Spiders get a bad rap. Seen as terrifying pests by many, these eight-legged creatures may be under-appreciated: a new study has estimated just how much spiders consume annually and how essential they are to our environment.

Researchers calculated that the world's spiders consume between 400 and 800 million tonnes of insects and other critters annually. In comparison, humans consume about 400 million tonnes of meat and fish each year.

That's a lot of grub.

But they are also vital to our environment.

While other studies have suggested that spiders play an important role in reducing crop pests, the authors of the study, published in the journal The Science of Nature, say their findings suggest the ecological importance of spiders is more significant in forests and grasslands.

Co-author of the study Martin Nyffeler said it's likely that spiders hunt more efficiently in natural areas like grasslands rather than highly disturbed agricultural fields.

On the dinner plate

Although spiders certainly dine on insects, the study found that more than 90 per cent of their prey consists of small, soil-surface-dwelling organisms that are closely related to insects.

The most common insect groups spiders eat are flies, mosquitoes, aphids, leafhoppers, beetles and ants, among others. They can also dine on lizards, fish, birds and even bats. And they're anything but loyal: spiders often feed on other spiders.

It also turns out that spiders may be helping your garden grow.

"Spiders kill large numbers of herbivorous insects — and by doing so they help to protect the plants from herbivore damage," Nyffeler said.

Not only do spiders contribute to the ecological web by consuming prey, they also provide a meal to some 3,000 to 5,000 species of birds.

"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," Nyffeler said.

Calorie count

So how exactly does one calculate how much arachnids eat annually?

The researchers used two different methods to make the calculation: in the first, they used published data about the average spider biomass per square metre in seven different biome types. Armed with that data, they theoretically calculated the annual food needs of the spiders contained in each biome.

Second, they used published data on the annual prey kill per metre of spider communities in each biome.

The jumping spider Phidippus apacheanus feeding on a red-banded leafhopper (Graphocephala coccinea). (David E. Hill/Peckham Society, Simpsonville, S.C.)

It's estimated that there are roughly 45,000 different species of spiders on this planet, which accounts for an estimated weight of around 25 million tonnes.

The wide variation in the estimate of how much spiders consume derives from the fact that prey kill can vary within ecosystems, which themselves vary.

So the next time you think about killing a spider, you might want to reconsider.