Definitions

Acid Deposition: A complex chemical and atmospheric phenomenon that occurs when emissions of sulfur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from the original sources, and then deposited on earth in either wet or dry form. The wet forms, popularly called "acid rain," can fall as rain, snow, or fog. The dry forms are acidic gases or particulates.

Assessment: A study designed to estimate or determine the significance and effects of factors or events

Biodiversity: The variety and variability among living organisms and the ecosystems in which they occur. Biodiversity includes the number of different items and their relative frequencies; these items are organized at many levels, ranging from complete ecosystems to the biochemical structures that are the molecular basis of heredity. Thus, biodiversity encompasses expressions of the relative abundances of different ecosystems, species, and genes.

Community: The assemblage of populations of plants and animals that interact with each other and their environment. The community is shaped by populations and their geographic range, the types of areas they inhabit, species diversity, species interactions, and the flow of energy and nutrients through the community.

Correlation Analysis: An analysis of the relationship of one variable to another.

Discrete Variable: A variable for which the values are not observed on a continuous scale because of the existence of gaps between possible values. Examples are the number of petals on a flower or the number of insects caught in the sweep of a net.

Disturbance: Any event, such as a forest fire or insect infestation that alters the structure, composition, or functions of an ecosystem.

Ecological goods and services: The direct and indirect benefits received from ecological processes such as filtered water and air, pollination of crops, land for recreation, nutrient cycling, raw materials, food production, soil erosion control and soil formation.

Ecological system: The most complex level of organization is the ecological system. An ecosystem includes the plant and animal communities in an area together with the non-living physical environment that supports them. Ecosystems have physically defined boundaries, but they are also dynamic: their boundaries and constituents can change over time. They can import and export materials and energy and thus can interact with and influence other ecosystems. They can also vary widely in size.

Ecology: The relationship of living things to one another and their environment, or the study of such relationships.

Ecoregions: A large area whose boundaries are fixed by geography, topography, climate, vegetation, and other easily recognized natural patterns of the landscape. Ecoregions contain many landscapes with different spatial patterns of ecosystems.

Edge Habitat: The outermost belt (ranging from a few to several hundred feet) encompassing a patch that has an environment very different from the interior of a patch.

Effector: A physical, chemical or biological factor that indirectly causes stress. It indicates a possibility for future stress yet it does not directly cause stress.

Endpoints: A technical term used to describe the environmental value that is to be protected. An environmental value is an ecological unit and its characteristics. For example, salmon are valued ecological units; reproduction and age class structure are some of their important characteristics. Together "salmon reproduction and age class structure" form an endpoint.

Exposure: The contact or co-occurrence of a stressor with an ecological unit.

Forest productivity: The conversion of light energy and carbon dioxide into living organic material often measured by forest inventories by species.

Geographic Information Systems: A GIS is a system of hardware and software used for storage, retrieval, mapping, and analysis of geographic data. It is a computer technology that brings together all types of information based on geographic location for the purpose of query, analysis and generation of maps and reports. GIS is both a database designed to handle geographic data as well as a set of computer operations that can be used to analyze the data. In a sense, GIS can be thought of as a higher order map.

Hydrologic Unit: A member of the hierarchical system for identifying and subdividing river-basin units of the United States. Hydrologic units are used for the collection and organization of hydrologic data. The levels of the hierarchy, listed in order of largest to smallest in area, are: region, subregion, accounting unit, and subbasin. region — Mid-Atlantic region subbasin — The fourth level of subdivision of hydrologic units. A subbasin represents the geographic area of part or all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature. Subbasins are uniquely identified with an eight-digit hydrologic unit code. - subregion — Upper Chesapeake. accounting unit — Upper Chesapeake. Delaware, Maryland,Virginia, and Pennsylvania. subbasin — Chester-Sassafras. Delaware, Maryland, Pennsylvania. Each hydrologic unit is identified uniquely with a hydrologic unit code.

Hydrologic unit code (HUC): A hierarchical, numeric code that uniquely identifies hydrologic units. The first two digits identify the region, the first four digits identify subregions, the first six digits identify accounting units, and the full eight digits identify subbasins. For example, from the example provided with the definition of hydrologic unit, the hydrologic unit codes are:

• 02 - the region (Mid-Atlantic)
• 0206 - the subregion (Upper Chesapeake)
• 020600 - the accounting unit (Upper Chesapeake. Delaware, Maryland,Virginia, and Pennsylvania.)
• 02060002 - the cataloging unit or subbasion (Chester-Sassafras. Delaware, Maryland, Pennsylvania.)

Hydrography: The description, study, and mapping of the waters of the Earth's surface (the seas, lakes and rivers) including their forms and physical features.

