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Extent and Distribution of Ecological Systems
Ecological Condition Chapter
- Introduction
- Overview
- List of Questions
- Introduction
- Overview
- Discussion
- HD National Trend
- List of Indicators
- References
Extent and Distribution of Ecological Systems
- Jay Messer
Office of Research and Development
roe@epa.gov
Chapters
- ROE Database Home
- Air
- Water
- Land
- Human Health
- Ecological Condition
What are the trends in the extent and distribution of the Nation's ecological systems?
Ecological systems,7 ranging from forests and watersheds to wetlands and coral reefs, are the foundation of the environment. An ecological system can be defined as a spatially explicit unit of the Earth that includes all of the organisms, along with all components of the abiotic environment, within its boundaries. Ecological systems are not isolated but blend into and interact with other systems. The spatial coverage and arrangement of ecological systems influence the types of animals and plants that are present; the physical, chemical, and biological processes in the system; and the resiliency of the systems to perturbations.8 Ecological systems influence water and nutrient cycles, the building of soils, the production of oxygen, sequestration of carbon, and many other functions important for the health of the planet and people who depend on them.
This section examines trends in the extent and distribution of ecological systems. Extent refers to the physical coverage of an ecological system; it can be reflected as area or percent compared to a baseline or total area. Distribution includes the pattern or arrangement of the components of an ecological system and is dependent on the scale of analysis. For example, the national distribution of forests can be estimated by a percent coverage, but within a stand of trees the distribution may involve patterns of gaps, species, and edge/interior ratios. As noted in Section 6.1.1, ecological systems can be defined by predominant biota, spatial scales, and physical characteristics. Extent indicators typically are based on physical and biological characteristics that are observable by remote sensing, with indistinct boundaries operationally defined according to some scientific or resource management construct.9
As noted in Chapter 1, safeguarding the natural environment is an integral part of EPA’s mission. EPA traditionally has been most concerned with maintaining the quality of air, water, and land necessary to support balanced biological communities and the processes that support them; however, the success of these efforts requires that ecological systems not be altogether lost or fragmented. The potential influences of pollutants on the extent and distribution of ecological systems are a prime concern, and, in turn, the extent and distribution of ecological systems have far-reaching influences on air and water quality.
Apparent trends in extent and distribution of ecological systems depend on the temporal and spatial scale of assessment. For this reason, both National and Regional Indicators are particularly valuable. Temporal changes occur naturally over long time scales, such as those associated with geological and climatological forces (e.g., glaciation). Change can also occur more quickly as a result of direct shifts in land use (e.g., forest to development and historical filling of wetlands), alterations of nutrient and hydrological cycles (e.g., dam removal), introduction of invasive species (e.g., Asian carp), pollutant exposure (e.g., acid rain), or extreme weather events, which all act over comparatively short time periods. Thus, trends can be the result of natural forces or may be accelerated by human activity.
The spatial scale of alterations also represents a significant factor in tracking ecological condition. Alterations that are short in duration and local in nature (e.g., seasonal droughts or a windfall in a closed forest canopy) may not have large-scale or lasting effects on ecological systems. Alterations that are chronic in nature and occur over large areas may affect entire ecosystems over long periods of time, especially if they affect soil formation, microclimate, refugia for recolonizing species, etc. Particularly relevant discussions of the importance of scale in ecological processes, monitoring, and management can be found in a number of relatively recent publications.10,11,12
Different regions and different ecological systems respond to stressors in different ways, resulting in unique regional distributions of species and habitats. The result is that across any slice of landscape the extent and distribution of ecological systems may shift.13 In the case of habitat loss, large impacts may occur and the extent of coverage may be reduced or eliminated altogether. More subtle changes in ecological systems can occur that are not captured in simple metrics of extent and distribution. These changes are discussed in later sections of this chapter.
Fragmentation, the division of previously uninterrupted habitat, can have either negative or positive impacts on communities.14 Examples of fragmentation include building highways through a forest, damming a river in a manner that limits migration of fish, or developing waterfronts in a manner that splits apart bordering marshlands. Fragmentation and the increasing area of edge habitat may force migrating species to find new transport corridors, may allow new species (e.g., competitors, pathogens, weeds) to enter areas previously blocked from immigration, and in some cases may actually increase biodiversity.15 Regardless of the impact, fragmentation likely will result in shifting distributions of species.
Trends in ecological system extent and distribution are highly dependent on the evaluation scale. At one scale, coastal wetlands may appear to be uninterrupted and uniform. However, at a more refined scale, edges, patches, corridors associated with tidal creeks, and discontinuous distributions of species become evident. Defining systems in terms of local organization or predominant species facilitates discussion and analysis, but may also obscure the important linkages among systems across landscapes. Therefore, while it is helpful to discuss trends in the extent and distribution of systems such as wetlands or forests, each system is tied into global water, nutrient, carbon, and energy cycles.
The indicators discussed in this section fall into three broad categories: indicators of the extent and distribution of forests, indicators of the extent and distribution of wetlands, and indicators of land use.