Biological integrity is associated with how "pristine" an environment is and its function relative to the potential or original state of an ecosystem before human alterations were imposed. Biological integrity is built on the assumption that a decline in the values of an ecosystem's functions are primarily caused by human activity or alterations. The more an environment and its original processes are altered, the less biological integrity it holds for the community as a whole. If these processes were to change over time naturally, without human influence, the integrity of the ecosystem would remain intact. The integrity of the ecosystem relies heavily on the processes that occur within it because those determine what organisms can inhabit an area and the complexities of their interactions.
The concept of biological integrity first appeared in the 1972 amendments to the U.S. Federal Water Pollution Control Act, also known as the Clean Water Act. The United States Environmental Protection Agency (EPA) had used the term as a way to gauge the standards to which water should be maintained, but the vocabulary instigated years of debate about the implications of not only the meaning of biological integrity, but also how it can be measured. The first conference about the term occurred in March 1975 called "The Integrity of Water" and provided the first accepted definition of biological integrity (see below). In 1981, EPA assembled a field of experts from the U.S. Fish and Wildlife Service, academia, and its own staff to further refine the definition and identify key indicators to quantitatively measure biological integrity. The conference not only identified a definition, but also methods to evaluate the community, and they established that multiple sites should be used to determine the condition of the environment
Today, the accepted definition is "the capability of supporting and maintaining a balanced, integrated, adaptive community of organisms having a species composition, diversity, and functional organization comparable to that of the natural habitat of the region." This definition was adapted from Frey (1977). The implications of this definition are that living systems have a variety of scales relative to which they exist, that one can quantify the parts that sustain or contribute to a system's functioning and that all systems must be seen in the context of their environments and evolutionary history. This term primarily refers to aquatic environments because the vocabulary is derived from the Clean Water Act, but the concepts can be applied to other ecosystems.
In order to quantify and evaluate the biological integrity of a system, the Index of Biological Integrity (IBI) was created. In this index the baseline biological integrity (its function before human influence) and the current functions of an ecosystem are measured against one another to evaluate how much of ecosystem's function has been preserved. The IBI evaluates the ecosystem by utilizing biosurveys and comparing species richness, indicator taxa, hybrids, and invasive species. IBIs are used primarily to evaluate aquatic ecosystems even though one could technically apply any measurement of biological integrity to any natural ecosystem.
- "The objective of this chapter is to restore and maintain the chemical, physical, and biological integrity of the Nation's waters." Clean Water Act, section 101(a), 33 U.S.C. § 1251(a).
- Ballentine, R.K. and L.J. Guarraia (editors)(1977). The Integrity of Water. Proceedings of a Symposium, March 10–12, 1975, U.S. Environmental Protection Agency, Washington, D.C. EPA Publication No. 832-R-75-103.
- Hughes, R.M., J.H. Gakstatter, M.A. Shirazi.and J.M. Omernik (1982). "An approach for determining biological integrity in flowing waters." Archived June 12, 2010, at the Wayback Machine. Pages 877-888 in T.B. Brown (editor), In Place Resource Inventories: Principles and Practices, A National Workshop. Paper presented at the workshop August 9–14, 1981. Society of American Foresters, Bethesda, MD.
- Karr, J.R. and D.R. Dudley (1981). "Ecological perspectives on water quality goals." Environmental Management 5:55-68.
- Frey, D. 1977. "Biological integrity of water: an historical approach." Archived December 17, 2008, at the Wayback Machine. Pages 127-140 in Ballentine and Guarraia, op. cit.
- Karr, James R. (1981). "Assessment of biotic integrity using fish communities." Archived June 13, 2010, at the Wayback Machine. Fisheries[dead link] 6:21–27.
- Karr, James R. 1991. "Biological integrity: A long-neglected aspect of water resource management." Ecological Applications 1:66–84.