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Review the indicators
at a glance
America’s ecosystems are enormous, and enormously diverse.
They range from deep ocean trenches to wide grassy plains, from
above the Arctic Circle to the tip of Florida. In this chapter,
rather than focusing on specific ecosystems, as we do in succeeding
chapters, we present ten indicators that describe key characteristics
of the entire array of America’s ecosystems.
These ten core national indicators provide a broad, yet succinct,
description of the condition and use of ecosystems in the United
States. They describe and track changes in key aspects of the area
and configuration of ecosystems, significant chemical and physical
conditions, biological components, and the goods and services that
people derive from these systems. In doing so, they parallel the
indicators presented in the six following chapters, each of which
focuses on a single ecosystem. These chapters also cover ecosystem
area and configuration, chemical and physical properties, biological
components, and human uses, but they do so using a larger number
of indicators that focus on a subset of the nation’s lands
and waters.
What can we say about the condition and use of U.S. ecosystems,
based on these core national indicators?
Partial or complete data are available for seven of the ten core
national indicators. Four of the seven have data from a long enough
period to judge trends, and one uses federal benchmarks to help
readers judge the significance of ecosystem conditions. The three
indicators for which data are not presented require further development.
After the following brief summaries of the findings and data availability
for each indicator, the remainder of this chapter consists of the
indicators themselves. Each indicator presentation offers a graphic
representation of the available data, defines the indicator and
explains why it is important, and describes either the available
data or the gaps in those data.
System Dimensions
The national indicators include two measures of extent and pattern.
The first is the most basic description of the state of our nation’s
lands and waters, the area of each of the component systems and
how they change through time. The second measure, not yet developed,
will describe the intermingling of the various system types across
the national landscape.
- What is the area of the six major ecosystem types? Grasslands
and shrublands and forests each occupy about a third of the area
of the lower 48 states, and farmlands about a quarter. The area
of forest and grasslands and shrublands has declined since European
settlement, as has the area of freshwater wetlands, and the extent
of cropland and urban and suburban areas has grown. More recent
trends show decline in forest, croplands, grassland and shrublands,
and freshwater wetlands, and increases in urban and suburban areas
(Table 4.1).
The area of ecosystems is a very basic characteristic but, for
various reasons, is complex to report. The main reason is that
the area of different ecosystems is often tallied by different
agencies, using different methods and definitions of the systems.
Satellite remote sensing, which can provide an integrated view,
is available at the appropriate scale for only one time period
(1992) and thus cannot provide information on changes in the area
of different ecosystems.Table 4.1 presents estimates from multiple
sources, which means that care must be taken in comparing and
adding data about different ecosystems and in tracking gains and
losses from one system to another.
| Table 4.1.
Core National Extent Measurements (lower 48 states) |
| Ecosystem |
Core National
Extent Measurements |
Area in Millions
of Acres |
Percent of Land Areaa |
Estimated Presettlement Area (as % of Total Land Area) |
Changes from 1950s, Millions of Acres (%)a |
| Grasslands and Shrublands |
Total area
(not including pastures) |
683 |
36% |
52% |
Declining, amount and rate unknown |
| Forests |
Total area |
622 |
33% |
48% |
–6 (–1.0%) |
| Farmlands |
Area of croplands |
455 |
24% |
— |
–23 (–4.8%) |
| Freshwater |
Area of Freshwater
Wetlands |
94 |
5% |
11% |
–11 (–10%) |
| Urban and Suburban areas |
Urban and
suburban lands |
32 |
1.7% |
— |
Increasing, amount and rate unknown |
| Coasts and Oceans |
Coastal
brackish water |
Unknown |
— |
Unknown |
Unknown |
Source: Grasslands and shrublands and urban and
suburban areas: Multi-Resolution Land Characterization Consortium
and the U.S. Geological Survey; data are for 1992. Forests:
USDA Forest Service Forest Inventory and Analysis program;
data are for 1997. Croplands: USDA Economic Research Service
(see
farmlands total cropland
indicator
for estimates from other agencies); data are for 2002. Freshwater
and coastal wetlands: U.S. Fish and Wildlife Service; data
are for mid-1990s. Coverage: lower 48 states.
a This table does not include 100% of lands in the
United States. For example, urban and suburban areas, as defined
in this report, do not include all developed areas (some developed
areas are too small to be considered “suburban” or
“urban”). Thus, declines in the area of forests, grasslands
and shrublands, croplands, and freshwater wetlands are not—nor
should they necessarily be—offset by corresponding gains
in urban and suburban lands. In addition, the area of wetlands
and portions of urban and suburban areas may also be counted
as croplands, forests, or grasslands and shrublands. For these
reasons, the figures in this table should not be added to obtain
an overall estimate of U.S. land area. |
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