Suitable living areas seem to be in short supply in Switzerland not just for humans, but also for colony-building ants. This is apparent from the first nationwide census of red wood ant-mounds carried out by WSL during the fourth National Forest Inventory (NFI). Ant-mounds built by one of the five species of red wood ant (Formica rufa group) occurring in Swiss forests were found on only one in twenty of the 6500 plots sampled. When extrapolated for the whole of Switzerland, this amounts to about 1.4 mounds per hectare of forest, whereas in Europe the average is five. The number of mounds found on the Central Plateau was ten times lower than the Swiss average.
Wood ants like to build mounds in places with conifers, morning sun, a thick herb layer on the ground and an open forest structure. There they can find aphids in abundance, whose sweet excretions are what they mainly eat. A surprising finding was that ants do not need large continuous sections of forest. Most nests were found in areas above 900 metres in altitude. “We have just started to find out why there are so few mounds on the Central Plateau,” says Anita Risch from the Research Group ‘Animal-Plant Interactions’, who is in charge of the study. “I am assuming it is because the forests there are more strongly influenced by humans.” This is why the researchers are currently focussing on human activities in the forest: “We are assessing how strongly, for example, forest management, accessibility improvements and recreational behaviour influence the distribution of wood ants,” says Anita.
It is not possible to say at the moment whether the number of ant-mounds is increasing or decreasing. “Up until now little was known about their distribution,” says Beat Wermelinger, an insect specialist at WSL. The census provides the first reliable data to use as a basis for monitoring the future development of red wood ants. (Beate Kittl, Diagonal 2/18)