Posted by: chrismaser | May 30, 2012

REPAIRING ECOSYSTEMS—MENDING THE PRAIRIE THROUGH FIRE AND GRAZING

The reintroduction of fire and grazing by large ungulates, alone or in combination, has increasingly been recognized as central to the repair of North American mixed-grass and tall-grass prairies. Fire (a physical disturbance) and grazing (a biophysical disturbance) are major forces that shape the patterns of diversity among native and exotic species in many types of grassland; yet these disturbances have notoriously variable effects.

To examine these two types of disturbances, studies were conducted in a mosaic of serpentine and non-serpentine soils in California. Both fire and grazing increased total species richness on both soils. However, on the one hand, fire enhanced the diversity of exotic species more on non-serpentine soils and increased native species richness disproportionately on serpentine soils. Grazing, on the other hand, increased native species richness on serpentine soils but not on non-serpentine soils. The reason cattle grazing increases the species richness of indigenous grasses, as shown by fertilization experiments, is that invasive grasses are limited by the amount of nitrogen in the soil.1

CATTLE

Several lines of evidence indicate that deposition of dry nitrogen from smog is responsible for the invasion of non-native grasses. For example, the estimated rate at which nitrogen is deposited in grasslands south of San Jose is between 22 and 33 pounds of nitrogen per 2.5 acres per year; whereas the grasslands on the San Francisco Peninsula experience nitrogen deposition of 9 to 13 pounds of nitrogen per 2.5 acres per year. Moreover, cattle selectively graze on grasses over forbs, and their grazing leads to a net export of nitrogen—a nitrogen sink, if you will—as cattle are removed for slaughter. Although poorly managed cattle can significantly disrupt native ecosystems, in this case moderate, well-managed cattle grazing is essential for the maintenance of indigenous biodiversity in the face of invasive, exotic grasses and external inputs of nitrogen from nearby urban areas.2

In the prairie remnants in the Loess Hills of Iowa, grazing by domestic livestock promoted the greatest overall species richness, whereas grazing and burning resulted in the lowest cover by woody plants. Burning, however, achieved the best overall increase in the cover and diversity of native species while simultaneously reducing exotic forbs and grasses, the latter being predominantly cool-season in habit, such as cheatgrass.3

In contrast, livestock grazing appears to be an exotic ecological force in grama grasslands of southeastern Arizona, one destructive to certain components of the native flora and fauna. The destructiveness of livestock grazing could result from the absence of extensive grazing by indigenous ungulates in the Southwest since the Pleistocene. Thus, the tolerance of particular grasslands to their use by domestic livestock may depend on their historic association with native grazing animals, as well as on the pervasiveness of certain exotics, like cheatgrass.4

CHEATGRASS

Cheatgrass, an annual, was originally confined to roadways and abandoned farmlands but is now pervasive in the shrub-steppe ecosystem of the western United States, where it’s expanding into shrub, ponderosa pine, and piñion-juniper ecosystems. The spread of cheatgrass causes fires to become more frequent and larger in scale, which has a dramatic effect on indigenous plants and animals. Therefore, although a mosaic of burning and grazing (alone and in combination) may provide the greatest landscape-level species richness, this strategy may also promote the persistence of exotic species, such as cheatgrass, in some areas because species-specific responses appear to be idiosyncratic. 5

FIRE

With respect to fire, entomologists have expressed concern that prescribed burning is incompatible with the conservation of insect diversity on small prairie sites. To evaluate this anxiety, a research project was conducted over seven seasons within small, isolated remnants of tall-grass-prairie in northern Illinois, northwestern Indiana, and southeastern Wisconsin. The study focused on the responses of remnant-dependent and remnant-independent species to multiple fires. Sweep nets, light traps, sticky traps, and visual searches were used to gauge population responses and to track negatively affected populations to recovery.

Most species (93%) responded consistently to prescribed fires. Postfire responses, however, ranged from 26% positive to 40% negative for 151 species representing 33 families and 7 orders. Three attributes—remnant-dependence, upland inhabitance, and nonvagility—were significant predictors of negative postfire response. (“Vagility” is the capacity or tendency of an organism or a species to move about or disperse in a given environment.) Among negatively affected populations, 68% recovered within a year, and all 163 populations tracked to recovery did so in two years or less. Therefore, a judicious use of rotational, cool-season burning can be a workable tool in helping to mend small, isolated prairie remnants.6 Now the question is whether, in addition to fire, grazing by large ungulates can be used to repair a prairie remnant?

TULE ELK

Many successful reintroductions of large mammalian herbivores have taken place throughout the world, but remarkably little attention has focused on how these actions affect native and exotic vegetation. One such herbivore is the tule elk, endemic to western California, which was on the brink of extinction in the mid-1800s, but now has several stable populations.

