NRCA: A tool for natural resource condition assessment

America’s National Parks protect abundant and diverse natural resources. The excitement of seeing wildlife, the opportunity to touch a glacier, and the serenity of standing in the woods draw millions of visitors into our parks every year. But no natural resource exists independently; rather, they are parts of complex ecosystems that interact with each other and respond to drivers and stressors at various scales. With the resources themselves ranging from microscopic bacteria to canyon-carving rivers, how can park managers assess the status and trend of resources? How can understanding the condition and drivers on resources inform appropriate and efficient stewardship activities?
A great white heron stands among green foliage.
A great white heron (Ardea herodias occidentalis) stands amidst mangroves in Everglades National Park.

NPS Photo

Historically, the National Park Service (NPS) has sought to maintain natural resources in their historical or “pristine” condition. As ecosystems evolve and adapt to large-scale changes, park ecosystems change via natural processes as well. How then can we know if a resource is in “good” condition, if it is fundamentally changed from its condition 100 years ago? Some resources have clear benchmarks for comparison and clear thresholds for identifying impacts. For example, water quality can be easily characterized by chemical properties, and there are state and federal standards for clean water. Amphibian health, for example, may be more difficult to define. Amphibian field data may be sparse, and observation methods may vary by scientist or location. It may be hard to determine trends in the data. How then can we decide if our amphibian populations are in good condition?

Translating these broad concepts of resource condition and trend down to the level of specific, measurable, and management-useful resource condition information is an ongoing challenge. The effort remains a necessary one to pursue because ecological resources can’t be effectively managed without reliable condition-status data. The Natural Resource Condition Assessment (NRCA) Program helps park managers address this challenge with technical guidance and funding for specialized types of condition assessment studies.

Some Natural Resource Condition Assessment (NRCA) projects are more comprehensive providing a state of the resources report. These summary projects are designed to provide an efficient, yet defensible, “status of knowledge” review and update on current conditions, critical data gaps, and selected condition influences for a select number of important park ecological resources. These assessments consider selected park resources and synthesize data about specific measures and indicators that may inform a resource’s overall condition.

  • A gap analysis is provided for resources that lack adequate data for credible evaluations of their current conditions. A literature review, combined with expert input, produces a management-useful summary of the resource: its ecological importance, the general status of knowledge regarding factors influencing conditions, and useful data, indicators, and studies to consider in the future for improving managers’ understanding of the resource.
  • A condition assessment is provided for resources that have adequate data to credibly assess current conditions for one or more indicators of condition of that resource. For each indicator, data for one or more measures are evaluated either qualitatively and/or quantitatively and combined to report a condition rating at the indicator level. Rating statements accompany each color classification, and indicate the combined measures’ qualitative characteristics, and when available, quantitative values, for a range of good to poor conditions for each indicator. In some instances, the indicator of condition is unknown and is depicted in gray. The rating statements need to include logical and defensible criteria for assigning a condition level. A condition rating is not reported for the resource itself due to the complexity of adequately characterizing condition at that level.

Drivers and Stressors: Forces Contributing to Ecosystem Change

What do a coral reef, desert, tropical rainforest, and grassland all have in common? They are all types of ecosystems. Ecosystems are defined by the network of interactions among organisms and between organisms and their environment. Both internal factors (such as food webs, competition, and decomposition) and external factors (such as climate and topography) control ecosystems. Ecosystems are dynamic — they are in the process of recovering from past disturbances and are periodically exposed to new disturbances. In addition to natural disturbances, human-caused disturbances, or stressors, can affect the integrity or health of ecosystems. To assess the condition of resources in an ecosystem, we need to understand the key drivers, ecological processes, and stressors.

Drivers and Stressors are important parts of the conceptual framework used to show ecosystem context in NRCAs and other reports that assess resource condition in parks.So What are Drivers and Stressors?


Drivers are human-related factors/activities and/or natural-system dynamics that help “drive” ecological resource conditions in parks. Drivers tend to be large scale, long term, and not easily controlled or changed. They are influencers of change in natural systems.

Examples of drivers include:
  • Climate change
  • Energy production and mining
  • Biological resource use
  • Human intrusion and disturbance
  • Invasive and problematic species and pathogens
  • Agriculture


Stressors are the specific physical, chemical, or biological disturbances to which park ecosystems and resources respond. They can be foreign or natural to the system and can be positive or negative.

Examples of stressors include:
  • Changes in temperature and precipitation
  • Air pollution
  • Water contaminants
  • Altered hydrology
  • Resource exploration and extraction
  • Accidentally started fire
  • Forest pests
  • Off-road vehicle use (noise and dust)
Leaves blowing in the wind representing resources being influenced by stressors.

How are Drivers and Stressors used?

In an NRCA, we seek to understand important existing, or newly emerging drivers and stressors affecting the resources the park selected to evaluate. Park managers need to understand the current state of their resources and whether conditions are improving, stable, or deteriorating, and they need to know what is responsible for changes. Assessing current conditions and trends of park resources strengthens knowledge to manage or mitigate stressors and impacts. Once condition has been assessed, park managers can respond with management strategies and activities to maintain, improve, or restore desired ecological resource conditions in parks. Such responses might include strategic planning, on-the-ground management actions, partnerships, interpretive/educational programs for visitors, and/or studies or monitoring of park resources.

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Last updated: September 19, 2023


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