London: Spiders devour up to 800 million tonnes of prey each year – more than the amount of meat and fish humans consume over the same period – making them one of the world’s most voracious predators, a new study has found.
More than 90 per cent of that prey is insects and springtails, researchers said.
Large tropical spiders occasionally prey on small vertebrates (frogs, lizards, snakes, fish, birds, and bats) or feed on plants.
With over 45,000 species and a population density of up to 1,000 individuals per square metre, spiders are one of the world’s most species-rich and widespread groups of predators.
Due to their secretive lifestyle – many spiders are nocturnal or live well camouflaged in vegetation – it was previously difficult to demonstrate their ecological role.
Now, zoologists at the University of Basel and Lund University in Sweden have used calculations to conclude that spiders indeed have an enormous ecological impact as natural enemies of insects.
Researchers used two calculation methods based on different models, which consistently showed that the global spider population (with a weight of around 25 million tonnes) wipes out an estimated 400-800 million tonnes of prey every year.
The large range of the global prey kill estimate is due to the fact that rates of prey kill can vary widely within specific ecosystems, and these variations must be taken into account for ecological projections, researchers said.
Compare this to the fact that the worldwide human population consumes around 400 million tonnes of meat and fish every year.
The spider’s eating habits can even be compared to those of the whales (Cetacea) in the world’s oceans, which eat an estimated 280-500 million tonnes of prey a year.
The zoologists also showed that spiders kill many times more insects in forests and grasslands than in other habitats.
Spiders in these areas catch huge numbers of forest and grassland pests, whereas spiders in desert regions, in the Arctic tundra and in annual crops kill fewer insects in comparison.
The spiders’ impact is lower in agricultural areas because these are intensively managed areas that offer unfavourable living conditions for spiders.
“Our calculations let us quantify for the first time on a global scale that spiders are major natural enemies of insects,” said Martin Nyffeler from the University of Basel, lead author of the study.
“In concert with other insectivorous animals such as ants and birds, they help to reduce the population densities of insects significantly,” said Nyffeler.
“Spiders thus make an essential contribution to maintaining the ecological balance of nature,” he added.
The research was published in The Science of Nature journal.
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