A Wealth in Decline

While it is doubtful whether any new species are still being added through speciation, there is no doubt at all about the continuing losses. The biological wealth of our planet has been declining rapidly, and the accusing finger points clearly at human activities.

The scale of past damage is sobering. The colonisation of tropical Pacific Islands by humans alone is said to have caused the extinction of more than 2,000 species of native birds. Closer to our own times, the IUCN Red List (2004) documents the extinction of 784 species in the last 500 years — made up of 338 vertebrates, 359 invertebrates and 87 plants.

Faces We Have Already Lost

Some recent extinctions have become almost symbolic of the crisis. Among them are the dodo of Mauritius, the quagga of Africa, the thylacine of Australia, Steller's Sea Cow of Russia, and three subspecies of tiger — the Bali, Javan and Caspian tigers. The last twenty years alone have witnessed the disappearance of 27 species.

A careful look at these records reveals something important: extinctions across taxa are not random. Some groups are far more vulnerable than others — amphibians, for instance, appear especially prone to extinction. So the loss is not spread evenly across the tree of life; it hits certain branches harder.

Species Under Threat Right Now

Adding to this grim picture is the number of species standing at the edge. More than 15,500 species worldwide currently face the threat of extinction. Broken down by group, the figures are alarming: presently 12 per cent of all bird species, 23 per cent of all mammal species, 32 per cent of all amphibian species and 31 per cent of all gymnosperm species in the world are threatened.

Those percentages are worth committing to memory, because they show at a glance just how widely the danger is spread — from birds and mammals to amphibians and even ancient plant groups like the gymnosperms.

The Sixth Extinction

Fossil records tell us that large-scale loss of species is not new. Over the long history of life on earth — more than 3 billion years — there have already been five episodes of mass extinction, all before humans ever appeared.

So how is the 'Sixth Extinction' now under way any different? The difference lies not in the fact of extinction but in its rate. The current species extinction rate is estimated to be 100 to 1,000 times faster than in pre-human times, and human activities are responsible for this acceleration. Ecologists warn that if present trends continue, nearly half of all the species on earth could be wiped out within the next 100 years.

Why does losing biodiversity in a region matter for the region itself? Three consequences stand out: (a) a decline in plant production; (b) lowered resistance to environmental perturbations such as drought; and (c) increased variability in ecosystem processes such as plant productivity, water use, and pest and disease cycles.

Quick Recap

  • Biodiversity is declining rapidly, driven by human activities; colonisation of tropical Pacific Islands caused the loss of more than 2,000 native bird species.
  • IUCN Red List (2004): 784 species extinct in the last 500 years — 338 vertebrates, 359 invertebrates and 87 plants.
  • Recent extinctions: dodo (Mauritius), quagga (Africa), thylacine (Australia), Steller's Sea Cow (Russia) and the Bali, Javan and Caspian tigers.
  • Extinctions are not random — amphibians are especially vulnerable; more than 15,500 species are threatened (12 per cent of birds, 23 per cent of mammals, 32 per cent of amphibians, 31 per cent of gymnosperms).
  • There have been five mass extinctions before; the current 'Sixth Extinction' differs in rate — 100 to 1,000 times faster than pre-human times.
  • Loss of biodiversity causes declining plant production, lowered resistance to perturbations, and greater variability in ecosystem processes.

Solved Examples — Section 6

Q1. How many species does the IUCN Red List (2004) record as extinct in the last 500 years, and how are they distributed?

Answer: 784 species — 338 vertebrates, 359 invertebrates and 87 plants.

Q2. Name any four recently extinct animals mentioned in the chapter.

Answer: The dodo (Mauritius), quagga (Africa), thylacine (Australia) and Steller's Sea Cow (Russia).

Q3. Which three subspecies of tiger have become extinct?

Answer: The Bali, Javan and Caspian tigers.

Q4. Which vertebrate group appears most vulnerable to extinction?

Answer: Amphibians.

Q5. How does the current 'Sixth Extinction' differ from the earlier five mass extinctions?

Answer: It differs in rate — the present extinction rate is 100 to 1,000 times faster than in pre-human times, and is driven by human activities.

Q6. List the three consequences of losing biodiversity in a region.

Answer: (a) A decline in plant production, (b) lowered resistance to environmental perturbations such as drought, and (c) increased variability in ecosystem processes such as plant productivity, water use, and pest and disease cycles.