Fewer species on Earth than thought

By Melissa Leong 1 June 2010
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Scientists have greatly revised their estimate of the number of spiders, insects and other arthropod species, according to a new study.

SCIENTISTS HAVE GREATLY REVISED their estimate of the number of spiders, insects and other arthropod species, according to a new study. These are the most numerous groups of organisms on the planet.

Previously, scientists thought that between 30 and 100 million species of tropical arthropods — invertebrates with exoskeletons, segmented bodies and jointed limbs — existed in the world.

The new study, led by biologists Dr Andrew Hamilton and Professor Nigel Stork from the University of Melbourne, indicates that as few as two million and as many as seven million species exist. The scientists’ best estimate is that the world has 3.7 million species of arthropods. This takes the total of species on the planet to about 5.5 million.

Extinction rates

The more accurate picture of species numbers provides a better idea of how quickly arthropods may die out, the scientists say. “Extinction rates are typically estimated through knowing the area of habitat that has been lost,” Andrew says. “But to know the absolute number of species that have been lost, we need to know how many are present in the first place.”

Tropical rainforests are home to the overwhelming majority of arthropods, most of which are insects. Spiders, mites, scorpions, millipedes and centipedes are also included in the group.

The findings of the research, published in the current edition of The American Naturalist, highlights the lack of knowledge about arthropods — only about 30 per cent are formally identified.
Modelling numbers

To estimate the number of arthropod species, the scientists calculated the total number of herbivorous beetles (a dominant group that account for about a quarter of all described arthropods) in a canopy in Papua New Guinea and used a technique known as uncertainty modelling, which incorporates different environmental and biological variables.

Previous studies did not include a range of variables, providing an inaccurate appraisal of arthropod numbers. The over-inflated estimate also meant that extinction rates were incorrect.

The new evaluation gives scientists much more reliable information they can now use to predict how many species the world is losing.

“This is just a starting point with lots more room for improvement,” Andrew says. “As more data comes in, we can feed more into the model and generate more accurate results.”

The results suggest that extinction of species is not as prolific as previously thought. “Only about 1000 species of fauna and flora [in the world] have become extinct in the past 200 years,” Nigel says. “The suggestion that we’ve got millions and millions of species and so we are losing millions and millions of them does not really follow.” 

However, the revised number paints a gloomier picture of biodiversity, the scientists say. “Obviously, if we are starting with fewer species, we may be worse off than we thought, and also be reducing the complexity of ecosystems even faster,” Andrew says. “I think it is a bit of a wake up call and people have probably been taking the number of species for granted or given it little thought.”

Biodiversity loss: threat to our health

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