Spiders can seem scary, but just imagine what life might be like without them.
Spiders are some of Earth's most important predators of insects (a broad group of creatures that, despite a common belief otherwise, does not include spiders). Like tigers or wolves, spiders' big appetites and predatory skills can make them a potent ecological force, helping keep a wide range of potential pest insects in check.
To calculate how important spiders are to their — and our — ecosystems, two researchers recently undertook the ambitious task of estimating the annual food intake of all spiders on the planet. Their study, published this week, suggests spiders worldwide eat somewhere between 400 million and 800 million metric tons of food every year, more than 90 percent of which are insects and springtails.
For context, humans eat an estimated 400 million metric tons of animal protein per year, which means spiders might eat even more meat than we do. Spiders also rival the diets of whales, which ingest 280 million to 500 million tons of seafood annually, and they blow seabirds' relatively paltry 70 million tons out of the water.
While many spiders set traps for prey, others (like this jumping spider) take a more proactive approach. (Photo: Zleng/Flickr)
Previous research has shown spiders can be valuable predators of agricultural pests, but the new study suggests they're much more effective in less disturbed ecosystems like forests and grasslands, where roughly 95 percent of their prey kills are thought to occur. The higher diversity of insects (and thus food options) likely creates a stronger safety net for spiders, so their pest-control services may improve at farms or gardens with more biodiversity — and with less use of broad-spectrum pesticides.
"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," lead author and University of Basel biologist Martin Nyffeler says in a statement.
The green lynx spider, native to North and Central America, is known to be an important predator in cotton fields. The species preys on pests such as corn earworms, cotton earworms and cabbage loopers. (Photo: Cathy Keifer/Shutterstock)
Before they could estimate how much spiders eat, Nyffeler and his co-author — Klaus Birkhofer, a biologist at Lund University in Sweden — had to figure out how many spiders exist on Earth. Using data from 65 previous studies conducted across seven biomes, they deduced there are roughly 25 million metric tons of spiders around the planet. Most of these arachnids live in a forest, grassland and shrubland biome, followed by farmland, deserts, urban areas and tundra.
Nyffeler and Birkhofer then used two models to calculate how much prey all those spiders kill per year. In the first, they considered the amount of food a typical spider must eat to survive, plus data on the average spider biomass per square meter in each of the seven biomes. In the second approach, they combined prey-capture observations from the field with estimates of spider biomass density.
The first model initially suggested spiders eat about 700 million metric tons per year, but then the researchers recalculated to account for rain — "assuming that it rained during one-third of the feeding season, with no prey being captured on rainy days" — which reduced that estimate to 460 million tons. The second model estimated a global annual prey kill of 400 million to 800 million tons.
While the vast majority of these meals come from forests and grasslands, the study's authors note that, on farms without much pesticide use, spiders can help manage hemipteran insects like aphids, leafhoppers or shield bugs. "[I]n wheat-, rice- and cotton-growing areas with no or very low pesticide usage, the presence of spiders (in combination with other predators) may at times have a beneficial effect in slowing down the population growth of hemipteran pests," they write.
Spiders may be some of the planet's top predators, but suppressing insects is only part of the ecological repertoire they've been honing for 300 million years. While spiders seem like monsters to many people (a perception likely shared by more than a few insects), they're also key sources of food for a wide range of wildlife.
Between 8,000 and 10,000 predators, parasitoids and parasites feed exclusively on spiders, the authors point out, an impressive level of biodiversity built on the arachnids' backs. And aside from supporting all those specialists, spiders are also a dietary staple for about 3,000 to 5,000 bird species. Given the agricultural value of some parasitoid wasps and birds, that boosts the benefits of spiders even more.
"We hope that these estimates and their significant magnitude raise public awareness," Nyffeler says, "and increase the level of appreciation for the important global role of spiders in terrestrial food webs."