Down, but not out

Correction

In 2021 it came to light that software errors during data extraction and initial processing led to a substantial corruption of the version of the Rothamsted Insect Survey (RIS) dataset that was used in analyses for the paper on which this news story is based.

This corruption systematically affected a quarter of the total time series (1967–1979 and 2017, but not 1980–2016), and was responsible for the large increase in estimated biomass that was observed between 1978 and 1980.

Peak biomass is now in 1976 rather than 1982 and evidence for a decline over the full timescale of the dataset is ambiguous – but the authors point out there has been a clear decline of 8% per decade since 1976. The correction published by the journal can be found here: https://www.nature.com/articles/s41559-021-01449-5

How the corrected analysis affects the news story can be found within the text below.

May 2021

Rothamsted Insect Survey data on moths shows no evidence of the supposed ‘Insect Armageddon’, according to a new paper in Nature Ecology and Evolution.

Despite widespread reports that the UK is experiencing an alarming plunge in insect numbers, the report shows more ups than downs in the moth population over a period of half a century. ( *This is no longer true. The authors found that biomass was stable. Abundance is known to have declined.) 

The authors, including head of the Rothamsted Insect Survey, Dr James Bell, suggest changing weather patterns linked to climate change could be an explanation for the pattern.

Using nationwide light trap data for the period 1967 - 2017, the findings reveal there is around twice the combined biomass of moths in the present day compared with the 1960s. (*There is the same amount of biomass now compared to the 1960s.)

While there has been a roughly 10% decline in moths per decade since the early 1980s, this came after a steep increase between the late 1960s and 1982.(*The peak in biomass has been re-estimated as 1976 rather than the 1980s. Post-1976 declines were 8% per decade overall. There was no rise and fall in biomass but rather a linear trend.)    

And whilst light pollution or pesticides have often been blamed for decreases in the insect population, it was woodland and grasslands - and not urban areas or farmland - that suffered the greatest declines.

Lead author of the study, Dr Callum Macgregor, from the University of York, said: “Moths are a good indicator of what may also be happening in other insect populations as they are the second most diverse group of insect herbivores, with a full range of species from extreme habitat generalists to extreme specialists.

“Our study does not support the narrative that insects are vanishing en masse before our eyes, because there has been a net increase in biomass over the last 50 years. However, the clear decline we observed since the 1980s is still a cause for concern.

“Moths come in all shapes and sizes, so measuring their combined weight allowed us to analyse changes in their populations that are relevant to their predators and food plants.” 

Co-author Professor Chris Thomas, University of York, added: “Biomass is particularly important because it is linked to ecosystem processes, such as the total amount of plant material consumed by insects and food availability for other animals that eat insects. 

“Moths provide pollination nocturnally to a wide range of plants, and they provide food for birds, bats and small mammals. Their survival is crucially important to the rest of life on earth.”

The authors investigated the possible reasons behind the fluctuations in insect populations, and why the results of the study might differ from the conclusions of previous research.  

According to the researchers, the use of better and longer-term data may be part of the answer: to put ‘Insectageddon’ to the test, they analysed data from “perhaps the best insect population database available anywhere in the world” -- the Rothamsted Insect Survey’s national network of light-traps. 

The data allowed them to estimate the weight of moths caught each year at 34 different sites around the UK between 1967 and 2017.  

Dr Bell explained: "Since 1964, the Rothamsted Insect Survey has been at the forefront of the insect declines research, exploiting the longest standardised terrestrial insect time series in the world.  Even though we have reported on population change in aphids, moths, ladybirds, wasps and general insect biomasses for decades, this study represents a major advance in our understanding.” 

Two of the most commonly suggested causes of ‘Insectageddon’ are agricultural intensification and, for night-flying insects like moths, light pollution. However, the researchers found no evidence that agricultural practices or urban light pollution are the main driving force behind this recent decline. 

The researchers categorised the moth trap sites into four land use types: woodland, grassland, arable and urban, and found that land use type (as well as changes over time in land use) had little impact on the pattern of increase and decline.  

Dr Macgregor commented: “If pesticides were causing the problem you would expect to see the biggest decline in arable landscapes; likewise, if it was light pollution, the biggest decline would be in urban environments. We found neither of these to be the case – in fact, the habitats with the biggest decline were woodland and grassland.”

Changing weather patterns and climate change could be an explanation, the researchers say.  Sharp increases in the amount of moths in the late 1970s came soon after one of the hottest, driest years on record. 

Disturbance from extreme weather events can sometimes cause equally extreme and unpredictable population changes in insects, including both increases and declines.

Whether climate is contributing to the more recent decline requires further investigation; rising average annual temperatures and rainfall patterns did not match up with the peaks and troughs in moth populations, but extreme weather events – from deluge and flooding to heatwaves and drought – are often not reflected in average climate records. 

The post-1982 decline appears to be real, but the researchers concluded that the reasons for this ecological decline are not as straightforward as is sometimes suggested.  

“There is no simple explanation and more research on long-term data sets is required. We found that short-term studies and infrequent sampling can give erroneous estimates of biomass change. The complexity of insect population change requires more, better and longer-running data if we are to draw robust conclusions, particularly for parts of the world where insect data are limited,” said Professor Thomas.