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For semi-natural and natural areas, however, there are two
problems. First, species composition is monitored for some
systems (e.g., percent cover of non-native plants in forested
areas), but not for all systems or for all types of species
alterations. Just as important, ecologists have not agreed
on how to classify a particular natural or seminatural area
as disturbed, less disturbed, or undisturbed. Again, measures
exist for some ecosystem types (for example, see Status
of Freshwater Animal Communities) but not for most.
Discussion The biological communities found
on and in much of the nation’s lands and waters today
are very different from those of presettlement times. Much
of this change is the result of deliberate human intervention:
forests have been cleared for farms, streams dammed to form
lakes or to generate power, and land covered with housing
and roads. Clearly, the goods and services derived from these
altered lands and waters differ considerably from those derived
from wilderness areas and other lands in a natural or semi-natural
condition. Until better means are developed to determine what
conditions exist on the two-thirds of U.S. lands that are
natural to semi-natural, both reporting on and interpreting
this indicator will remain difficult.
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