Science unravels spiders' monstrous food web

The world's spiders eat 400-800 million tonnes of insects every year -- as much meat and fish as humans consume over the same period, a study said Tuesday.

In the first analysis of its kind, researchers used data from 65 previous studies to estimate that a total of 25 million metric tonnes of spiders exist on Earth.

Taking into account how much food spiders need to survive, the team then calculated the eight-legged creatures' annual haul of insects and other invertebrates.

"Our estimates... suggest that the annual prey kill of the global spider community is in the range of 400-800 million metric tons," they wrote in the journal The Science of Nature.

This showed just how big a role spiders play in keeping pests and disease-carriers at bay -- especially in forests and grasslands where most of them live.

"We hope that these estimates and their significant magnitude raise public awareness and increase the level of appreciation for the important global role of spiders," the study authors wrote.

For context, the study points out that humans consume about 400 million tonnes of meat and fish every year, while whales feed on 280-500 tonnes and seabirds about 70 million tonnes of seafood.

There are some 45,000 known spider species, all of them meat-eating.

And the critters can travel far to feed, swinging from place to place on silken threats that allow them to cover up to 30 kilometres (19 miles) in a day.

Spiders are found everywhere from the Arctic to the most arid of deserts, in caves, on ocean shores, sand dunes and flood plains, the study authors said.

Aside from being masterly hunters, spiders serve another important role -- as food.

More than 8,000 species of birds, other predators and parasites feed exclusively on spiders, said the researchers.