Hungry, hungry spiders eat more than 400 million tons of insects a year

In his 1958 book "The World of Spiders," British arachnologist William Bristowe made a jarring speculation: The weight of the insects killed each year by Britain's spiders exceeded the weight of Britain's total human population.

One of that book's readers, Martin Nyffeler, went on to pursue a career in arachnology, and has just found that Mr. Bristowe may have been on to something. In a new study, he and other scientists have estimated that spiders kill 400 million to 800 million tons of insects annually, more than the 400 million tons of meat and fish that humans put away each year.

While their work doesn’t confirm Bristowe’s proposal, it makes a similar comparison – and Dr. Nyffeler, now at the University of Basel in Switzerland, credits his British predecessor with inspiring this study, and the decades of work that made it possible.

"These 40 years of gathering experience – spending thousands of hours dealing with spider prey capture rates and prey selection – was needed to be able to write this paper on the global annual prey kill of the spiders," he told the BBC in an email.

Despite Nyffeler’s diligence, spiders’ small size, reclusive habits, and global spread ruled out a Biggest Loser-style look at their eating habits. Instead, he and his colleagues turned to data, drawing on 65 previous studies to estimate that there were 25 million metric tons of spiders on the planet.

With this figure in hand, they then calculated spiders’ total prey using two different methods. The first combined data on the average spider biomass in each of the world’s seven biomes with knowledge of how much food those spiders need; the second approach drew on field observations and estimates of spider numbers per square meter.

Using these two estimates, the researchers estimated that spiders need to kill between 400 million and 800 million tons of insects annually to keep up their numbers – a weight that likely surpasses that of humanity’s yearly kill.

When you factor in other food sources such as grains, vegetables, and dairy, humans still consume far more than spiders: According to the UN’s Food and Agriculture Organization, people ate more than 1 billion tons of cereals alone in 2007.

But for a taxonomic order that relies on predation to survive, and whose biggest species have leg spans of about 1 foot, the ability to bring home 400 million to 800 million tons of food is still respectable.

It’s also remarkably beneficial to humans. Left unharmed, many of those ill-fated insects would otherwise eat their fill of our crops. In 2014, arachnologist Norman Platnick went so far as to say, “If spiders disappeared, we would face famine.”

The study's authors don’t see that happening anytime soon. But they also hope that their work helps preserve these spiders' benefits. "These estimates emphasize the important role that spider predation plays in semi-natural and natural habitats, as many economically important pests and disease vectors breed in those forest and grassland biomes," Nyffeler explained.

"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 in terrestrial food webs," he added.