Invasive Species
View Chapter 2 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
Exotic annual Bromus species are widely recognized for their potential to invade, dominate, and alter the structure and function of ecosystems. In this chapter, we summarize the invasion potential, ecosystem threats, and management strategies for different Bromus species within each of five ecoregions of the western United States. We characterize invasion potential and threats in terms of ecosystem resistance to Bromus invasion and ecosystem resilience to disturbance with an emphasis on the importance of fire regimes.
View introductory chapter of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
The spread and impacts of exotic species are unambiguous, global threats to many ecosystems. A prominent example is the suite of annual grasses in the Bromus genus that originate from Europe and Eurasia but have invaded or are invading large areas of the Western USA. This book brings a diverse, multidisciplinary group of authors together to synthesize current knowledge, research needs, and management implications for Bromus. Exotic plant invasions are multifaceted problems, and understanding and managing them requires the biological, ecological, sociological, and economic perspectives that are integrated in this book. Knowing how well information from one geographic or environmental setting can transfer to another is a key need for broadly distributed Bromus species especially given ongoing climate change.
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The objective of this study was to investigate how climate, land use and community structure may explain these patterns of species dominance. We found that differences in summer precipitation and winter minimum temperature, land use intensity, and shrub size may all contribute to the dominance of annual species in the Great Basin, particularly cheatgrass. In particular, previous work indicates that summer precipitation and winter temperature drive the distribution of cheatgrass in the Great Basin. As a result, sites with wet summers and cold springs, similar to the Chinese sites, would not be expected to be dominated by cheatgrass. A history of more intense grazing of the Chinese sites, as described in the literature, also is likely to decrease fire frequency, and decreases litter and shrub dominance, all of which have been demonstrated to be important in cheatgrass establishment and ultimate dominance. Further research is necessary to determine if other annuals that follow the same pattern of scarcity in the Junggar Basin and dominance in the Great Basin are responding to the same influences.
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Time-controlled, short-duration, high intensity sheep or cattle grazing for several days in early spring removes substantial amounts of alien annual plant seed while it is still in inflorescence and opens up the sward canopy to allow light to penetrate to young, short-statured seedling perennials. This grazing event must be timed to allow perennial grass regrowth, flowering and seed set before spring soil moisture is exhausted. It must be intense enough to graze off the grass inflorescences of most alien annual grasses. The result is increased live crown cover for mature perennial grasses, reduced decadent dead-center growth forms in bunchgrasses, and improved light availability to tiller bases which promotes basal bud activation and new vegetative and reproductive tiller formation. These perennial grass responses constitute what managers term improved plant vigor.
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Fire risk in western North America has increased with increasing cover of cheatgrass, an invasive alien annual grass. The relationship between cheatgrass cover and fire risk was determined in a historically burned shrub-steppe community where cheatgrass cover ranged from 5 to 75%. Fire risk ranged from about 46% with an average of 12% cheatgrass cover to 100% when cheatgrass cover was greater than 45% based on prediction confidence limits. Reflectance of the green and red bands of aerial photographs, were related to senescent cheatgrass cover to create fine resolution cheatgrass cover and fire risk maps. This assessment technique will allow land managers to prioritize lands for restoration to reduce fire risk in the shrub-steppe.
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Cattle grazed a cheatgrass-dominated pasture during the fall dormant period for four years (2006-2009) and were provided a protein nutrient supplement to improve their distribution, uptake of dry feed and production performance. Cheatgrass standing crop was reduced by 43 percent to 80 percent each year, and cattle weight and body condition score increased each year. The fall-grazed site had less cover from cheatgrass than the ungrazed site had. The fall-grazed site also had no decline in perennial grass cover. Cheatgrass density was 64 percent less on the grazed site after two years, and had 19 fewer plants per square foot than the adjacent ungrazed area. The seedbank potential for cheatgrass decreased much more on grazed areas than on the adjacent ungrazed areas, with a 95 percent or greater reduction in the seedbank potential. The difference was due to the grazing treatment.
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The widespread abundance of cheatgrass on the foothill and semidesert ranges of the Intermountain Great Basin region causes concern to range managers of this area. During the past 20 years, crested wheatgrass has been used to replace cheatgrass on some of the more favorable sites. However, many soils supporting extensive areas of cheatgrass are so low in productivity it is believed that they should be managed as an annual grass type and used for the most effective purpose which appears to be spring range for livestock.
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Yearling cattle gained weight satisfactorily on cheatgrass range under rotational (moderate) and continuous (moderate and heavy) grazing systems during a 3-year study. This study was designed to determine effects of these systems on the rangeland-not on individual plant species. Assignment of these systems to different pastures each year precluded evaluation of long-term vegetal response to the treatments. Weight gain was greatest in late spring. Grazing capacity of the range and cattle gain per acre increased through the summer, then declined. Yearly variation in production of forage and beef was apparently due to weather. Grazing capacity and beef production increased under continuous heavy grazing, but possible vegetation changes not evaluated in this study make heavy grazing undesirable.
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Comparisons were made of grazing capacities and sheep gains on both cheatgrass range and on native bunchgrass range during early spring. Capacities were similar on these two range types in wet years, but were almost 60% greater on bunchgrasss range during dry years. Sheep gained an average of 0.32 lb. per head per day over three spring periods on the two vegetation types. In 2 of the 4 study years, they lost weight during the first 2-week grazing period. Sheep gains per acre were similar on the two range types.
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Grazing management can be complicated with very sophisticated grazing system designs, but in this document we discuss a simple method for managing livestock to control annual grasses while allowing perennial grasses to reoccupy the sites and generating more animal production. It’s called “Green and Brown” grazing to manage annual grasses: graze when invasive annual grasses are green and desired
species are brown. This strategy is also known as time-controlled, short-duration, high-intensity grazing.