Biodiversity is not distributed uniformly across the Earth but shows strong geographical patterns, with the highest species richness concentrated in the tropics and decreasing toward the poles — a pattern called the latitudinal biodiversity gradient, one of the most consistent and striking ecological patterns known. The tropics between the Tropic of Cancer and Tropic of Capricorn contain approximately 70-75% of all known species despite covering only about 40% of Earth's land area, reflecting the combined effects of higher solar energy availability, more stable climatic conditions over geological time, greater primary productivity supporting more complex food webs, and longer evolutionary history without the disruptions caused by repeated glaciation events that affected temperate and polar regions. Among tropical regions, the Amazon basin of South America contains the greatest biodiversity of any terrestrial ecosystem on Earth, estimated to harbour approximately 10% of all species on the planet, followed by the Congo basin tropical forests of Africa and the tropical forests of Southeast Asia as the three major centres of global terrestrial biodiversity.

The Amazon rainforest, spanning approximately 5.5 million square kilometres across Brazil (60% of the total), Peru, Colombia, Venezuela, Ecuador, Bolivia, Guyana, Suriname, and French Guiana, represents not just the world's largest tropical rainforest but the single most biodiverse terrestrial region on Earth by virtually any measure of species richness. Documented diversity includes approximately 40,000 plant species (including roughly 16,000 tree species — more tree species in a single hectare of Amazon forest than in the entire British Isles), approximately 1,300 bird species (about 15% of all bird species globally), over 3,000 freshwater fish species (greater than the freshwater fish diversity of any other river system), 430 mammal species, 1,000 amphibian species, 400 reptile species, and an almost incomprehensible diversity of insects and invertebrates estimated to reach millions of species. This extraordinary biodiversity arises from the convergence of several favourable factors: the vast continuous area of suitable habitat (one of the most important predictors of species richness), consistently warm and humid climate throughout the year supporting high primary productivity, and millions of years of relatively stable environmental conditions during which continuous evolutionary diversification has occurred, with the complex heterogeneous Amazon landscape — including diverse soil types, flooding regimes, microhabitats, and elevational gradients — further promoting ecological specialisation and speciation.

Africa's tropical forests, concentrated in the Congo basin of central Africa (the world's second largest tropical forest after the Amazon), West Africa, and East African coastal forests, represent the second major global centre of tropical forest biodiversity. The Congo basin alone covers approximately 3.4 million square kilometres of continuous tropical forest, second only to the Amazon. African tropical forests contain substantial plant diversity (estimated 30,000+ plant species across the continent), significant mammal diversity including the great apes (chimpanzees, bonobos, gorillas — humanity's closest living relatives, found only in African tropical forests), forest elephants, okapis, and many endemic species. African savannas add additional biodiversity beyond forests, with the famous large mammal diversity of the African savanna (elephants, lions, wildebeest, zebras) representing a globally unique and extraordinarily diverse megafauna assemblage, explaining Africa's overall very high biodiversity ranking despite the Amazon's greater forest biodiversity.

India, despite ranking 4th in the listing given (b, after Amazon, Africa, and Central America), contains exceptionally high biodiversity for its relatively modest land area, supporting approximately 7-8% of all known species (45,000 plant species, 90,000 animal species) despite covering only 2.4% of global land area. India has two of the world's 36 biodiversity hotspots — the Eastern Himalaya (part of the Indo-Burma hotspot) and the Western Ghats + Sri Lanka hotspot — both characterised by extremely high plant endemism. India is among the 17 mega-diverse countries identified as containing an disproportionate share of global biodiversity. Conservation challenges in India include habitat loss due to agricultural expansion and urbanisation, human-wildlife conflict, overexploitation, invasive species, and climate change impacts, with approximately 132 species listed as Critically Endangered on the IUCN Red List as of recent assessments including vulture species, the Great Indian Bustard, gharial, freshwater dolphins, and several endemic plant species.