Just a few weeks ago, everyone was talking about plummeting insect numbers. Academic discourse focused on three main causes: the destruction of habitats, pesticides in agriculture and the decline of food plants for insects.
Just a few weeks ago, everyone was talking about plummeting insect numbers. Academic discourse focused on three main causes: the destruction of habitats, pesticides in agriculture and the decline of food plants for insects. A team of researchers from the Universities of Bonn and Zurich and the Swiss Federal Institute for Forest, Snow and Landscape Research WSL have now demonstrated for the first time that the diversity of food plants for insects in the canton of Zurich has dramatically decreased over the past 100 years or so. This means that bees, flies and butterflies are increasingly deprived of their food base. The study, which is representative for all of Central Europe, has now been published in the journal "Ecological Applications".
"Over the past 100 years, there has been a general decline in food plants for all kinds of insects in the canton of Zurich," says Dr. Stefan Abrahamczyk from the Nees Institute for Biodiversity of Plants at the University of Bonn. The homogenization of the originally diverse landscape has resulted in the disappearance of many habitats, especially the wetlands, which have shrunk by around 90 percent. Human settlements have spread more and more at the expense of cultivated land, and the general intensification of pasture and arable farming has led to a widespread depletion of meadows and arable habitats. The researchers compared the abundance of food plants of different insect groups, based on current mapping for the years 2012 to 2017, with data-based estimates from the years 1900 to 1930 in the canton of Zurich (Switzerland).
The food plants of specialized groups of flower visitors are particularly affected by the decline. For instance, the Greater Knapweed (Centaurea scabiosa) is pollinated by bumblebees, bees and butterflies, as their tongues are long enough to reach the nectar. The decline is particularly dramatic for plant species that can only be pollinated by a single group of insects. In the case of Aconite (Aconitum napellus), for example, this can only be done by bumblebees because the plant's toxin evidently does not affect them.
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Photo: A Narrow-bordered five-spot burnet sitting on a brown knapweed. CREDIT: Beat Wermelinger
