A term that has been widely used since the United Nations Conference on Environment and Development in 1992. Biodiversity refers to the totality of species that make up an ecosystem, the integrity and sustainability of which is determined by the diversity of the species that make it up. All species are interrelated, and protecting individual species is often futile and pointless.

Biodiversity is the variety of life in all its manifestations and an indicator of the complexity of a biological system, the heterogeneity of its components. Biodiversity is also understood as diversity at three levels of organisation: genetic diversity (diversity of genes and their variants - alleles), species diversity (diversity of species in ecosystems) and finally ecosystem diversity, i.e. the diversity of ecosystems themselves.

Robert Whittaker's work proposed an organisation of levels of ecosystem diversity and investigated the dependence of biodiversity on environmental factors. According to his ideas, a distinction is made between

alpha diversity - diversity within a community

beta diversity - diversity between communities

gamma diversity - diversity of the supra-cenotic system along environmental gradients.

Subsequently, these ideas have been developed and a number of different classifications have been proposed. All this typological diversity boils down to two types of diversity: inventory diversity, i.e. diversity within a biosystem, and differentiation diversity, i.e. diversity between biosystems. Inventory diversity is usually assessed using unary indices (e.g. diversity measures), while differentiation diversity is assessed using n-ary (often binary) measures.

The level of biodiversity, both within a species and within the biosphere as a whole, is recognised in biology as one of the main indicators of the viability (survivability) of a species and an ecosystem as a whole, and has been termed the 'biodiversity principle'. Indeed, when the characteristics of individuals within a species are very similar (this applies to humans, plants and micro-organisms alike), any significant change in external conditions (weather, epidemics, changes in food, etc.) will have a more critical effect on the survival of the species than in the case of a greater degree of biodiversity. The same applies (on another level) to the richness (biodiversity) of species in the biosphere as a whole.

Biodiversity is a key concept in the conservation discourse. This definition has become an official legal definition as it has become part of the UN Convention on Biological Diversity, which has been adopted by every country in the world except Andorra, Brunei, the Holy See, Iraq, Somalia and the USA. The UN has established an International Day for Biological Diversity.

It is difficult to objectively define the need to conserve and maintain biodiversity, as it depends on the perspective of who is assessing the need. Nevertheless, there are four main reasons for conserving biodiversity:

From the consumer's perspective, elements of biodiversity are natural treasures that already provide visible benefits to people today or may prove useful in the future.

As such, biodiversity has both economic and scientific benefits (e.g. in the search for new medicines or cures).

The decision to conserve biodiversity is an ethical one. Humanity as a whole is part of the planet's ecological system and depends on its well-being, and should therefore respect the biosphere.

The importance of biodiversity can also be characterised in terms of aesthetics, essence and ethics. Nature is celebrated and honoured by artists, poets and musicians throughout the world; for human beings, nature is an eternal and enduring value.