Ráðunautafundur - 15.02.1989, Side 13

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nutritious plant species, and management of rangelands for grazing is directed
toward sustained yield of desirable plant species. Hence, the condition, or health, of
rangelands is tied closely to knowledge of plant syneclogy (Stoddart et al„ 1975).
Although plant species may occupy distinct niches in terms of their resource uses,
herbivores can and to mediate community organization through differential use of
plants variously susceptible to herbivory (Archer and Tieszen, 1986). The abundance
of nutritious plants thus depends on the interaction between plant
tolerance/avoidance of defoliation and level of use by livestock, each of which may
vary along environmental gradients (Archer and Tieszen, 1986; Bryant et al., 1987).
To the extent that we can understand the mechanisms that underlie plant
synecology, we can manage plant assemblages to our benefit.
Historically, much scientific effort has gone into describing changes in vegetation
on both a spatial and a temporal basis. The concepts of range site and range
condition and trend have developed as an outgrowth of this research (Stoddart et
al„ 1975). Grazing management occurs at the plant community and landscape levels
of organization. However, most research on plant responses to grazing has focused
on individual organisms and single species of organisms. Future research should
emphasize population and community level processes if grazing systems are to be
understood at spatial and temporal scales appropriate for management (Archer and
Tieszen, 1986). This research should lead to greater insights into the functioning of
plant communities and as a result enhance our ability to manage rangelands.
Diet selection by livestock affects both livestock production and range condition.
By selecting some plant species/parts and avoiding others, animals obtain a
nutritious diet. They can also cause those plant species that are avoided to increase
in abundance relative to those that are selected (Stoddart et al„ 1975). To the
extent that we can understand how and why livestock select/avoid the plant they
do, we increase our potential to manipulate diet selection to our benefit (Malechek
et al„ 1986; Provenza and Balph, 1987, 1988; Provenza et al„ 1988).
Research on diet selection has generally been descriptive and has focused on
which plant species livestock select and their physical and chemical characteristics.
Such research has lead to important recommendations on when to graze and when
to supplement on rangelands. However, little attention has been given to the plant
species/parts that livestock avoid, their physical and chemical characteristics and
why livestock avoid them (Provenza et al„ 1988). Research on diet selection is
beginning to address the question of how herbivores obtain a nutritious diet by
selecting some plant species/parts and avoiding others, which will undoubtedly lead
to knowledge of why they select the diets they do. This research will of necessity
focus on the genetic and environmental determinants of diet selection and will
undoubtedly enhance management (Provenza and Balph, 1987, 1988; Provenza et al„