How many kinds of living things share the planet with us? Nobody knows, and the honest range of answers spans more than a factor of ten. Scientists have formally described and named a little over two million species. Most estimates of the true total run several times higher. How it is measured shapes what people believe about the state of nature.
What biodiversity means
The word itself is young. The biologist Walter Rosen shortened "biological diversity" to "biodiversity" while planning a 1986 conference in Washington, and E. O. Wilson used it as the title of the book that came out of the meeting in 1988. Biodiversity includes three levels: genetic diversity within species, the diversity of species, and the diversity of ecosystems. Genetic diversity matters because it gives populations the raw material to adapt. Florida panthers, reduced by the early 1990s to a few dozen inbred animals with heart defects and other problems, recovered after eight female pumas from Texas were released in 1995 to bring in new genes.
How many species?
About 2.1 million species have been described. A widely cited 2011 study estimated about 8.7 million species of plants, animals, fungi and other complex organisms, suggesting that most of the world's species have not yet been described, especially in the oceans, in tropical forest canopies and in soil. Microbes are another matter; some estimates run into the billions or more, though such numbers are highly uncertain.
Richness and evenness
The simplest measure is species richness, the number of species in an area. But richness alone can mislead. Imagine two forests, each with ten tree species. In one, the trees are spread fairly equally among the species; in the other, nine of every ten trees belong to a single species. Evenness measures how equally individuals are spread among the species present, and the first forest is far more even. Diversity indices, such as those named after Claude Shannon and Edward Simpson, combine richness and evenness into one number. In 1960 the American ecologist Robert Whittaker also distinguished diversity within a single site, diversity between sites, and diversity across a whole region.
Patterns across the planet
One of the oldest observations in biology is that species richness generally increases from the poles toward the tropics. A single hectare of Amazon forest can hold more tree species than all of Europe. Why this happens is still debated. Proposed explanations include more solar energy, a longer stable history, larger area, and faster rates of evolution in warm climates. No single answer has won.
Area matters too: larger areas hold more species. In 1967 MacArthur and Wilson built this into the theory of island biogeography. The number of species on an island reflects a balance between new arrivals and local extinctions, so large islands near a mainland hold more species than small, remote ones. Wilson and his student Daniel Simberloff tested the theory in the Florida Keys by tenting and fumigating tiny mangrove islands to remove their insects and spiders. Within about a year the islands had been recolonized to roughly their original number of species, though not always the same species. Nature reserves surrounded by farms and cities behave like islands too.
In 1988 the British ecologist Norman Myers proposed focusing conservation on biodiversity hotspots, places rich in species found nowhere else and already heavily damaged. Conservation International now lists 36 such hotspots, covering a small share of the land surface but holding a large share of plant species found nowhere else. Critics note that the scheme centers on plants and may overlook places such as large wildernesses that matter for other reasons.
Tools of the count
Ecologists count with quadrats, small framed plots in which every plant is recorded; with transects, lines walked at set intervals; with mark and recapture, in which animals are tagged, released and caught again; and increasingly with camera traps and sound recorders. A newer method, environmental DNA, detects species from traces of genetic material they leave in water or soil, so a single bottle of river water can reveal the fish living upstream.
Large indices summarize trends. The Living Planet Index tracks thousands of monitored vertebrate populations, and its 2024 report found an average decline of 73 percent since 1970. That figure is often misread. It is an average of changes in population size, not a finding that 73 percent of all animals have disappeared, and critics have shown that a small number of steep declines can pull the average down sharply.
Worked example: counting North America's birds
In 2019 a team led by Kenneth Rosenberg combined decades of bird surveys with weather radar that tracks migrating flocks. They estimated that the United States and Canada had about 2.9 billion fewer breeding birds than in 1970, a drop of roughly 29 percent. Few species had vanished, so richness barely changed. The loss was in abundance, and much of it was among common birds such as sparrows, blackbirds and warblers. Some groups grew: ducks and geese benefited from wetland protection, and birds of prey recovered after DDT was banned. The study shows why measurement choices matter. A count of species would have missed a collapse that a count of individuals made obvious.