TheImprovement ofTropical and Subtropical Rangelands

34 IMPROVEMENT OF TROPIOAL AND SUBTROPIOAL RANGELANDS
and seasonally inundated depressions. Perennial grasses, once dominant
in many areas, are now localized within a predominantly annual
grassland. Growth is confined largely to the seasonal flushes characteristic
of summer therophyte vegetative communities, and grazing
systems are generally based on pastoralism. Populations ofboth wild
and domesticated animals are restricted by temperature, forage, and
available moisture (Schmidt-Nielsen, 1964).
Systems of range classification should be regionally adjusted to
include descriptions ofthe existing vegetation in physiognomic terms,
with subdivisions by species composition.
SOCIAL SYSTEM-ECOSYSTEM INTERACTIONS
Most environmental systems are highly modified by human activity.
Hence, an understanding of the biological and use potential of
these systems benefits greatly from analyses ofenvironmental change
over time (National Research Council, 1981). Such analysis is also
important in defining ecosystems and in identifying cause-effect relationships
that have contributed to changes in the composition and
productivity of these systems.
Indigenous social systems, through selection and adaptation, are
functionally associated with local ecosystems through flows of energy,
material, and information· (Rambo and Sajise, 1984). Changes
in either the social or environmental system result in changes in the
other. Hence, each system must be thoroughly understood ifpositive
change is to be realized. In many, perhaps most, instances, highly
disruptive changes are responses to external stimuli. Many examples
could be cited. For example, the highly regulated land-use systems
of many societies (see the discussion of the AemfJ system in case
study 9, Part II) were commonly transformed into open-access systems
through the imposition of European public-domain law often
combined with land expropriation, a situation that, in many regions,
has led to intense use pressure and severe environmental degradation.
Similarly, colonial era introductions of cattle into inappropriate
areas (such as Zone 5 of the above classificatory system) has led to severe
degradation and zonal compression (National Research Council,
1983b). The fixing ofboundaries, at national and sub-national levels,
has reduced or eliminated strategies of mobility that are crucial to
these areas. In addition, increasing market integration has converted
highly conservative systems of land use into opportunistic systems
that impose greater pressure on available resources. In some cases,