1988). These and other adaptations of Great Plains grasses are often cited as an indication that the Great Plains grasses evolved with large grazing mammals. While there is no doubt that many of these adaptations help plains grasses to cope with herbivory, it’s a stretch to then argue that they “need” to be grazed by livestock or even native herbivores like bison.
Some livestock advocates while unwilling to suggest that grasslands benefit from livestock may nevertheless argue that livestock herbivory under proper management does little harm (Laurenroth et al. 1994), thus should not preclude livestock grazing on public or private lands.
While most evolutionary ecologists would have no problem with the statement that adaptations to herbivory may help a plant to cope with herbivory pressure, they do not necessarily agree that grasslands “need” to be grazed or they will somehow become degraded in the absence of grazing influences. Indeed, the overwhelming evidence is to the contrary.
For instance, Belsky (1986) finds that while herbivory may benefit certain plants by reducing competition or removing senescent tissue, no convincing evidence supports the theory that herbivory benefits grazed plants. Belsky and Painter (1993) warn that uncritical acceptance of research purporting to demonstrate benefits from grazing may result from misinterpretation of evolutionary ecology.
Other authors warn that the differences in height, leaf length, and other morphological characterizations of grazed vs. ungrazed plants often used to indicate greater “vigor” may be misleading because of “possible grazing-related genetic differences between plants in sites with different grazing histories” (Painter et al 1989).
Finally, lack of overwhelming evidence of harm as reported by Lauenroth et al. (1994) from livestock herbivory under certain conditions does not necessarily mean no harm is occurring. This is an especially convenient position for livestock proponents to take since range professionals who are usually livestock proponents conduct most research. Such researchers can not be perceived to be unbiased observers. The kinds of questions and research one conducts often determine the kinds of answers one finds. Range professionals are not necessarily going out of their way to find examples of harmful impacts from livestock. Their findings are countered to some degree by others such as Licht (1997) that focuses on ecosystem harm resulting from livestock production.
PLANT ADAPTATIONS ARE ANTAGONISTIC RESPONSE TO HERBIVORY
Indeed, the presence of high levels of silicates, sharp awns, low nutritional quality, and vegetative modes of reproduction may be a consequence of a long association with herbivores, but these adaptations suggest an antagonistic relationship not a beneficial one. Much as alarms, multiple locks and bars over windows in a high crime neighborhood may suggest a long association with thieves, one would never claim that the presence of these devices indicates a “beneficial” relationship between criminals and victims of criminal activity. The fact that some businesses may persist in the presence of high crime does not necessarily mean they need it. Indeed, all these “compensatory” factors “cost” the business more energy and resources. That some businesses may be successful in spite of these costs should not be taken as evidence that they “need” or “thrive” in the presence of thieves.
Similarly the production of sharp awns, vegetative reproduction (which is inferior to sexual reproduction with seeds), and the ability to replace lost photosynthetic material are adaptations to herbivory that have a real cost to plants (Holland and Detling 1990). These authors state “the decrease in root biomass along the grazing chronosequence is probably caused by repeated grazing and reduced allocation of carbon belowground.” Even grazing by bison has a similar effect. Johnson and Matchett (2001) found that bison “grazing decreased root growth, especially in heavily grazed patches.” Even grazing by smaller mammals like prairie dogs can substantially reduce root biomass (Whicker and Detling. 1988) In other words, plants respond to grazing pressure by reduction in the amount of roots produced. This has real consequences to the plants living in arid and semi arid climates including the Great Plains since over all root biomass determines the ability of plants to capture soil moisture and thus survive periodic drought common in these environments.
Even when such negative overall effects on plant biomass is not found, does not mean that grazing enhances grasslands. Evidence such as increased growth rates, higher total biomass, increased seed production occasionally (very rarely) found in clipped or grazed plants used by livestock proponents (Savory 1999, Knight et al. 2001) to argue that herbivory “benefits” plants can be more properly termed responses to injury (Belsky et al. 1993).
NEW LEAVES COME AT THE EXPENSE OF ROOTS
Plants respond to clipping by herbivores in the same way that your body responds to hypothermia. It shuts down blood flow to less “vital” parts of the body and tries to maintain a core temperature. After losing its leaves to a herbivore, the plant says “May Day” May Day”-- I need to grow some new leaves because I’ll “stave” if I can’t photosynthesize. I have to maintain my “core” food production abilities. A grass plant responds by translocating energy from roots to leaf production much as the body shuts down blood flow to extremities when core temperatures are declining due to hypothermia. But just as it comes at a cost to other bodily functions, the translocation of energy from roots (where extra carbohydrates are usually stored) to the production of new leaves comes at a cost in root growth. Less root mass, for instance, can cause such plants to die during drought periods. In making the “decision” about responses and resource allocation, a plant places leaf replacement first since it needs to “eat” over the reduction in root mass “gambling” that it can grow enough new roots in time to compensate for these changes before the next drought.
Indeed, all studies that demonstrate a “benefit” from herbivory are done in laboratory conditions where water and nutrition are unlimited and competition does not exist. Or as Jaramillo and Detling (1988) have pointed out findings of reduced production of individual plants in field exclosures may be as much the result of greater competition among plants than due to any positive direct effects on plants as a consequence of herbivory.
Furthermore, many of these plant adaptations such as the ability to replace lost photosynthetic material by translocating resources from roots to leaves or ground level meristems may also be adaptations to other influences like fire that may have some positive benefits for grazed plants as well. Such adaptations do not always demonstrate a long continuous association with herbivory pressures.
