Diversity and Biological Balance
What are the trends in the diversity and biological balance of the nation's ecological systems?
Importance of Diversity and Biological Balance
The health of an ecological system can often be judged by its biological diversity and balance.
- Diversity can be viewed in terms of both the number of species present in an ecological system and the extent to which some species are threatened and endangered. Diversity arises over time as adaptation results in new species that fill available niches in the environment—a dynamic process involving colonization, evolution, and extinction of species.
- Biological balance refers to the interrelationships among organisms, including the structure of food webs and the ability of ecological systems to sustain themselves over time. Balance is a dynamic characteristic rather than a fixed state.
Diversity and biological balance may influence the functioning and stability of ecological systems.1,2 Scientists generally agree that as the number of species in any particular type of ecological system declines, that system can potentially lose its resilience (i.e., its ability to rebound after it has been stressed).3 However, determining the number of species, their types, and their distribution necessary for ecosystems to be resilient has been an exceedingly complex and difficult scientific challenge.4
In addition, for many people, biological diversity contributes to quality of life.5 The diversity of some species contributes to the diversity of important human commodities (e.g., different tree species for different woods, such as pine, walnut, or hickory, or different fish species for food, such as tuna, red snapper, or catfish).
Several factors contribute to species diversity, including habitat diversity, competition among species, and genetic diversity. Genetic diversity within a species not only is necessary to maintain diversity among species,6 but also contributes to the diversity of food, fiber, and medicines available from nature. For many people, “charismatic” species, such as the California condor or American bison, have value in and of themselves.
Biological diversity and balance can be impacted by both natural forces and human activities happening over different time and spatial scales. Changes can occur over millions of years, or be rapid and dramatic.
Ecological systems that are stable in the short term may evolve into different systems in the long term. This process has occurred over thousands or millions of years over large geographic areas, punctuated occasionally by periods of more rapid change due to events such as large meteor impacts, periods of intense volcanism, and ice ages.
Decreases and increases in biological balance and diversity have occurred throughout U.S. history in response to changes in land use, water management, environmental pollution, and intentional or unintentional introduction of new species into ecosystems. For example:
- Introduced plants and plant pathogens can rapidly transform landscapes. The American chestnut tree has been lost because of the accidental introduction of the chestnut blight, while kudzu, which was intentionally introduced, has dramatically overgrown the landscape in the South. Introduction of the sea lamprey to the Great Lakes led to sweeping changes in the entire food chain, from lake trout all the way down to the phytoplankton.7
- Declining otter populations led to loss of marine kelp forests, as the sea urchins that the otters preyed upon grazed the kelp down to the sea floor.8
- The decimation of grazers such as the American bison, and predators such as wolves, has had cascading impacts on upland vegetation, wetlands, fish, and other species.9
- Toxic chemical pollution can create wastelands where only the most resistant species can survive.
- Nutrients and acid rain have had indirect effects on diversity and balance by causing sweeping changes in the chemical habitat.
Disturbances that reduce biological diversity or disrupt balance on a small scale may or may not affect ecological function on a larger scale or over longer time periods.
The ROE presents four national-scale indicators (Benthic Macroinvertebrates in Wadeable Streams, Bird Populations, Coastal Benthic Communities, and Fish Faunal Intactness) and two regional-scale indicators (Non-Indigenous Estuarine Species in Pacific Northwest and Submerged Aquatic Vegetation in Chesapeake Bay) to address the diversity and biological balance question.
Since few national programs track diversity and biological balance, ROE indicators are available for only a limited set of ecosystem and population types. The substantial variation in ecological systems across geographic regions makes it difficult to develop national-scale diversity and balance indicators. Biodiversity, in particular, is challenging to characterize at the national scale.
Invasive species are an important but poorly quantified source of stress to the diversity and balance of native species. The Non-Indigenous Estuarine Species in Pacific Northwest indicator provides some insight into the potential importance of invasive species in that region; however, no indicators that meet the ROE criteria are currently available to capture the full significance of this issue at the national level.
Biological variation is expected at annual, decadal, and even longer time scales. Because of the limited time frames over which observations are made, it is difficult to parse normal fluctuations in diversity and balance from longer-term trends.
 Chapin III, F.S., B.H. Walker, R.J. Hobbs, D.U. Hooper, J.H. Lawton, O.E. Sala, and D. Tilman. 1997. Biotic control over the functioning of ecosystems. Science 277(5325):500-504.
 Wilson, E.O. 1992. The diversity of life. Cambridge, MA: Belknap Press.
 McCann, K.S. 2000. The diversity-stability debate. Nature 405(11):228-233.
 Srivastava, D.S., and M. Vellend. 2005. Biodiversity-ecosystem function research: Is it relevant to conservation? Annu. Rev. Ecol. Syst. 36:267-294.
 Norton, B. 1988. Commodity, amenity, and morality: The limits of quantification in valuing biodiversity. In: Wilson, E.O., ed. Biodiversity. Washington, DC: National Academies Press.
 Lankau, R. A., and S. Y. Strauss. 2007. Mutual feedbacks maintain both genetic and species diversity in a plant community. Science 317:1561-1563.
 Eck, G.W., and L. Wells. 1987. Recent changes in Lake Michigan's fish community and their probable causes, with emphasis on the role of the alewife (Alosa pseudoharengus). Can. J. Fish. Aquat. Sci. 44(Suppl. 2):53-60.
 Estes, J.A., and J.F. Palmisano. 1974. Sea otters: Their role in structuring nearshore communities. Science 185:1058-1060.
 Pritchard, J.A. 1999. Preserving Yellowstone's natural conditions: Science and the perception of nature. Lincoln, NE: University of Nebraska Press.