Arthropod decline in grasslands and forests is associated with landscape-level drivers

Abstract

Recent reports on local extinctions of arthropod species1 and of massive declines in arthropod biomass 2 point to land-use intensification as a major driver of decreasing biodiversity. However, there are no multi-site time-series of arthropod occurrences across land-use intensity gradients to confirm causal relationships. Moreover, it remains unclear which land-use types and arthropod groups are affected and whether the observed declines in biomass and diversity are linked to one another and continue. Here we analyzed arthropod data on more than 1 million individuals and 2,700 species from standardized inventories from 2008 to 2017 at 150 grassland and 140 forest sites in three regions of Germany. Overall gamma diversity in grasslands and forests decreased over time indicating loss of species across sites and regions. In annually sampled grasslands, biomass, abundance and species number of arthropods declined by 67%, 78%, and 34%, respectively. The decline was consistent across trophic levels, mainly affected rare species, and its magnitude was independent of local land-use intensity. However, sites embedded in landscapes with higher cover of agricultural land showed a stronger temporal decline. In 30 forest sites with annual inventories, biomass and species number, but not abundance, decreased by 41% and 36%, respectively. This was supported by analyses of all forest sites sampled in 3year intervals. The decline affected rare and abundant species and trends differed across trophic levels. Our results show that there are widespread declines in arthropods that concern biomass, abundance and diversity across trophic levels. Declines in forests demonstrate that arthropod loss is not restricted to open habitats. Our results 4 suggest that major drivers of arthropod decline act at larger spatial scales, and are, at least for grasslands, associated with agriculture at the landscape level.This implies that land-use relevant policies need to address the landscape scale to mitigate negative effects of land-use practices. Main textMuch of the debate on the human-induced biodiversity crisis has focused on vertebrates 3 , yet population decline and extinctions may be even more substantial in small organisms such as terrestrial arthropods 4 . Recent studies report declines in biomass of flying insects 2 , diversity of insect pollinators 5,6 , butterflies and moths 1,7-10 , hemipterans 11,12 and beetles 7,13,14 . Owing to the associated negative effects on food webs 15 , ecosystem functioning and ecosystem services 16 , the insect loss has spurred an intense public debate. However, time-series data on arthropods are rather limited and studies have so far focused on a small range of taxa 11,13,14 , few land-use and habitat types 12 or even on single sites 1,17 . In addition, many studies lack species information 2 or high temporal resolution 2,12 . Hence, it remains unclear whether reported declines in arthropods are a general phenomenon that is driven by similar mechanisms across land-use types, taxa and functional groups.The ...

Interpreting insect declines: seven challenges and a way forward

“…A salient example of just how important cross-validation could be is the recent study of arthropod decline in Germany over a 10-year time series from 2008 to 2017 (Seibold et al, 2019). The overall time series trend, as well as region-specific and taxon-specific trends, are heavily influenced by one or two time-points in the data (see Fig.…”

“…1 and fig. S3-1 in the study by Seibold et al, 2019). To their credit, the authors attempt a sensitivity analysis by dropping 1 year of the time series at a time, which "showed that the decline was influenced by, but not solely dependent on, high numbers of arthropods in 2008" (Seibold et al, 2019, p. 672).…”

“…To their credit, the authors attempt a sensitivity analysis by dropping 1 year of the time series at a time, which "showed that the decline was influenced by, but not solely dependent on, high numbers of arthropods in 2008" (Seibold et al, 2019, p. 672). This is equivalent to a very shallow leftcensoring of the time series in the case of the 2008 data point, which (by visual inspection of the evidence in the study by Seibold et al, 2019) will have removed a large component of the apparent trends, by itself alone. A full cross-validation would certainly nullify any remaining evidence for a general decline trend in their data.…”

Glowing, glowing, gone? Monitoring long‐term trends in glow‐worm numbers in south‐east England

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