Blood lust: Scientists say spiders eat up to twice as much meat as humans | Science| In-depth reporting on science and technology | DW | 15.03.2017
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Blood lust: Scientists say spiders eat up to twice as much meat as humans

Ever been told by your doctor it's time to eat less meat? It's healthier, apparently. And perhaps it's time spiders cut back too. They eat way more meat than humans do.

Love them or fear them, but spiders are hungry little things. All the spiders in the world eat more meat per year than humans do.

Spiders scoff between 400 and 800 million metric tones of insect meat and other "prey kill." That's up to twice as much as people eat. Scientists from Switzerland, Germany and Sweden say the large range accounts for the fact that prey kill varies from ecosystem to ecosystem.

But at the very least, spiders eat the same amount of animal food as we do.

Humans manage 400 million tons of meat and fish per year. Whales, meanwhile, eat an estimated 280-500 million tons of seafood per year.

Extrapolated figures

The scientists estimate the global spider community weighs a combined 25 million metric tons.

They used two methods to extrapolate the figures. The first used published data on the average spider biomass per square meter in seven different environments (biomes) and the food requirements of spiders in those environments. The seven environments included tropical forests, temperate forests, grasslands and savannas, pastures and meadows, cropland, deserts and Arctic tundra. These data were then used to estimate the global annual prey kill.

The second method used published data on the annual prey kill per square meter of spider communities in different biomes. These were likewise extrapolated to give a global estimate.

So there was a lot of guesswork involved - no one counted each individual spider in the world or monitored what they consumed to the nearest calorie. But still the scientists say their findings are significant for the way we view the role of spiders in crop fields. 

The bottom line

Until now, scientists have tended to think spiders play an "essential role as biological control agents of insect pests" in crop fields.

However this study by Martin Nyffeler, which was published in "The Science of Nature," suggests spiders play a relatively small role in pest control. It seems they consume fewer crop pests than we thought.

They appear to play a greater ecological role as "predators of insects in forests and undisturbed grasslands," write the researchers.

About 90 percent of spiders' diet consists of insects and springtails, tiny soil-dwelling organisms. They are also partial to flies, mosquitoes, aphids, leafhoppers, bugs, beetles, butterflies and wasps. Some tropical spiders will eat frogs, lizards, snakes, fish, birds and bats. And then there are those that eat other spiders - some especially after sex.

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