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Report on the Environment

Diversity and Biological Balance

What are the trends in the diversity and biological balance of the Nation's ecological systems?

Trends in the biological diversity of the nation’s ecological systems can be viewed in terms of both the numbers of species present in an ecological system and the extent to which some of the species are threatened or endangered. “Biological balance” refers to the interrelationships among organisms, including the structure of food webs and the ability of ecological systems to maintain themselves over time. Balance is a dynamic characteristic rather than a fixed state.

The biological diversity and balance within ecological systems are often used to judge the health of the system, and their reduction often represents a response to pollutants or other stressors. Restoring biodiversity and biological balance has been a focus of EPA’s attention over the past three decades. Reversing declines of species such as the brown pelican (caused by pesticides) and brook trout (caused by acid rain), replacing nuisance algal blooms caused by excess nutrients with balanced communities of phytoplankton, replacing beds of sludge worms below wastewater discharges with balanced communities of benthic invertebrates, and restoring biological communities previously decimated by improper handling of toxic and hazardous wastes are well-known examples.

The significance of biological diversity also stems from the fact that, for many people, biological diversity contributes to the quality of life.19 Everyone recognizes the importance of species as commodities (if those species produce products that can be bought and sold), and some argue that species have moral value in and of themselves.

Diversity and biological balance are also of interest because of how they may influence the functioning and stability of ecological systems.20,21 While scientists debate the exact relationship between the diversity and the functioning and stability of ecological systems, it is generally agreed that as the number of species in any particular type of ecological system declines, there is a potential loss of resilience within that system.22 It is also recognized that these relationships are not straightforward and can vary in degree depending on the types of species introduced to or removed from a system.23

Diversity and balance have important time and space components. Diversity arises over time as adaptation results in new species that fill available niches in the environment. This is a dynamic process involving colonization, evolution of species adapted to new conditions, and extinction of species that are less well adapted to a changing environment. This process has occurred over thousands or millions of years over large geographic areas, punctuated occasionally by events such as large meteor impacts, periods of intense volcanism, and ice ages. Ecological systems that are stable in the short term evolve into different systems in the long term. Disturbances that reduce biological diversity or disrupt balance on a small scale may not have an effect on a larger scale or over longer time periods.

Changes (decreases and increases) in biological diversity have likely occurred throughout the history of the U.S. in response to regional land use changes, water management, intentional and unintentional introductions of species, and environmental pollution. Other changes in diversity and the composition of the biological community can be rapid and dramatic. Introduced plants and plant pathogens can rapidly transform landscapes as some species, such as the American chestnut, are lost and others, such as kudzu, thrive. 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.24 Declining sea otter populations led to loss of kelp forests, as sea urchins formerly preyed upon by otters grazed the kelp down to the sea floor.25 The decimation of grazers such as the American Bison or predators such as grizzly bear or wolves has had cascading impacts on upland vegetation, wetlands, fish, and other species.26 Toxic chemical pollution can create wastelands where only the most resistant species can survive, and nutrients and acid rain have had indirect effects on diversity and balance by causing sweeping changes in the chemical habitat.

Indicators of diversity and biological balance incorporate information about primary producers and invertebrate and vertebrate consumers, especially keystone species that play critical roles in structuring habitat or serve major roles as primary producers, top predators, or important prey species. Indicators of invasive species are also important with respect to assessing trends in diversity and biological balance because these species can alter the nation’s ecological systems by displacing indigenous species, potentially changing the structure of biological communities.

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