The ecological impact of spiders

ARACHNOPHOBIA is a common and powerful fear. Spiders sit high in the pantheon of species that have an outsized terror-to-danger ratio. But, unsettling though they may be, the eight-legged do excel at keeping six-limbed pests in check. They prey upon insects in vast quantities, while, for the most part, leaving people alone. Indeed, in 1957 William Bristowe, a British arachnologist, wondered whether British spiders might kill prey equivalent in mass to all of the people then living in Britain.

In research published this week in the Science of Nature, Martin Nyffeler of the University of Basel, in Switzerland, and Klaus Birkhofer of Lund University, in Sweden, attempt to put some numbers on spiders’ dining habits. Starting with the available data on the mass of spiders found per square metre in Earth’s main habitat types—forests, grasslands, fields of crops and so on, they calculated the amount of prey required in each habitat to support the weight of spiders there, based on spiders’ known food requirements per unit of body weight. That done, they extrapolated their habitat-based results to the whole planet, in light of what is known about the total areas of such habitats.

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. Approximately speaking, then, Bristowe was right. Arachnophobes, meanwhile, should consider this: without spiders, there would be an awful lot more other creepy-crawlies around.