Biodiversity can have three general meanings. (see http://www.usaid.gov/our_work/environment/biodiversity/ for more information) First, it can be used to describe the complexity of biological species that populate a specific ecosystem. At a macro level, biodiversity is influenced by the natural environmental features found within an ecosystem. These features may include lakes, oceans, forests, mountains, deserts, and swamps. Within these macro-environmental features there usually exists overlapping or distinct micro-ecosystems. A micro-ecosystem is defined by the diversity of species living within it, and how this species population differs from what’s found in neighboring micro-ecosystems. Micro-ecosystems emerge in nature because such things as temperature patterns, soil composition, moisture levels, altitude, and sunlight exposure can vary considerably over short distances within a macro-ecosystem. The physical characteristics of macro- and micro-ecosystems therefore determine biological diversity.

Biodiversity also describes the complexity of interactions that occur between biological species populating macro- and micro-ecosystems. For example, on the eastern bank of a river there may be three types of salamanders dining on 50 different species of insects, while on the west bank there may only be two salamander types dining on 30 different species of insects. The different number of salamander types on each bank of the river could be due to differences in food availability. Biodiversity can therefore be used to describe how complex causal interactions are between species within an ecosystem, interactions that ultimately influence species survival.

The word biodiversity can also be used to describe the genetic complexity within a species. For example, there are 521 different species of salamanders in the world and 5093 sub-species. The species Ambystoma tigrinum, or Tiger Salamander, ranges widely across the United States and has six distinct subspecies that can be distinguished by pigment patterns and colors. By comparison, the species Ambystoma barbouri, or the Streamside Salamander, does not have any subspecies and resides in a relatively limited area along the Ohio River. The genetic diversity that allows Tiger salamanders to survive in habitats as diverse as the Sonoran Desert or the upper Great Lakes region would be predicted to be much greater than the genetic diversity of Streamside Salamanders residing in a fairly homogenous ecosystem.