Their conclusion was that there are 25m tonnes of spiders around the world and that, collectively, these arachnids consume between 400m and 800m tonnes of animal prey every year. This puts spiders in the same predatory league as humans as a species, and whales as a group. Each of these consumes, on an annual basis, in the region of 400m tonnes of other animals.
Somewhere between 400m and 500m tonnes is also the total mass of human beings now alive on Earth.
Here is the Economist article.