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Species- or other taxon-based measures of biodiversity, however, rarely capture key attributes such as variability, function, quantity, and distribution—all of which provide insight into the roles of biodiversity.(See ) Ecological indicators are scientific constructs that use quantitative data to measure aspects of biodiversity, ecosystem condition, services, or drivers of change, but no single ecological indicator captures all the dimensions of biodiversity () Ecological indicators form a critical component of monitoring, assessment, and decision-making and are designed to communicate information quickly and easily to policy-makers.Even among the larger and more mobile species, such as terrestrial vertebrates, more than one third of all species have ranges of less than 1,000 square kilometers.
Ecological indicators are founded on much the same principles and therefore carry with them similar pros and cons ( Biodiversity is essentially everywhere, ubiquitous on Earth’s surface and in every drop of its bodies of water.
The virtual omnipresence of life on Earth is seldom appreciated because most organisms are small ( Documenting spatial patterns in biodiversity is difficult because taxonomic, functional, trophic, genetic, and other dimensions of biodiversity have been relatively poorly quantified.
Even knowledge of taxonomic diversity, the best known dimension of biodiversity, is incomplete and strongly biased toward the species level, megafauna, temperate systems, and components used by people.
(See ) This results in significant gaps in knowledge, especially regarding the status of tropical systems, marine and freshwater biota, plants, invertebrates, microorganisms, and subterranean biota.
Biodiversity is the foundation of ecosystem services to which human well-being is intimately linked.
No feature of Earth is more complex, dynamic, and varied than the layer of living organisms that occupy its surfaces and its seas, and no feature is experiencing more dramatic change at the hands of humans than this extraordinary, singularly unique feature of Earth.
For these reasons, estimates of the total number of species on Earth range from 5 million to 30 million.
Irrespective of actual global species richness, however, it is clear that the 1.7–2 million species that have been formally identified represent only a small portion of total species richness.
Functional diversity (the variety of different ecological functions in a community independent of its taxonomic diversity) shows patterns of associations (biota typical of wetlands, forests, grasslands, estuaries, and so forth) with geography and climate known as biomes (see ).
These can be used to provide first-order approximations of both expected functional diversity as well as possible changes in the distribution of these associations should environmental conditions change.