Biodiversity describes the variety of living things at multiple organizational levels: genetic, species and ecosystem diversity. Species diversity includes all branches of the evolutionary tree from single-celled prokaryotic organisms, bacteria, and archaea, to the eukaryotic kingdoms: plants; animals; fungi; and protists. To date, there have been about 1.75 million species identified, and new species are discovered every week.

Biodiversity also includes the interactions that connect organisms to each other and to the environment in which they live. Organisms have evolved together to create the intricate webs of life that involve both cooperative (symbiotic) relationships and predator-prey relationships. Biodiversity is, therefore, a much broader concept than the simple collection of species to which it is often reduced.

All living things depend on the existence and activities of other living things. Groups of populations of different species interacting with one another and with their physical environment constitute an ecosystem. Ecosystems themselves are very diverse: for example forests, ponds, deserts, coral reefs, and even the human intestinal flora. Scientists who study biodiversity are not only interested in the number of different species in an ecosystem, but also in how many individuals of each species is present. Studying biodiversity indicates in which ways organisms are interacting with one another and how local human activities or global climate change are affecting the system in question.

The health of a species and its ability to survive in the face of changing environmental conditions is dependent on biodiversity at the genetic level. Small changes in the genetic makeup of specific individuals within a population can allow them to succeed where others may fail. In sexually reproducing species, genetic biodiversity is essential for maintaining the health of the species so that mating can occur between genetically dissimilar individuals. It is well established that inbreeding results in the expression of unfavorable traits, decreasing the fitness of the species. For this reason, maintaining the genetic biodiversity of a species is essential before it reaches a level of critical endangerment.