Spiders could theoretically eat every human on Earth in one year

The world's spider population weighs 29 million tons - as much as 478 Titanics. Photo / The Washington Post

There's an insane amount of spiders on this planet, and they're all hungry.

Spiders are quite literally all around us. A recent entomological survey of North Carolina homes turned up spiders in 100 percent of them, including 68 percent of bathrooms and more than three-quarters of bedrooms. There's a good chance at least one spider is staring at you right now, sizing you up from a darkened corner of the room, eight eyes glistening in the shadows.

Spiders mostly eat insects, although some of the larger species have been known to snack on lizards, birds and even small mammals. Given their abundance and the voraciousness of their appetites, two European biologists recently wondered: If you were to tally up all the food eaten by the world's entire spider population in a single year, how much would it be?

Martin Nyffeler and Klaus Birkhofer published their estimate in the journal the Science of Nature earlier this month, and the number they arrived at is frankly shocking: The world's spiders consume somewhere between 400 million and 800 million tons of prey in any given year. That means that spiders eat at least as much meat as all 7 billion humans on the planet combined, who the authors note consume about 400 million tons of meat and fish each year.

Or, for a slightly more disturbing comparison: The total biomass of all adult humans on Earth is estimated to be 287 million tons. Even if you tack on another 70 million-ish tons to account for the weight of kids, it's still not equal to the total amount of food eaten by spiders in a given year, exceeding the total weight of humanity.

In other words, spiders could eat all of us and still be hungry.

To arrive at this number Nyffler and Birkhofer did a lot of sophisticated estimation based on existing research into A) how many spiders live in a square meter of land for all the main habitat types on Earth, and B) the average amount of food consumed by spiders of different sizes in a given year.

These numbers yielded some interesting factoids on their own. For instance, one study estimated that global average spider density stands at about 131 spiders per square meter. Some habitats, like deserts and tundra, are home to fewer spiders. On the other hand, spider densities of 1,000 or more individuals per square meter have been observed under certain "favorable" conditions - since Nyffler and Birkhofer don't define what "favorable" means in this context, I'm going to assume it refers to dark, dusty places like the area under my bed.

If you gathered up all the spiders on the planet and placed them on a very large scale, together they'd weigh about 25 million tons, according to Nyffler and Birkhofer. For comparison, the Titanic weighed about 52,000 tons. The mass of every spider on Earth today, in other words, is equivalent to 478 Titanics.

Spider biologists have also generally found that spiders consume approximately 10 percent of their body weigh in food per day. That's equivalent to a 200-pound man eating 20 pounds of meat each day.

Conversely, it would take approximately 2,000 pounds of spiders to consume a 200-pound man in one day.

In the end, spiders' voracity actually works out to mankind's benefit. Since they primarily feast on bugs, their hunger means fewer pests in the garden, fewer mosquitoes in the yard and fewer flies in the house.