ABOVE GROUND BIOMASS ONLY PART OF THE PICTURE
The usual evidence cited by most livestock proponents to demonstrate that herb ivory “benefits” grasslands involve the measurements of overall ABOVE GROUND biomass production. This is partially due to the fact that measuring roots and other below ground parts is difficult to do without killing the plant. Furthermore, most range researchers are primarily interested in the growth of leaves and stems since these are the parts of the plant that are forage for livestock.
Above ground biomass (which excludes roots, which is the largest part of the grass plant biomass) can increase under herbivory pressure. If you crop a grass during the growing season, it will respond by growing more leafy material. If you measure the leafy material prior to cropping and measure the new material produced in respond to cropping as well, the overall biomass of this leafy material we be greater than what you had if you had not cropped the plant at all. In addition, the remaining unclipped leaves on a plant will sometimes increase their photosynthetic output by up to 35% in response to the loss of other leafy material.
ECONOMIC VS BIOLOGICAL VALUES
If your concern is production of above ground biomass to feed livestock, than one might be willing to call these responses a “benefit”. However, we must be clear to note that this is an economic value, not a biological or evolutionary value. The young trees that are planted after clearcutting also produce more biomass per acre annually than old growth forests, but few would argue that clearcutting old growth forests is entirely a biological benefit. All of these “compensatory” factors can be thought of as “coping” mechanisms to injury or disturbance.
The response of grasses to cropping is somewhat analogous to the respond of coyotes to control. There is much evidence in the scientific literature that coyotes can increase pup reproduction and survival in the face of persecution. If we humans shoot, trap and poison coyotes, they respond to this “pressure” in the same way that plants respond to grazing. They compensate to survive. Exploited coyotes reproduce at a younger age and produce more pups that survive into adulthood. So if you are using overall pup production as a measurement or “indicator” of benefit, than shooting, trapping and poisoning coyotes could be argued to “benefit” coyotes. Of course, most environmentalists would be loath to argue that we should shoot, trap and poison coyotes because we wind up getting “more” coyotes overall. Few would argue this is a “benefit” except for Wildlife Services, which gets paid to kill coyotes.
DEFINATION OF HEALTH NEEDS CLARIFICATION
As explained previously above ground biomass production or even a large number of species recorded for a site does not necessarily make the site “healthy.” Sites that are disturbed may actually record more species in total numbers but deviate greatly from expected natural conditions. Most grassland ecologists agree that a “healthy” landscape tends to low rates of soil erosion, low rates of exotic species invasion, dominance by natural ecological processes as periodic wildfire that are expected on that site, wildlife and plant species in some kind of natural range of abundance and distribution again expected on that particular site and other criteria that broadens the definition of what exactly is ecological health.
GREAT PLAINS ECOSYSTEMS DO NOT DEGRADE IN THE ABSENCE OF LARGE MAMMAL HERBIVORY
Even the presence of large migratory grazing herds of bison and other native ungulates as occurred on the Great Plains doesn’t mean that these plant communities “need” to be grazed or somehow will become degraded from herbivory pressure. To see the fallacy of argument, observe the right of ways along rural highways and railroads throughout the Great Plains. Many thousands of miles of these rights of ways are neither grazed by large ungulates nor cropped in any other way (i.e. mowing). The fact that these remain luxuriously clothed in grasses suggests that herbivory is not necessary for “ecosystem health.” Similar examination of the tops of isolated buttes (Square Butte in Montana), rugged side hills and slopes of badlands as along the Missouri River Breaks, Badlands National Park and other sites where no large herbivores regularly grazed demonstrates that even on the Great Plains herb ivory is not necessary to maintain ecologically stable plant communities.
EXAMPLES OF NON-GRAZED GREAT PLAINS SITES
The following natural areas have neither bison nor livestock yet some are used, as research natural areas because of they remain as ecological standards against which livestock grazed sites are measured. These non large herbivore grazed Great Plains sites include many isolated buttes, mesas, steep cliff faces along rivers such as the Yellowstone, and tops of badlands found throughout the plains that are inaccessible to large herbivores for a variety of reasons.
Among the sites that I personally know have no large herbivory pressure but exhibit intact and healthy grasslands are the top of Square Butte, Montana, Devil's Tower National Monument, Wyoming, Makoshika State Park, Montana, Terry Badlands WSA, Montana, Agate Fossil Beds NM, Nebraska, Cherry Creek State Park, Colorado, Seedkadee NWR, Wyoming (this is on the edge of the plains and may be considered part of the Great Basin by some), Fort Stevenson State Park, ND, Badlands NP. (Badlands has bison grazing about 65,000 acres of the park, but according to the resource person I interviewed there is no “major ungulate grazing on much of the park."), Medicine Lake NWR Montana (some is ungrazed mid-grass prairie), Niobrara NWR Nebraska (bison are found on a small part of this mostly grassland area, but the majority has no livestock or bison). Scotts Bluff NM, Nebraska, Chatsworth State Park, Colorado, Boyd Lake SP Colorado, Boulder City Open Spaces, Colorado, Greycliffe Prairie Dog Town State Park, Montana, Ulm Pishkun State Park, Montana, Guadalupe National Park, Texas, and Springer Wildlife Management Area, near Torrington, Wyoming.
The debate over whether grasslands require herb ivory by large animals like livestock is at the center of much debate over continued grazing of public lands. Those who argue that grasslands require grazing misinterpret or conveniently ignore a great deal of scientific research that suggests that moderate to heavy grazing pressure is detrimental to grazed plants—even though plants do have coping adaptations that enable them to survive herb ivory effects.
At the very least the abundance of these sites suggests that the absence of large herbivore grazing pressure is not necessary for ecosystem health and calls into question the assertions that Great Plains grasslands or other grassland ecosystems require or need large mammal herbivory to remain “healthy.”
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