Study: Spiders eat more annually than weight of all human adults

Study: Spiders eat more annually than weight of all human adults
A wolf spider preparing to eat some insects (probably)
A wolf spider preparing to eat some insects (probably)
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A wolf spider preparing to eat some insects (probably)
A wolf spider preparing to eat some insects (probably)

Some larger types might opt for a lizard or even a frog, but the great majority of spiders love to eat insects. We mean really, really love to eat insects. So much so, that the world's spiders consume somewhere between 400 and 800 million tons of biomass each year consisting almost entirely of creep crawlies, a new study has shown. That's around twice the combined weight of all the world's adults, in case any arachnophobes were wondering.

With 45,000 individual species spread across the globe, spiders may well be abundant but their nocturnal and secretive nature has made studying their role as prominent predators difficult. Scientists at Switzerland's University of Basel and Sweden's Lund University overcame this by studying the existing literature on spiders and using mathematical models to gauge their ecological impact.

This actually involved two separate methods, each based on separate sets of studies with no overlap between the two. The first is based on spiders' food requirements combined with total spider biomass, while the second is based on spiders' annual prey kill rates, using data from prey censuses in the field combined with web density estimates.

Both of the calculation methods showed that the world's spider population, which itself weighs around 25 million tons, takes out between 400 and 800 million tons of prey each year. More than 90 percent of this food consists of insects, although lizards, snakes, frogs and even bats eaten by larger tropical spiders were taken into account, too.

The team says the large range of the prey kill-estimate is because kill rates vary wildly within certain ecosystems. But still, even at the conservative end, that is a whole lot of bugs for breakfast. The UN estimates that us humans consume around 400 million tons of meat and fish every year, while whales swallow an estimated 280 to 500 million tons.

Another useful revelation from the study was just how much more productive spiders are as hunters in forests and grasslands compared to other habitats. Spiders living in these areas account for more than 95 percent of the annual prey kill, while those hunting in crops kill less than two percent, likely due to the tightly managed lands that are hostile to their survival.

"Our calculations let us quantify for the first time on a global scale that spiders are major natural enemies of insects," says Martin Nyffeler from the University of Basel, lead author of the study. "In concert with other insectivorous animals such as ants and birds, they help to reduce the population densities of insects significantly. Spiders thus make an essential contribution to maintaining the ecological balance of nature."

The work of the researchers has been published in the journal The Science of Nature.

Source: University of Basel

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