Index: A number or measurement that serves to guide, point out, or otherwise facilitate reference.

Interior Habitat: Habitat necessary for insulation from edge effects (e.g., noise, wind, sun, predation) which occurs within the interior of a patch.

Land Cover: Anything that is visible from above the Earth's surface. Examples include vegetation, exposed or barren land, water, snow, and ice.

Land Use: The way land is developed and used in terms of the kinds of anthropogenic (human-induced) activities that occur (e.g., agriculture, residential uses, industrial uses).

Land use planning: The process of organizing the use of lands and their resources to best meet people's needs over time, according to the land's capabilities.

Landscape: A large land area composed of interacting ecosystems that are repeated due to factors such as geology, soils, climate, and human impacts.

Landscape Metric: A characteristic of the environment that is measured to provide evidence of the biological condition of one or more resources at the ecosystem level.

Migratory bird stopovers: Crucial areas where birds feed and rest for a short period of time during seasonal bird movements.

Model: A representation of reality used to simulate a process, understand a situation, predict an outcome, or analyze a problem.

Natural state: The set of conditions under which an ecosystem evolved.

Non-indigenous species: A nonnative plant and/or animal species introduced into a region that then overwhelms and crowds out native species, degrades habitats, and contaminates the gene pools of indigenous species.

Non-point source pollution: Pollution whose source is not specific in location. The sources of the discharge are dispersed, not well defined, nor constant. Rainstorms and snowmelt often make this type of pollution worse. Examples include sediments from construction sites and chemical-bearing runoff from road surfaces and agricultural fields.

Parameter: A quantity that characterizes a statistical population and that can be estimated by calculations from sample data.

Pathogens: Microorganisms (e.g., bacteria, viruses, or parasites) that can cause disease in humans, animals and plants.

Population: An aggregate of individuals of a species within a specified location in space and time.

Quality of life: The level of enjoyment and fulfillment derived by humans from the life they live within their local economic, cultural, social, and environmental conditions.

Receptor: A receptor is an ecological entity exposed to a stressor. This term may refer to tissues, organisms, populations, communities, and/or ecosystems. While either "ecological component" (U.S. EPA, 1992a) or "biological system" (Cohrssen and Covello, 1989) are alternative terms, "receptor" is usually clearer in discussions of exposure where the emphasis is on the ecological entity exposed to the stressor.

Reflectance: A measure of the ability of a surface to reflect light or other electromagnetic radiation.

Region: In the ReVA context, a large, multi-state geographic area corresponding to an EPA region such as the Mid-Atlantic, Northeast, Southeast or Pacific Northwest regions within the United States.

Relative vulnerability: The vulnerability of an ecological entity in relation to another ecological entity.

Risk: A measure of the probability that damage to human life, health, property, and/or the environment will occur as a result of a given hazard.

Risk management: The process of evaluating and selecting alternative regulatory and non-regulatory responses to a given hazard. It requires the consideration of legal, economic, and behavioral factors.

Runoff: The flow of water, usually from rainfall and snow melt, which is not absorbed into the ground. It flows across the land and eventually runs to stream channels, lakes, oceans, or depressions or low points in the Earth's surface. The characteristics that affect the rate of runoff include rainfall duration and intensity as well as the ground's slope, soil type, and ground cover. Runoff can pick up pollutants from the air and land and carry them into streams and lakes.

Scale - The spatial or temporal dimension over which an object or process can be said to exist, as in, for example, a landscape, forest ecosystem or community.

Spatial variability: The different ways things are arranged on a map. For example, the number, size, shape and distance between patches, can describe the pattern of forest patches. The spatial pattern exhibited by a map can also be described in terms of its overall texture, complexity, and other indicators.

Stressors: A physical, chemical or biological factor that can disrupt, change or otherwise alter ecosystem health and human health in a negative way. For example, pesticides used in agriculture are stressors both to ecosystem health and human health.

Synergistic effects: The effect of several variables whose combined effects are more than the sum of each individual effect.

Threshold: An ecological threshold can be defined as a condition beyond which there is an abrupt change in a quality or property of the ecosystem. Previous research has established that ecosystems often do not respond to gradual change in forcing variables in a smooth way. Instead, they respond with abrupt, discontinuous shifts to an alternative state as the ecosystem exceeds a threshold in one or more of its key variables or processes. It is the point at which an effect can be seen.

Watershed: A watershed is defined as an area of land that is drained by a single stream, river, lake, or other body of water. Ridges form the dividing lines between watersheds. Water on one side of the ridge flows into one stream, while water on the other side may flow into a different stream. Thus, watersheds are a natural unit defined by the landscape.

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