SOUTHWEST SAVANNAS: DESERTIFICATION BY WHITEFACES
April 26, 2002
Robert M. Dixon, Ph.D
This essay is an effort to counter the Southern Arizona movement to demonize exotic grasses. It's about land desertification by two exotic whiteface mammals -- cows and humans.
During the past 150 years the perennial grass component has been all but eradicated from the savannas in the southwestern states including the Sonoran Desert of southern Arizona . The native perennial grasses not having co-evolved with large herbivorous cowlike critters such as cattle and buffalo, couldn't cope and succumbed to heavy grazing in just a few years, especially during short term droughts.
When the grass component was lost, infiltration dropped off rapidly, runoff and erosion accelerated, and downslope flooding and silting became rampant. After more than a century of accelerated erosion of surface soil, the habitat for native grasses has been lost. Exotic perennial grasses were introduced that could cope with both the degraded habitat and grazing by exotic cows to regain hydrologic function. Two good matches have been proven in southern Arizona , namely Lehmann lovegrass and buffelgrass. These two grasses coevolved with cows or cowlike critters in Africa and thus can cope with light grazing where annual rainfall exceeds 12 inches-that is where the elevation is greater than Tucson 's. At the Tucson elevation (2500 ft.) and lower these grasses can survive but not thrive without grazing.
These exotic grasses also occur west of Tucson along I-10 & I-8 where rainfall is less than 12 inches because they receive runoff from the blacktop. These exotics function hydrologically just like the eradicated native grasses by increasing infiltration, soil moisture, and groundwater recharge. They also rebuild the topsoil habitat for native perennial grasses. This soil rebuilding involves feeding many native soil organisms, literally thousands per tablespoon of topsoil. The savanna restored with these two exotic grasses not only has many times the number of organisms per acre but much greater species diversity as well, if one considers the soil microbial population.
The food chains recreated by these exotic grasses are numerous and quite obvious to even a casual observer. Insect, bird and rodent populations increase abruptly not to mention snakes and lizards. Hawks, owls, coyotes and bobcats feed on the rodents, snakes and lizards, while the snakes and lizards feed on the rodents and insects, respectively. Populations of soil invertebrates also increase. Soil insects such as ants, termites and solitary bees burrow deep into the soil, thereby greatly increasing rainwater infiltration rate and deep percolation to recharge groundwater. The increased hydration of the soil resulting from these two exotic grasses provides the moisture required to drive the interlinked life cycle of the many diverse organisms in a functional savanna.
Many plant species coexist because of complimentary interactions. An example is the deep infiltration induced by grasses serves the deep-rooted leguminous species quite well especially during drought years. On the other hand these grasses feed on the atmospheric nitrogen that legumes fix in the soil. Thus the exotic grasses and leguminous shrubs such as mesquite, paloverde, and the acacias live side-by-side in a biomass productive savanna.
The ratio of grass to shrubs can be maintained by fire and the type of livestock or wildlife. Fire has always maintained the extent of grass in savannas and prairies by periodically killing small shrub and tree seedlings. Trees & shrubs often reside in and near watercourses. Likewise, livestock foraging by sheep and goats will kill small woody seedlings. In contrast cattle, by selectively grazing grass, will tend to increase the ratio of shrubs to perennial grass. This ratio is close to infinity for most of the Southwestern savannas, except for the areas where the adapted exotic grasses have been introduced. Exotics are usually seeded between the dry washes, but not in the washes many of which are deeply gullied. Since cattle concentrate in the seeded areas with abundant forage, remnant colonies of native perennial grasses are expanding in washes where rainwater runoff is available.
The Sonoran Desert Conservation Plan requires sustainable cattle grazing. In view of the above comments, sustainable grazing of exotic cattle requires the establishment and maintenance of exotic perennial grasses in many land areas above Tucson in elevation. Yet several well-known conservation organizations are currently attempting to eradicate adapted exotic grasses including bufflegrass and fountaingrass which, in turn, are attempting to colonize the barren areas where cattle eradicated the native perennial grasses. These organizations claim that such exotic perennial grasses are threatening the extinction of selected endangered species.
Nothing could be farther from the truth when in fact it is the whitefaces that are threatening a mass extinction not only in the Southwestern savannas but in many savannas, globally. These organizations are led by green vigilantes that are out to destroy the exotic grasses in southern Arizona using destructive methods. They look at only the negative aspect of grasses, not the positive ones mentioned above. They don't consider that these grasses have already integrated into the savanna ecosystem, filling many of the functional roles of the natives that were grazed out by whiteface cattle and their close relatives. They don't consider that these grasses are providing habitat for many native plants and animals, both above and below ground. They don't consider that these grasses are providing important wind and water erosion control that is literally holding landscapes together, which was the role of the natives before they were grazed out by the whitefaces. They in other words are propagandizing the public with only a small part of the total picture. They are demonizing the exotic grasses to justify their destruction. Just as the pioneer whitefaces demonized the native Americans and their native wildlife to justify the brutality that ensued. Native Americans were called savages and their food source, the buffalo (bison), was nearly eradicated from the prairies of the Great Plains .
Certainly the foregoing essay is overly simplistic. The faces are not all white or black for that matter. In fact the faces are usually many shades of brown.
Now for the 3 directors of the conservation organizations that are calling the public to arms to eradicate buffelgrass and fountaingrass I have some suggestions.
First let's call a moratorium on this green vigilante activity and study the situation further.
Let's see what the sciences have to offer-disciplines like hydrology, hydrobiology, edaphology, soil microbiology, evolutionary biology and restoration ecology.
Let's go on local field tours to see fountaingrass controlling erosion on steep roadside slopes and to see where Lehmann lovegrass and buffelgrass are reclaiming retired farmland from bumper crops of dust storms and tumbleweeds in the aftermath of irrigated cotton.
Let's take a close look at the many native species that are benefiting from the exotic grasses, as well as the exotic species that are deriving benefits such as the whitefaces.
Let's remember what Ralph Waldo Emerson said about weeds: Weeds are plants whose virtues have yet to be discovered. Exotic grasses do have many proven virtues as described above.
Let's also remember what Albert Einstein said about problems: No problem is ever solved at the same levels of awareness at which it was created. The exotic grasses were created by whiteface disturbances. More whiteface disturbances will only create another generation of exotics indefinitely into the future ensuring that the gallant weed warriors will never be unemployed, or without volunteer work.
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