Biodiversity, most simply put, is the variety of life on Earth—everything from genes, to species, to entire ecosystems. Shorthand for “biological diversity,” the concept is most frequently applied to the array of plant and animal species that occur in a particular place, or region. The notion, however, captures not only the diversity of species in an area, but also the genetic variation within those species, as well as the organization of these species into biological communities and the variety of ecosystems across a landscape. Biodiversity conservation must take each of these levels into consideration.
As might be expected of a term that encompasses the vast array of life forms inhabiting the Earth, numerous definitions for biodiversity have been offered. Perhaps the most widely used definition is contained in the Convention on Biological Diversity, the international undertaking that grew out of the 1992 “Earth Summit” in Rio de Janeiro. The Convention defines biodiversity as:
…the variability among living organisms from all sources including, among other things, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems.
Looking across the various definitions that have been proposed, four key concepts
emerge that address different aspects of biodiversity: variety, variability, multiple
biological levels, and sustaining processes.
- Variety. The number of different biological units of interest—for example, the number of distinct plants, animals, and microorganisms occurring on a particular installation, or the types of different ecosystems found across a state or region.
- Variability. The differences both within and among those biological units—for example, the genetic variation within a rare plant population, or the distinctions and differences found across the range of a widespread bird species.
- Multiple biological levels. The different levels of biological organization, including genetic, species, and ecosystem levels. The levels of this hierarchy are occasionally more finely subdivided, and landscape levels are often included.
- Sustaining Processes. The processes that sustain the variety and variability of life forms at these different biological levels. This can include ecological processes, such as the role of fire in maintaining many forest ecosystems, as well as evolutionary processes, such as the gene flow resulting from the dispersal of animals to other populations.
Another widely used framework for characterizing biodiversity distinguishes among three distinct components—composition, structure, and function—that apply at each of the biological levels (genes, species, ecosystems) described above (Noss 1990). In this framework, composition refers to the variety of different entities in a defined area, for example the identity and variety of species in a region, or the types of different ecosystems on an installation. Structure refers to the physical organization or pattern in a system, such as the vertical structuring of a vegetation type (e.g., grassland, shrubland, forest), the organization of populations or subpopulations across a species range, or the pattern of habitat patches across a landscape. Function includes ecological and evolutionary processes such as gene flow, pollination or seed dispersal syndromes, fire or flooding regimes, and energy and nutrient cycling in an ecosystem.
Consistent with these concepts, the DoD Conservation Instruction (DoDI 4715.03) defines biodiversity as:
The variety of life forms and the ecological processes that sustain it. Biodiversity includes the number and variety of living organisms, the genetic differences among them, the communities and ecosystems in which they occur, and the ecological and evolutionary processes that keep them functioning…