Kjær, C., Ehlers, B., Bruus, M., Hansen, M.D.D., Hansen, R.R., Holmstrup, M., Høye, T.T., Jensen, J., Offenberg, J., Strandberg, B., Strandberg, M. & Wiberg-Larsen, P. 2020. Insekters tilbagegang. Hvilke insekter går tilbage, hvorfor og hvad kan der gøres? Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 106 s. - Videnskabelig rapport nr. 388. http://dce2.au.dk/pub/SR388.pdf

Summary

In this report, we have examined the background for the observed decline in diversity and insect populations. To do this, we have reviewed the scientific literature for observations of developmental tendencies for species that are indigenous to north-western Europe. In addition, the recently published Danish Redlist has also been used to identify the endangered species in Denmark.

The majority of the studies showed that insects are in decline, regardless of the method of collection. This is most obvious for butterflies, bees and ground beetles. For groups in decline, it does not matter where the study was conducted. Generally, the species that are sedentary, are specialists, are associated with nutrient-poor habitats or live in areas with low landscape diversity are all in decline. Species that are stable or prosper are mobile generalists or species that live close to their northern boundary. The general declining trend is also reflected in studies that are not categorized by species group.

Analyses of Redlist data support the trends described in the literature, but add more insect orders to the series of endangered insect groups. In particular, Redlist assessments show that certain specific lifeforms are endangered. Overall, data from the Redlist show that the presence of the following food resources is important for the endangered species:

• Flowers (pollen and nectar)
• Dead branches and wood infected by fungi
• Host plants of specialised herbivores
• Carrion
• Dead organic matter (leaf matter)
• Dung (from large herbivores).

In addition, some species require specific physical conditions. They include riparian species and species that are associated with dry, sunny and nutrient-poor habitats.

The report examines the factors that are significant to whether an insect thrives. In this review, we focus on the species' resource habitat. A resource habitat is an area that contains all the necessary resources to ensure that a given species can complete a full life cycle. The concept stems from the species’ ecological niche. In this context, the habitat is described by the spatial distribution and temporal incidence of the resources in relation to each other, as well as the volume and quality of food.

On the basis of this review of the living conditions that are important for insects, the external factors that may affect the insect are discussed. These factors include: Habitat destruction and fragmentation, pollution (pesticides, organic matter, nitrogen/phosphorus, veterinary medicines and light pollution), invasive species, climate change and the management of habitats. 

Based on the above, the following general recommendations are made for measures that can improve the conditions for endangered species and species that are in decline:

• Ensure larger and more connected natural areas. It is also important that existing open natural habitats and woodland are protected to accommodate the species with a limited dispersal (it has been shown that some species have a very limited dispersal) and the species that exploit resources that are found specifically in the mentioned habitats.
• Avoid felling veteran trees. In addition, create more “minimum intervention forests”.
• Establish grazing on larger areas and adjust the intensity in accordance with the area's productivity/nature type.
• Reduce the impact of nitrogen and herbicides from agriculture in transitional zones between agriculture and natural areas. This also encompasses expansion of buffer zones around watercourse habitats and temporary water holes, thereby reducing the intake of nitrogen and pesticides.
• Ensure as large green areas as possible with varied vegetation in urban areas and reduce light pollution as much as possible, e.g. by using the type of bulbs in street lighting that attract the fewest insects.
• Optimise small biotopes in arable land and urban landscapes by creating larger areas, reducing mowing and planting/sowing indigenous flowers. Furthermore, ensure that hedges are made wider with a developed hedge bottom, and that hedges are shielded from the effects caused by the application of pesticides and nitrogen by use of buffer zones.