Spiders devour up to 800 million tonnes of prey every year

Enlarge ImageA house spider hangs out in a web.                  Amanda Kooser  CNET

"Our calculations let us quantify for the first time on a global scale that spiders are major natural enemies of insects".

Sometimes just being a spider is scary enough to insects: In recent years, researchers have shown spiders not only eat bugs, they can also intimidate them to the point where insects eat less in the presence of spiders.

Earth's arachnids devour 400 to 800 million tonnes of prey each year, according to a new study which establishes their critical role in the insect food chain.

With more than 45,000 species and a population density of up to 1,000 individuals per sq. m., they are among the most common and abundant predators in terrestrial ecosystems. Whales feed on 300 to 550 million tons of seafood, while the world's total seabird population eats an estimated 77 million tons of fish and other seafood.

For the study, the researchers used two calculations based on different models.

Meanwhile, humans consume a mere 400 million tons in meat and fish.

But if you're now worrying that the world might be inherited by spiders, remember that in turn, they are a source of prey as well. Furthermore, large tropical spiders occasionally prey on small vertebrates (frogs, lizards, snakes, fish, birds, and bats) or feed on plants.

Further calculations revealed that spiders living in forests and grasslands take up more than 95 percent of the yearly prey kill of its global community.

"Spiders thus make an essential contribution to maintaining the ecological balance of nature". The figure reflects the fact that forests, grasslands and savannas are less frequently disturbed than for instance agricultural or urban areas, and therefore allow for greater spider biomass.

It's hard to estimate the number of spiders in the world. Spiders in these areas catch huge numbers of forest and grassland pests, whereas spiders in desert regions, in the Arctic tundra and in annual crops kill fewer insects in comparison.

Dr. Martin Nyffeler hopes that the study would raise awareness about the importance of these critters in our food web. Some 8,000 to 10,000 other predators and parasites consume spiders exclusively, while up to 5,000 bird species have made these crawlies a part of their diet.

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