How many species, and where they live

How many kinds of living things are there?

Here is a question that has no sure answer. How many species live on the Earth? Biologists have described and named about 1.5 million. This is the figure that NCERT takes from the IUCN record of 2004. But nobody thinks that this is all.

The estimates

Scientists try to guess the total. The estimates run from 20 to 50 million. A more careful estimate by the ecologist Robert May gives about 7 million. The range is wide because many places, such as the deep sea and the canopy of tropical forests, are still hardly explored.

You should find that, with an estimate of 7 million, a little over a fifth is known. With 50 million, only a few per cent is known.

Who has the most species?

NCERT gives these shares for the species that are described.

• More than 70 per cent of the species are animals.
• Plants, which here include algae, fungi, bryophytes, gymnosperms and angiosperms, are no more than 22 per cent.
• Among the animals, insects are the most species-rich group. They are more than 70 per cent of the animals. Out of every 10 animals on the Earth, 7 are insects.
• The number of species of fungi is more than the number of all the fishes, amphibians, reptiles and mammals put together.

What about the prokaryotes? Biologists are not sure how many species there are. One reason is that the usual species concept is hard to use for bacteria. Another is that many microbes cannot be grown in the laboratory.

Diversity is not the same everywhere

The species are not spread evenly over the Earth. The diversity is highest in the tropics and falls towards the poles. This is the latitudinal gradient. NCERT gives bird species as the example.

• Colombia, near the equator, has nearly 1400 species of birds.
• India, with much of its land in the tropical latitudes, has more than 1200.
• New York, at 41 degrees north, has 105.
• Greenland, at 71 degrees north, has only 56.

The same is true of plants. Take a forest in Ecuador, in the tropics. It has up to 10 times as many species of vascular plants as a forest of equal area in a temperate region. An example of such a region is the middle of the United States.

The richest place on the Earth is the Amazon rain forest in South America. It has more than 40,000 species of plants, 3000 of fishes, 1300 of birds, 427 of mammals, 427 of amphibians, 378 of reptiles and more than 125,000 invertebrates. Scientists think that at least two million insect species there are waiting to be found and named.

Why are the tropics so rich?

Ecologists give three ideas. These are hypotheses, which means that they are explanations that are still being tested.

1. Time. Speciation takes time. Temperate regions were often covered by ice in the past, so their species were wiped out or pushed away. The tropics were undisturbed for millions of years. This gave a long period for new species to form.
2. A stable climate. The tropical environment is less seasonal, and it is more constant and predictable. This promotes the niche specialisation of species and leads to greater diversity.
3. More sunlight. There is more solar energy in the tropics. This means a higher productivity, which may lead indirectly to greater diversity.

Try it in Python

The program uses the numbers above. It finds the animals and the insects among the 1.5 million, and the share that is known for three estimates of the total.

Practise with new numbers

Common mistakes

• Writing that the plants are the largest group. Animals are more than 70 per cent of the described species, and insects are the largest group among them.
• Saying that only 1.5 million species exist. That is the number that is described. The real number is much higher.
• Calling the hypotheses facts. Time, a stable climate and sunlight are ideas that may work together.
• Mixing up the figures of the birds. Colombia is 1400 and Greenland is 56. The higher the latitude, the fewer the species.

Remember these

• Described: about 1.5 million (IUCN, 2004). Estimates: 20 to 50 million, and about 7 million by Robert May.
• Animals: more than 70 per cent. Plants: no more than 22 per cent. Insects: more than 70 per cent of the animals.
• Latitudinal gradient: the diversity is high in the tropics and falls towards the poles.
• Birds: Colombia 1400, India more than 1200, New York 105, Greenland 56.
• Three hypotheses: a long time for speciation, a less seasonal and predictable climate, and more solar energy.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. About how many species have been described, and why is the real number thought to be higher?

2. Which group has the largest number of described species?

3. Colombia has 1400 bird species and Greenland has 56. About how many times as many?

4. Give the three hypotheses for the high diversity of the tropics.

5. Which place has the greatest biodiversity on the Earth, according to NCERT?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 13, Biodiversity and Conservation, sections on how many species there are and on patterns of biodiversity.
• OpenStax Biology 2e (2018), chapter 47, Conservation Biology and Biodiversity, section on biodiversity.
• Campbell Biology, 12th edition (2021), Pearson, chapter 54, Community Ecology, section on latitudinal gradients of diversity.