All the varieties of living organisms on earth constitute biodiversity. Three levels of biodiversity have been recognized.

1. Ecological or Ecosystem Diversity

Organisms evolved features which helped them adapt to their surroundings or the ecosystems in which they live. There are different ecosystems and even related organisms living in different ecosystems may differ vastly from each other. For example tortoises are terrestrial and turtles are aquatic. Both are related but differ much especially in their feet.

There is diversity of ecosystems - terrestrial ecosystems include forests, plains, deserts and mountains and aquatic ecosystems are sea, river, pond. Organisms living in these have evolved suitable adaptations. India has very diverse terrestrial and aquatic ecosystems.

2. Species Diversity

Variety of species living in a certain geographical area constitutes species diversity. Individual organisms belonging to a particular species are similar and are able to undergo reproduction to produce fertile offspring. They cannot interbreed with another species. There is an enormous number of species of organisms. It refers to the variety of genes contained within species of plants, animals and microorganisms.

3. Genetic Diversity

Organisms are made of cells and cells in their nuclei contain chromosomes which bear the genes. Genes control the features of a particular species. Genes of individuals belonging to the same species are similar. Every species has a gene pool. Gene pool means all the different kinds of genes found in a species. The gene pool of a species differs from that of another species.