By Garry Rogers
Livestock and Plant Invasions
Using natural landscapes for any purpose requires caution to prevent plant invasions. An essential task for nature conservation is explaining this to the public.
Most investigations of plant invasions assign responsibility to Humans. In our ignorance, we introduce potentially invasive plants from foreign ecosystems, and then we disturb native ecosystems and help the introduced plants get established and spread. We have learned that diseases, predators, competitors, and supportive soil microorganisms control plant growth. Move plants to new locations where their natural controls aren’t present and they sometimes explode across the landscape.
After introduction, the principal human disturbances responsible for plant invasions in the desert are domestic livestock grazing, construction, fire, recreation, and climate change. Scientists have concluded that soil disturbance and seed dispersal by domestic livestock are the principal historic events that introduced and spread weeds into natural ecosystems of the western U. S. (Mack 1981, Belsky and Gelbard 2000, McAuliffe 1998, Rutman 1998, Morrison et al. 2003 and others).
According to Belsky and Gelbard (2000: 3), domestic livestock contribute to plant invasions by:
- transporting weed seeds on their coats and feet and in their guts,
- grazing native plant species and not introduced species,
- creating patches of bare, disturbed soils that act as weed seedbeds,
- destroying micro biotic crusts that stabilize soils and inhibit weed seed germination,
- creating patches of nitrogen-rich soils, which favor some weed species,
- reducing concentrations of soil mycorrhizae required by most western native species,
- accelerating soil erosion to create vacant sites for invasive species to occupy.
Livestock grazing began soon after Europeans reached western North America in the 1500’s (reviewed by Turner et al. 2003). Since then, introduced plants have become abundant across all arid habitats, and new ones continue to appear. When Jeff Steele and I began our study of desert fires in 1974 (Rogers and Steele 1980), the principal weeds in the Sonoran Desert were Red Brome (Bromus rubens) and Alfilaria (Erodium cicutarium). Since then, Buffelgrass (Pennisetum ciliare), and Sahara Mustard (Brassica tournefortii, aka Asian Mustard) have become serious problems. And there are others.
Range managers once believed that plants were interchangeable between locations. They imported foreign species to provide better livestock forage or fill other needs. For instance, they introduced Buffelgrass and Lehmann Lovegrass (Eragrostis lehmanniana) to protect arid sites denuded by livestock grazing. As recently as 1988, managers continued to plant both species on desert ranges (Cox et al. 1988). Now we know that these species are among the most harmful in our deserts (AZ-WIPWG 2009, Bock et al. 1986, Bock et al. 2007, and the S. AZ Buffelgrass website).
Perhaps the greatest damage by livestock has been to biological soil crusts (BSCs). Numerous studies report that even light cattle grazing damages BSCs (Kleiner and Harper 1977, Anderson et al. 1982, Brotherson et al. 1983, Johansen 1986). BSCs stabilize soils and block invasive species. These benefits plus the slow recovery of BSCs after livestock trampling should place them in the forefront of concerns for range managers. Strategies for managing livestock grazing to protect BSCs are described by Belnap et al. (2001), Marble and Harper (1989), Memmott et al. (1998), Miller et al. (1994), Burkhardt (1996), Kaltenecker and Wicklow-Howard 1994, and Kaltenecker et al. (1999b).
Livestock grazing and fire prone invasive species are an irresistible force for change. In the Great Basin Desert, the small Eurasian Cheatgrass has replaced more than 100 million acres of native Great Basin vegetation. When Hull (1965) wrote about the positive feedback loop connecting Cheatgrass and fire in the Great Basin, he was echoing earlier comments by Aldo Leopold (Leopold 1949: 154-158) who identified livestock grazing as the principal disturbance that admitted weeds, and fires as the chief force behind Cheatgrass replacement of native species.
Clear cases of similar changes are found throughout the U. S. arid lands. Invasive plants increase fire frequency, and native plants begin dropping out. In many instances, range managers allow livestock grazing on recently burned lands. Since the cows often go after native plants, the loss of native species accelerates. On Black Mesa, not far from my home, fires and continued grazing have allowed invasive species to almost completely replace native herbs and shrubs. Range ecologists have often pointed out that the weed vegetation on Black Mesa has reduced livestock carrying capacity, lowered biological diversity, and increased flammability.
Future of Livestock Grazing and Plant Invasions in North American Deserts
For North American deserts, three contemporary realities make it desirable and politically feasible to restrict livestock grazing. First, extensive research has confirmed that plant invasions and replacement of native ecosystems reduces forage production and increases fire danger (Leopold 1949, and many others in the References). Second, as housing and urban development have replaced productive lands, grazing resources have dwindled, and there has been a general decline in numbers of livestock on the public land (McClaran et al. 1992). Third, multiple uses of the public lands have begun to diminish the relative economic importance of grazing (e.g., Pope and Wagstaff 1987, Mathews et al. 2002).
According to the U. S. Department of Agriculture (Mathews et al. 2002), grazing on public lands of the 17 western states (cattle and sheep) amounts to about 1 percent of total domestic product for those states. During the period 1988-1997, BLM grazing permit receipts declined by about 30%, and in 1997 equaled 6.7% of BLM revenue from all non-government sources.
Livestock grazing is more important in most regions of the world than in North America. As the human population continues to grow and as long as people continue to consume animals, ecosystems will continue to deteriorate. Even if animal consumption ends, continued population increase will eventually be impossible to feed everyone. Water and irrigable land scarcity could replace energy resource scarcity as the principal cause of war. And then, as foretold by many dystopian novelists (e.g., Harrison 1966), the wars could shrink to small desperate struggles as Homo sapiens’ twilight falls.
References for Plant Invasions
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