The Three Levels of Biodiversity

Biodiversity is the variety of life on Earth, and it is described at three levels: genetic diversity, species diversity and ecosystem diversity. In brief: genetic diversity is the variety of genes within a species, species diversity is the number and abundance of different species in an area, and ecosystem diversity is the variety of habitats and communities. A forest with many tree species, many kinds of insects and many different soil and water habitats is high in biodiversity at all three levels.

Biodiversity underpins food, clean water and a stable climate, so measuring and protecting it is a central task of conservation. This guide explains each level, shows how species diversity is calculated, explains why biodiversity is important, works through a numerical example and corrects frequent mistakes.

What is biodiversity?

The word biodiversity combines “biological” and “diversity”. It includes all living things, from bacteria to whales, and the ecosystems they form. Scientists have described roughly two million species, but the real number is much higher and is not known precisely. Estimates range from several million to many millions, and many species, especially insects, fungi and microbes, remain unnamed.

Biodiversity is not spread evenly. Tropical regions, such as rainforests and coral reefs, hold far more species per area than polar regions. Places with a high number of endemic species that are also severely threatened are called biodiversity hotspots.

What is genetic diversity?

Genetic diversity is the variety of genes within a species or population. Individuals of the same species differ in their alleles, which gives variation in traits such as color, size, disease resistance and tolerance of heat or drought.

This variation is the raw material for natural selection. A population with high genetic diversity is more likely to contain individuals able to survive a new disease or a change in climate. A population with low diversity, such as one that has passed through a bottleneck, is more vulnerable. The cheetah is a commonly cited example of a species with low genetic variation.

Crop plants and livestock also show this level. Thousands of potato varieties are grown in the Andes, and a single disease can be devastating when farmers plant only one variety over a large area.

What is species diversity?

Species diversity combines two ideas. Species richness is the number of different species in an area. Species evenness describes how similar their numbers are. A community with ten species of equal abundance is more diverse than a community with ten species in which one species makes up 95 percent of the individuals.

Ecologists often use the Simpson index of diversity to combine both. One common version is D = 1 − Σ(n/N)², where n is the number of individuals of each species and N is the total number of individuals. The value ranges from 0 to nearly 1, and a higher value means a more diverse community. Some textbooks use a different form, D = N(N − 1) / Σn(n − 1), in which a higher value also means higher diversity. It is important to use the formula that your course specifies.

What is ecosystem diversity?

Ecosystem diversity is the variety of habitats, communities and ecological processes in a region. A landscape with forest, wetland, grassland, river and lake has higher ecosystem diversity than a monoculture farmland.

Different ecosystems support different species and provide different services. Wetlands filter water and store floodwater, forests store carbon, and coral reefs protect coasts. Losing an ecosystem type removes all the species that depend on it, even if each species is not obviously threatened elsewhere.

The three levels compared

Worked example: calculating species diversity

Two meadows were sampled. Each had 100 individual plants from 4 species. Compare their diversity using the Simpson index D = 1 − Σ(n/N)².

1. Meadow A: each fraction n/N is 0.25. Squared, it is 0.0625. The sum over four species is 4 × 0.0625 = 0.25. D = 1 − 0.25 = 0.75.
2. Meadow B: the fractions are 0.85, 0.05, 0.05 and 0.05. Squared: 0.7225, 0.0025, 0.0025 and 0.0025. The sum is 0.7225 + 0.0075 = 0.73. D = 1 − 0.73 = 0.27.

Both meadows have the same species richness, four species, but meadow A is far more diverse because its species are evenly distributed. This shows why counting species alone can be misleading.

Why is biodiversity important?

Biodiversity provides ecosystem services. These include pollination, soil formation, water purification, flood control, climate regulation and the supply of food, timber and medicines. Many drugs were first discovered in plants, fungi or microbes.

Diverse communities tend to be more resilient. If one species declines, another can partly fill its role. Biodiversity also has cultural and ethical value, as many people consider nature worth protecting for its own sake. Scientists warn that the current rate of species extinction is far above the natural background rate, and this concern is covered in articles on threats to biodiversity.

How did the concept develop?

Naturalists such as Alexander von Humboldt in the early nineteenth century described how species numbers change with latitude and altitude. The word biodiversity was popularized in the 1980s, partly through the 1986 National Forum on BioDiversity in the United States and the work of E. O. Wilson.

The Convention on Biological Diversity, agreed at the 1992 Earth Summit in Rio de Janeiro, made the three-level definition part of international policy.

Which mistakes are common?

Mistake 1: Equating biodiversity with the number of species only

Species richness is one component. Genetic and ecosystem diversity, and the evenness of species, also count.

Mistake 2: Thinking that a larger index always means more species

The Simpson index also reflects evenness. A community can have fewer species but a higher index if the species are more evenly distributed.

Mistake 3: Ignoring genetic diversity in rare species

A species may survive in numbers but have so little variation that it cannot adapt. Conservation therefore often tries to preserve genetic variety.

Mistake 4: Using the wrong form of the Simpson formula

Different textbooks use different but related formulas. Use the one given in your course and interpret high or low values accordingly.