Some ecosystems possess relatively large number of species as compared to others. This is particularly true of tropical forests which exhibit a very rich-biodiversity. For example, the 13.7 km² area comprising the La Selva Forest Reserve in Costa Rica (a tropical forest) contains almost 1500 plant species, which is more than the total number of plant species found in the 243,500 km² area comprising the whole of Great Britain.

Tropical moist forests are believed to be the richest terrestrial ecosystems on earth. In the marine environment, coral reefs also possess extremely rich biodiversity. It is now suspected, however, that the richness of species diversity on sea floor may be equal or even greater than coral reefs.