Knowing the condition of natural resources in national parks is fundamental to the National Park Service's ability to manage park resources unimpaired for the enjoyment of future generations. The challenge of protecting and managing a park's natural resources requires a broad-based knowledge of the status and trends of park resources and takes an ecosystem approach. Most parks are open systems, vulnerable to threats such as air and water pollution and invasive species, which originate outside the park's boundaries. Understanding the dynamic nature of park ecosystems and the consequences of human activities is essential for management decision making aimed to maintain, enhance, or restore the ecological integrity of park ecosystems and to avoid, minimize, or mitigate ecological threats to these systems.