New research shows that between 440 million and 880 million tons of insects are eaten every year by around 27 million tons of spiders.
The study, published in the journal The Science of Nature, estimates that the amount of food that spiders ingest annually can be compared to the volume of food that whales consume each year, a number that stands between 300 million and 550 million tons of food.
Martin Nyffeler, a biologist from the University of Basel in Switzerland, and Klaus Birkhofer from the Brandenburg University of Technology in Germany conducted the study.
The scientist evaluated spiders from seven terrestrial biomes: tropical forests, temperate and boreal forests, tropical grasslands and savannas, temperate grasslands and Mediterranean shrublands, annual cropland, deserts and Arctic tundra.
From the 45,000 species of spiders identified in the world, scientist estimate that there are around 131 spiders per square meter of land on the planet, and in some cases, the estimate rises to 1,000 spiders per square meter.
The employed methods
Spiders are predominantly carnivores; they feed on insects and in some cases other spiders. Researchers found it difficult to measure the impact of spiders on their prey because they often hunt at night or at places that are difficult to watch. For this reason, they started by measuring the number of spiders in the world.
Researchers used previously published data containing the weight of all the spiders (biomass) in the world from seven terrestrial biomes. Afterward, they used a global land cover for each type of environment to estimate spider biomass on the planet, arriving at 25 million metric tons (27 million U.S tons).
Once they estimated the spider’s biomass, Nyffeler and Birkhofer used two methods to determine the amount of food that the arachnids consume yearly.
The first method was calculating how much food a spider needs to survive, depending on its body mass. For most spiders, they estimate that the number stands at 0.1 milligrams of food per milligram of spider, or about 10 percent of the spider’s body weight on a daily basis.
However, for desert spiders, which have evolved to survive under difficult circumstances, the number stands between 0.01 and 0.04 milligrams of food per milligram of a spider. Researchers were able to estimate a range of food varying from 507 million to 772 million U.S tons (460 million and 700 million metric tons of prey per year).
The second method they employed consisted in extrapolating data from researchers on the field, meaning that they studied data from scientists that had counted the number of preys a spider eats. With this method, they estimated a food range between 435 million and 887 million U.S tons (395 million and 805 million metric tons).
Researchers can’t tell how many spiders there are in the world
The authors found that insects and collembolans -springtails, like fleas- make up for about 90 percent of spider’s prey. The other 10 percent -which is associated with larger species of spiders- is distributed among worms, small vertebrates like birds, bats or mice, and even snakes.
The study also shows that spiders from forests and grasslands kill almost 95 percent of the prey. This happens because spiders in forests and grasslands have more ground to cover and their habitats are rarely disturbed by humans. In turn, spiders living in annual crops, for example, have to deal with pesticides as well as human activities, and they only make up for less than 2 percent of the annual prey kill.
Martin Nyffeler talked to Live Science regarding his research, and he explains that for now, it is impossible to determine how many insects are needed to get to 800 million U.S tons, or how many spiders are required to put together 27 million tons.
He claims that there’s too much variation in the estimates, seeing that no one has been able to assess how many spiders there are in the world. He added that in 1947, a British arachnologist estimated that there were approximately 2.2 trillion spiders in England and Wales alone.
Nyffeler and Birkhofer concluded their research noting the spiders’ unique role in terrestrial food webs.
“The estimated standing biomass of the global spider community is impressive. There are few groups of terrestrial predaceous arthropods that can compare with spiders in terms of abundance and biomass. An exception are ants, but most species of ants are omnivorous” they wrote on the research.
Source: Live Science