The number of spiders with large marble runs in the Swiss plateau has decreased dramatically in the last 40 years. The main reason for this is the dwindling food supply for these insectivorous animals. This is shown in a study carried out by researchers from the University of Basel and the University of Ghent (Belgium), as reported in the journal Insects.
The approximately 48,400 known spider species worldwide are among the most important insectivorous animals on earth. One group of them, the spiders of the ball tracks, spin eye-catching wheel-shaped tracks. Perhaps the best known and so far very common spider species in Europe is the European garden spider Araneus diadematus. In addition to home gardens, their preferred habitats include parks, cemeteries, hedges, forest edges and clearings.
The spider’s average population density is about 140 times lower
The European garden spider can be easily identified by the bright, cross-shaped mark on the top of its belly. The females of this spider species, which as adults measure 10 to 18 mm, without legs, spin webs with a diameter of about 30 cm. In the late summer of 2019, the entomologists determined the population density of the European garden spider on the Swiss plateau in 20 representative habitats: They counted the spider webs in test plots from 200 to 1,000 square meters and calculated these values into the number of webs per square meter.
The results: Compared to data from the 1970s and 1980s, the frequency of this spider has declined alarmingly. The average population density in Switzerland, for example, is 140 times below the previous European average. No cobwebs at all were found in two thirds of the sample plots examined. The nets contained significantly fewer insects than in previous studies. In addition, it has been found that the weaving threads are considerably thinner, as is the case with malnourished spiders.
Small flying insects as food
Most species of ball spiders feed almost exclusively on small flying insects like flies, mosquitoes, and aphids, which are hardest hit by insect declines. A study carried out in Northern Belgium has shown that the population density of the European garden spider has recently fallen to a very low level. Not only the European garden spider, which can be found almost everywhere in the Swiss Plateau, but also other weaving spider species are less common today than in the last century. “The results of our study are a strong indication of an insect meltdown in large parts of Europe,” says lead author Dr. Martin Nyffeler from the University of Basel. The study also suggests that animals that feed on insects are also affected: “When the abundance of insects and their natural predators drops dramatically, ecosystems become impoverished and can eventually collapse.”
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