Animal behaviour
The ecological impact of spiders
Arachnids eat as much animal food as all of the humans on Earth
Mar 18th 2017
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.
This article appeared in the Science and technology section of the print edition under the
headline "Spider bites"
http://www.economist.com/news/science-and-technology/21718858-arachnids-eat-much-animal-food-all-humans-earth-ecological