How fast species are being lost · Biology Class 11–12 | StemPy Lab
Course 31.2·Lesson 1 of 3
How fast species are being lost
Species have always disappeared, but now they disappear far faster than before. This lesson gives the mass extinctions, the present rate, the numbers that NCERT quotes for the extinct and the threatened species, and what the loss of species does to an ecosystem.
about 30 min1 simulation1 practice question2 Python programs5 questions to check yourself
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Extinction is not new
Look at a fossil of a trilobite. The animal is gone, and so are millions of species that lived before us. Extinction is the end of a species. It has always happened. So why do ecologists worry now?
Five mass extinctions, and a sixth
Since life began and spread over the Earth, there were five episodes of mass extinction. In each, many species were lost in a short time. NCERT says that a sixth extinction is in progress now. It is different in two ways.
The present rate is 100 to 1000 times faster than it was before humans.
Human activities are responsible for the faster rate.
Ecologists warn that if the present trends continue, nearly half of all the species on the Earth might be wiped out within the next 100 years. This is a prediction, and not a measurement.
Some numbers from NCERT
Here are the figures that NCERT gives for the last 500 years.
784 species have gone extinct. Of these, 338 are vertebrates, 359 are invertebrates and 87 are plants.
Well known cases are the dodo of Mauritius, the quagga of Africa, the thylacine of Australia and the Steller's sea cow of Russia.
Three subspecies of tiger also went extinct. They are those of Bali, Java and the Caspian region.
In the last 20 years, 27 species have gone extinct.
A bar chart of the species that went extinct in the last 500 years, 784 in all. Vertebrates 338, invertebrates 359 and plants 87. The vertebrate and invertebrate bars are nearly equal, and the plant bar is the shortest.
Today, over 15,500 species of the world face the threat of extinction. The share of each group that is threatened is high.
12 per cent of all bird species.
23 per cent of all mammal species.
32 per cent of all amphibian species.
31 per cent of all gymnosperm species.
A bar chart of the per cent of the species of four groups that face the threat of extinction. Birds 12 per cent, mammals 23 per cent, amphibians 32 per cent and gymnosperms 31 per cent. The amphibian and gymnosperm bars are the tallest.
The numbers of this kind change from one edition of a book to the next. Use the figures of your own textbook.
What the loss does to a region
When biodiversity falls in a region, NCERT lists three results.
A decline in plant production.
A lower resistance to environmental troubles such as drought.
More variation in the working of the ecosystem, such as plant productivity, water use, and the cycles of pests and diseases.
A model of loss
This part is not in the NEET syllabus, but A-Level and IB use rate models.
Suppose a community lost half of its species in 100 years at a steady yearly rate. The rate is not 0.5 per cent a year. The loss acts on the species that are left, so each year a fixed share is kept. The share kept is 0.51/100=0.9931. So the loss each year is about 0.69 per cent.
You should find that the curve is steeper at the start and flatter later, because the loss acts on a smaller number.
Try it in Python
The first program adds up the extinct species of NCERT and finds their shares. The second finds the yearly rate of the model.
Practise with new numbers
Common mistakes
Saying that there have been six mass extinctions. There were five in the past, and the sixth is in progress.
Mixing up the rate. The present rate is 100 to 1000 times faster than before humans, not slower.
Thinking that the loss is mainly of mammals. Of the 784 extinct species, more are invertebrates than vertebrates.
Reading the share threatened as the share extinct. The 12, 23, 32 and 31 per cent are for threatened species.
Remember these
Five mass extinctions in the past. The sixth is in progress, 100 to 1000 times faster, and caused by humans.
Last 500 years: 784 extinct (338 vertebrates, 359 invertebrates, 87 plants). Dodo, quagga, thylacine, Steller's sea cow, three tiger subspecies.
Threatened: 12 per cent of birds, 23 per cent of mammals, 32 per cent of amphibians, 31 per cent of gymnosperms.
Loss of biodiversity: less plant production, less resistance to drought, more variation in the ecosystem.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
How many mass extinctions have there been in the past, according to NCERT?
How does the present rate of extinction compare with the rate before humans?
Name three animals that went extinct in the last 500 years, with the place of each.
Which group has the highest share of threatened species in the list of NCERT?
Give two effects of a loss of biodiversity in a region.
References
NCERT (2023). Biology, Textbook for Class XII, chapter 13, Biodiversity and Conservation, section on the loss of biodiversity.
OpenStax Biology 2e (2018), chapter 47, Conservation Biology and Biodiversity, section on threats to biodiversity.