Close-up of a spider on a spider web
March 15, 2017

Study puts spiders at the top of the global predator chart

It is a long-held belief that spiders are one of the most essential categories of predators of insects, and a recent study has revealed that spiders kill 400 to 800 million tons of insects around the world each year.

With over 45,000 species and a population density up to 1,000 individuals per square meter, spiders are among the world’s most species-rich and wide-ranging groups of predators. As a result of most spiders’ clandestine lifestyle, it has been challenging to show their ecological role.

Published in The Science of Nature journal, the study is based on two calculations derived from various models that reliably showed the global spider population wipes out about 20 times its weight in prey each year. Greater than 90 percent of that prey is insects and springtails (Collembola). In addition to eating insects, big, tropical spiders will also prey on small vertebrates like frogs and fish from time to time, or eat plants. The massive range of the global prey killed shows that rates of predation can vary widely inside of certain ecosystems, and these variants must be considered for ecological projections, the researchers said.

Studying Nature's Predators

The study's conclusion contrasts interestingly with data from the Food and Agriculture Organization of the United Nations (FAO) that shows the global human population eats around 400 million tons of meat and fish each year. Incidentally, whales eat between 280 and 500 million tons of prey each year.

The study team also revealed spiders kill a lot more insects in forests and savanna than in other environments. Spiders in these locations catch enormous numbers of forest and grassland pests, while spiders living in deserts, Arctic tundra and agricultural areas kill fewer insects. The spiders’ influence is reduced in agricultural areas due to the fact they are intensively maintained areas that provide undesirable living conditions.

“Our calculations let us quantify for the first time on a global scale that spiders are major natural enemies of insects. In concert with other insectivorous animals such as ants and birds, they help to reduce the population densities of insects significantly," study author Martin Nyffeler, a researcher from the University of Basel, said in a news release. “Spiders thus make an essential contribution to maintaining the ecological balance of nature,” he adds.

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