No recovery in the biomass of flying insects over the last decade in German nature protected areas

Abstract

Five years after a German study on insect biomass described a multi-decade decline in nature protected habitats, the DINA (Diversity of Insects in Nature protected Areas) project has investigated the status of insects in 21 selected nature reserves across Germany in the years 2020 and 2021. We used the same methods and protocols for trapping and measuring the biomass of flying insects as in the earlier study. Across two vegetation periods, we accumulated a comprehensive data set of 1621 data points of two-week emptying intervals to evaluate the insect biomass along gradients from arable land into nature reserves through transects of Malaise traps. On average, we observed an increase in maximum insect biomass per day along the transect from the edge to the centre of the nature reserve. Overall, the measured insect biomass remained at low levels, consistent with previous findings from the years 2007-2016. There were no significant regional differences. The results show that protected habitats have higher insect biomass compared to farmland and are therefore essential for insects but are unlikely to be sufficient to sustain insect biodiversity. Further measures need to be taken for better protection and sustainment of insects, which fulfil key functions in all terrestrial ecosystems.

Keywords: agriculture; biodiversity loss; insect biomass; insect decline; malaise traps; natura 2000; nature conservation.

FIGURE 1

Aerial view of a Malaise trap transect starting with MT1 on arable land (right in picture), MT2 at the boarder of the protected habitat and on the left MT5 towards the centre of the nature reserve (copyright: Entomological Society Krefeld).

FIGURE 2

Number of locations with increase (green) or decrease (orange) of mean daily biomass between the two monitoring periods 2020 and 2021 (the percentage indicates the difference in biomass).

FIGURE 3

Boxplots (median, interquartile range, whiskers and outliers) of daily insect biomass along the Malaise trap transect (MT1–MT5). Sampling year 2020 on the left, and 2021 on the right. Full circles connected by a dashed line at the top represent the maximum biomass.

FIGURE 4

Results of generalised additive models (GAMs) that examine the effects of mean maximum temperature, mean precipitation, continentality index and agricultural production area (2 km radius around NPA) on mean insect biomass per day within NPA (pooled data from MT3 to MT5). Model a includes all 21 sites for 2020, and model b for 2021. Model c excludes four sites with newly established ‘flower strips’ (Riedensee, Insel Koos, Bottendorfer Hügel, Schwellenburg) for 2020, and model d for 2021.

FIGURE 5

Daily insect biomasses in gram in German nature protected areas (NPA) of a 27‐year timeseries published in Hallmann et al. (2017) (dark grey = significantly different to 2020 and 2021; light grey = no significant difference to 2020 and 2021, Mann–Whitney U test) compared to our data (mean daily biomass of MT3–MT5) from 2020 to 2021 (orange). Some years had small numbers of samples (1991 n = 10, 2011 n = 4 and 2015 n = 10).