|The Improvement of Tropical and Subtropical Rangelands (BOSTID)|
|Criteria for plant selection|
Questions of reduced biological diversity have been raised in relation to range improvement projects carried out in the western United States during the 1940s and 1950s. Many of the early projects were designed for sites where the perennial bunch grasses were severely reduced by overgrazing, and where other shrubs of low palatability to livestock had come to dominate the sites. Remedial measures to remove or reduce the dominant shrubs, such as the application of broad-spectrum chemical herbicides or mechanical removal, also had an adverse impact on other species susceptible to the drastic control methods. The result was a severe reduction in the species diversity of the range vegetation.
In addition to the loss of diversity caused by the control measures, the number of species included in the seeding mix were very limited, sometimes consisting only of wheatgrasses. Experience in securing a satisfactory degree of establishment and in managing grazing on stands of varying species composition demonstrated that monospecific stands were easier to manage than stands with greater diversity (Valentine, 1979). Other factors, however, such as the susceptibility of a monoculture to insect infestations (Haws, 1978) and the need for species diversity to provide forage over various seasons or year-round, suggested that several species with diverse characteristics should be included in the seed mix. Plummer and colleagues (1968) recommend that the seeding mix consist of several plant types for restoring big-game range and areas of multiple use because of four advantages that can accrue to the project:
· A mixture is better suited to the varied terrain and
climatic conditions of mountain rangelands.
· A variety provides several nutritional sources for game animals and livestock.
· Including several species with different seasons of maturity prolongs the period during which green feed is available to animals.
· Mixtures provide a better overall degree of ground cover than does a single species.
Since the late 1970s, state and federal government regulations in the United States have required that a diverse and effective mixture of species be used for the reclamation of lands drastically disturbed by surface mining for coal. If seed supplies are available, species native to the area are to be given priority over non-natives. There is ample reason to argue that the emphasis should be placed on a high degree of adaptability and productive capacity in choosing species rather than their merely being native. Nevertheless, the critical factor is to provide the site to be revegetated with a sufficiently diverse spectrum of species that ecological stability will be assured during periods of environmental stress. Equally important is that various ecosystem functions that depend on a diverse vegetation will be supported.
An important issue is how range improvement operations that are planned in an often limited perspective can deal with the fundamental problem of maintaining biological diversity in rangeland ecosystems. Concern has been expressed in numerous conferences and symposia that the continued intensive development of agricultural and natural resource areas is seriously depleting the world of many valuable species upon which future genetic development and even the survival of mankind may depend (U.S. Department of State, 1982). Range improvement practices that require some degree of control over various competing species, so that they will not prevent the establishment of the seeded species, should be planned with a sensitivity for ways to maintain the overall diversity of the plant community. Further, range technicians who select species for seeding should include those that will have a high degree of adaptability and longevity on the site to be revegetated. Where possible, seeds of desirable species from the local area should be included in the seed mixture. Subsequent management practices should seek to avoid highly selective grazing that damages only certain species that are needed to maintain the diversity and the stability of the range ecosystem.