The elk significantly altered the species composition of coastal grasslands, where the response of annual species (dominated heavily by exotic taxa) was dramatically different from that of perennial species. The herbivory increased the abundance of both indigenous and exotic annuals, but either had no effect on perennials or caused them to decrease remarkable. Moreover, the elk decreased the cover of native shrubs, a finding that indicates that herbivores can play an important role in maintaining open grasslands. Further, elk dramatically reduced the abundance and biomass of velvet grass, which is a highly invasive exotic and thus a major problem in moderately moist perennial grasslands.7

Unlike the indigenous tule elk and bison, cattle are an exotic species that was introduced into North America more than two centuries ago. Whereas habitat and food items are partitioned among coexisting native herbivores, like the bison and pronghorn antelope, domestic cattle are generalists in their use of habitat and their foraging. Moreover, livestock grazing is perhaps the most ubiquitous use of land in the western United States.

As with grazing by native species, however, the biophysical results are highly variable. To understand some of the effects of livestock grazing, a study was carried out on the cover of plants, soil crusts, and plant-species richness at six sites with different potentially natural vegetation in the Chaco Culture National Historic Park in northern New Mexico. This park has a long history of human habitation and now is one of the largest grazing exclosures in the American West.

Species richness was higher under long-term protection than under current grazing at all sites. As with other studies, however, trends in the response of shrubs and grasses varied significantly among the sites, in that shrub cover increased with long-term protection at four upland sites, and grass cover increased with protection at four sites. The vegetative response to a discontinuation of grazing was determined partly by each site’s ecological potential of both its edaphic (soil) and topographic characteristics.

Like any disturbance, the successful reintroduction of a large grazing mammal can be expected to have extremely complex effects on the plant community, thereby giving rise to both desirable and undesirable outcomes from the perspective of repairing a particular ecosystem. The nuances in vegetative response at Chaco, for instance, are thought to represent site-specific ecological variation and thus challenge the notions of a widespread invasion of shrubs, as is often inferred.8 It is paramount, therefore, to pay careful attention to the existing heterogeneity of the ecological background in which one is proposing to work.

PLANNING HABITAT RECOVERY

Nevertheless, the cover of perennial grass has declined in many arid types of grassland over the past two centuries, while shrub density has increased. These changes, which are characteristic of desertification, are thought to have occurred most often after prolonged periods of intense grazing by domestic livestock. At many such sites, however, the subsequent removal of livestock for up to 20 years did not increase the cover of grasses.

To understand the time required for grasses to recover in historically arid grasslands dominated by shrubs, vegetation was examined at two desertified sites that differed in the length of time they had been free of livestock. There was little noticeable difference between vegetation at the site from which livestock had been fenced out for 20 years and the shrub-dominated vegetation just outside the exclusion fence. Nevertheless, there was a significantly higher cover of perennial grasses in the area from which livestock had been removed 39 years earlier, and all the increase had occurred within the last 20 years. It thus seems that perennial grasses in historic grassland ecosystems dominated by shrubs require a period of 20 or more years to recover from grazing once the domestic livestock have been eliminated.9

The grasses’ requirement of two decades or more for their recovery from livestock grazing is a beautiful illustration of the aforementioned dynamic shared by all ecosystems: cumulative effects, lag periods, and thresholds. It also demonstrates our limited powers of spontaneous observation, which can be thought of as the snapshot effect.

With respect to reintroducing large mammalian grazers into prairie remnants as part of the planned repair, Machteld Van Dierendonck and Michiel Wallis de Vries (both in the Department of Terrestrial Ecology and Nature Conservation, Agricultural University, Wageningen, The Netherlands) offer wise counsel based on experiences with the reintroduction of the Przewalski horse in the Mongolian steppe. For a planned reintroduction of any large mammalian grazer into an ecosystem being repaired, they recommend creating a framework for safeguarding the entire system with an integrated caretaking plan. Thus, each prairie site where such a reintroduction is anticipated requires a thorough assessment with respect to its suitability; the required information includes the area’s current size, habitat types, land use, socioeconomics, relevant legislation, and potential problems. In addition, each prairie site must contain one or more facilities for acclimating genetically and physically healthy, socially adapted animals in biologically sound groups.

From the human viewpoint, an organizational structure that incorporates a vision statement, goals, and objectives should be established for each reintroduction site. In turn, these elements need to be developed into an effective caretaking plan, one that includes carefully monitoring the population and its surrounding ecosystem, including research on and population control of the reintroduced ungulates to avoid unnecessary damage to the ecosystem. Finally, special attention needs to be given to local socioeconomic situations, community participation, and the training of a staff to manage the project.10

ONE SIZE DOES NOT FIT ALL

Returning to our example of the North American prairies, a graphic example in southeastern Oregon of the kind of problems Van Dierendonck and de Vries were talking about is illustrative of conflicting human desires that can erupt when communication and cooperation are lacking. A controversy arose between a rancher and a conservation organization while I was working in the rangelands of southeastern Oregon in the late 1970s. The subject of the debate was a population of rare plants that was discovered in an active grazing allotment. The conservationists wanted to enclose the plants within a fence to protect them from the cattle. The rancher, in turn, justified his grazing practices by arguing that the proposed “exclosure”—as he viewed the fence—was unnecessary because the plants and his cattle had coexisted for decades. The conservationists countered that the cattle not only are an introduced species but also are generalists in their eating habits and thus were a threat to the plants, whereas the indigenous grazers were more selective in their foraging behavior—like the bison of the American Great Plains.

