THURSDAY, 12 MARCH 2020Moths have declined over the last half century, but aphid numbers have remained steady.
A new study of insect population levels across Great Britain has provided a more precise view of the much-discussed “insectageddon” narrative, revealing a detailed picture of long-term trends over almost half a century. Data collected from over 24 million insects caught between 1969 and 2016 shows that British moths are in decline, while aphid numbers are relatively stable.
Lead author Dr James Bell says, “Although we have known anecdotally that insects have been declining in Great Britain for more than 100 years, these declines have only been properly estimated over the last 20 years.” The study, published in Insect Conservation and Diversity, reveals a fall of 31% in moth numbers, however this overall downward trend is complicated by several shorter periods of partial recovery. Dr Bell notes that many previous studies have failed to completely capture long-term trends in insect populations, due to their reliance on a limited set of data points over just a few years.
Although aphid numbers have fluctuated dramatically from year to year, their overall number has remained fairly constant. Notable exceptions are the three most common aphid species, which have all seen a decline in number, likely due to increases in pesticide use and the emergence of resistant plant traits. Understanding the factors behind the downturn in moth numbers is more difficult – Dr Bell explains that neither climate nor land use change alone can account for the trend, and more data is required to make sense of declines in seminatural environments, where other pressures such as light pollution and urbanisation may play an important role.
The study made use of data systematically collected from 137 insect traps across Britain, an extensive network run by the Rothamsted Insect Survey since 1964. These traps represent the most comprehensive source of standardised long-term data on insects in the world. Dan Blumgart, one of the authors of the paper, emphasizes the importance of this approach: “Long-term standardised data is by far the best way to measure changes in insect abundance, and the Rothamsted Insect Survey is unmatched in this regard.” Insect populations show significant fluctuations from one year to the next, so taking snapshots of abundance can result in the calculation of misleading trends.
Some intriguing findings appear in this study, with significant variation observed in the trends among different habitats. Moths seem not to have declined in farmland since 1968, with the main losses occurring in woodlands and urban areas. Similarly, while the most abundant cereal pest species of aphids have declined, aphid abundance in general has also remained stable in farmland. “This is counter to the narrative of farming practices being to blame for biodiversity loss”, says Blumgart. “As the amount of woodland has actually increased in the UK since 1968, we have no idea why moths should be declining in this habitat.” Blumgart’s PhD is aimed at understanding these differences in trends between habitats and identifying the mechanisms which have led to the decline, with the ultimate goal of maximizing biodiversity in highly cultivated areas.
More broadly, the study points towards the importance of formulating an accurate picture of population changes over time if insect decline is to be properly understood. Although many insect groups have declined since the 1960s when most records began, Blumgart maintains that “the idea of an insect apocalypse is sensationalist.” A paper by Sánchez-Bayo and Wyckhuys published in 2019 received extensive media attention for its claim that 40% of the world’s insect species might face extinction in the next few decades – yet according to Blumgart “this claim was completely unsupported by the data they presented”, and at least six peer-reviewed articles were subsequently published pointing out significant methodological flaws in the study.
At present, there is simply not enough known to make sweeping claims, says Blumgart. “The real message of the insect apocalypse narrative is that we don't have enough information on insect abundance to estimate changes in almost all groups in almost all of the world.” Similar long-term studies will be an effective way of working towards building a clearer picture.
Bell, Blumgart, & Shortall (2020) Are insects declining and at what rate? An analysis of standardised, systematic catches of aphid and moth abundances across Great Britain. ; Insect Conservation and Diversity; doi: 10.1111/icad.12412
Zak Lakota-Baldwin is a news editor at Bluesci.