TheImprovement ofTropical and Subtropical Rangelands

ORJTERIA FOR PLANT SELEOTlON 203
stands of varying species composition demonstrated that monOllpeeffic
stands were easier to manage than stands with greater diversity
(Valentine, 1979). Other factors, however, such as the sUlICeptibility
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 ofseveral 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 19708, state and federal govemment regulations in
the United States have required that a diverse and effective mixture
ofspecies be used for the reclamation oflands drastically disturbed by
surface mining for coal. Ifseed 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