Although the conservationists were correct about cattle being exotic organisms with catholic diets as compared to indigenous herbivores with selective diets, they did not mention their deep-seated dislike for cattle, which they regarded as unnatural intruders in the rangelands. Part of their dislike for the cattle was misplaced, however, because the cattle merely did whatever the rancher allowed them to do. Thus, as is often the case, the animals were blamed for the owner’s lack of responsible behavior.

At length, I was drawn into the controversy. After examining the area, I proposed that a portion of the plant community be fenced to exclude the cattle, but not all of it because the rancher just might be correct in his assessment. The conservationists, however, did not even consider my proposal and fenced the whole area.

Although the situation seemed fine for a while, the cumulative effects of fencing out the livestock began taking an unseen toll. By the time the decline in the community’s vigor became noticeable, the threshold of irreversibility had been crossed.

Had the conservationists been willing to test the rancher’s notion, they would have been able to work with him to ensure the community’s long-term survival. By unilaterally introducing the fence, however, they precipitated the demise of the very thing they wanted to save. The rare plants would probably still be flourishing if they had been given consideration commensurate with their special status.


Repairing Ecosystems:

• Historical Abuse

• Six Lessons From History

• Restoration, As We Currently Think of It

• Why Restoration Is Not Possible

• Basic Considerations

• Biophysical Dynamics

      1. Composition, Structure, And Function

      2. Cumulative Effects, Lag Periods, And Thresholds

      3. Habitat Components And Animal Behavior

      4. Habitat Configuration, Size, And Quality

      6. Special Considerations

• Monitoring Your Efforts

Related Posts:

• Principle 1: Everything is a relationship

• Principle 6: All relationships are self-reinforcing feedback loops

• Principle 7: All relationships have one or more tradeoffs

• Principle 8: Change is a process of eternal becoming

• Principle 9: All relationships are irreversible

• Principle 13: Systemic change is based on self-organized criticality

• Principle 14: Dynamic disequilibrium rules all systems

• Biodiversity—Our Social-Environmental Insurance Policy


ENDNOTES

1. S. Harrison, B. D. Inouye, and H. D. Safford. Ecological Heterogeneity in the Effects of Grazing and Fire on Grassland Diversity. Conservation Biology, 17 (2003):837–845.

2. Stuart B. Weiss. Cars, Cows, and Checkerspot Butterflies: Nitrogen Deposition and Management of Nutrient-Poor Grasslands for a Threatened Species. Conservation Biology, 13 (1999):1476–1486.

3. Lars A. Brudvig, Catherine M. Mabry, James R. Miller, and Tracy A. Walker. Evaluation of Central North American Prairie Management Based on Species Diversity, Life Form, and Individual Species Metrics. Conservation Biology, 21 (2007):864–874.

4. Carl E. Bock and Jane H. Bock. Cover of Perennial Grasses in Southeastern Arizona in Relation to Livestock Grazing. Conservation Biology, 7 (1993):371–377.

(1) 5. W. D. Billings. Ecological Impacts of Cheatgrass and Resultant Fire on Ecosystems in the Western Great Basin. Pp. 22–30. In: Proceedings Ecology and Management of Annual Rangelands. S. B. Monsen and S. G. Kitchen (eds.). Intermountain Research Station, USDA Forest Service, Fort Collins, CO. 1994; and (2) S. B. Monsen. The Competitive Influences of Cheatgrass (Bromus tectorum) on Site Restoration. Pp. 43–50. In: Proceedings Ecology and Management of Annual Rangelands. S. B. Monsen and S. G. Kitchen (eds.). Intermountain Research Station, USDA Forest Service, Fort Collins, CO. 1994.

6. The previous two paragraphs are based on: Ron Panzer. Compatibility of Prescribed Burning with the Conservation of Insects in Small, Isolated Prairie Reserves. Conservation Biology, 16 (2002):1296–1307.

7. Brent E. Johnson and J. Hall Cushman. Influence of a Large Herbivore Reintroduction on Plant Invasions and Community Composition in a California Grassland. Conservation Biology, 21 (2007):515–526.

8. The previous three paragraphs are based on: M. Lisa Floyd, Thomas L. Fleischner, David Hanna, and Paul Whitefield. Effects of Historic Livestock Grazing on Vegetation at Chaco Culture National Historic Park, New Mexico. Conservation Biology, 17 (2003):1703–1711.

9. Thomas J. Valone, Marc Meyer, James H. Brown, and Robert M. Chew. Timescale of Perennial Grass Recovery in Desertified Arid Grasslands Following Livestock Removal. Conservation Biology, 16 (2002):995–1002.

10. The two previous paragraphs are based on: Machteld C. Van Dierendonck and Michiel F. Wallis de Vries. Ungulate Reintroductions: Experiences with the Takhi or Przewalski Horse (Equus ferus przewalskii) in Mongolia. Conservation Biology, 10 (1996):728–740.


Text © by Chris Maser 2012. All rights reserved.

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