August 10, 2020

expert reaction to study of insect numbers in the US

A study, published in Nature Ecology and Evolution, looked at insect numbers in the USA.

Prof Jane Hill, Professor of Ecology, University of York, said:

“This new study provides a more careful analysis of patterns of decline – and increase – in insects, and shows that the suggestion of catastrophic worldwide declines which gave rise to worries of ‘insectageddon’ was too simplistic.

“This new study focuses on long term monitoring studies from the USA, from a scheme started in the 1980s, and shows that about 1/3 of species are declining, about ¼ increasing and the remainder showing no change. Thus the authors conclude that ‘losers’ balance out ‘winners’ to give no overall change when all the data from all the species are combined. Understanding why some species are losers but others winners will be key.

“The study highlights that precisely which data sets are selected for inclusion in analyses affects conclusions, which contributes to why studies differ in their conclusions, although species in the USA may be suffering less than elsewhere in the world. For robust analyses, it is important to consider when the monitoring scheme started (i.e. the baseline for assessing subsequent declines and increases), and the number of years the data sets cover (which need to extend over many decades given the huge year to year variation in insect numbers).”

Prof Michael Bonsall, Professor of Mathematical Biology at the University of Oxford, said:

“This is a comprehensive piece of work. While there are no net effects, there are clearly taxa specific changes – for example, this work shows that mosquitoes are on the increase and butterflies are declining. The devil is detail here for insect distributions and abundances.

“The broad spatial scales of these schemes is important for monitoring biodiversity changes. More nuanced studies are needed to unravel how changes in insect communities affect the structure and function of ecosystems – for example, changes in rarity as illustrated in this study, has implications for the role of invertebrates across different habitats.”

Prof Axel Hochkirch, Professor in Conservation Biology and Chair of the IUCN SSC Invertebrate Conservation Committee at Trier University in Germany, said:

“The authors combined a variety of different data sets to analyse trends in insect abundance and diversity across North America. Some datasets strongly dominate the complete data. For example, more than half of the time series were for urban insects in Phoenix (Arizona), mosquitoes in Baltimore (Maryland) and aphids in the US Midwest. However, they do not distinguish species qualities properly. If, for example, a non-native invasive species increases in abundance, while a rare or threatened species decreases, the net trend will be zero. The authors acknowledge this and argued that some of their data were strongly dominated by single invasive species. Numerous studies from Europe have already shown that the situation of insects in cities is often much better than in rural areas. Some insect groups are virtually absent from the study (e.g. the flying insects that have shown a 75% decline in biomass in Germany in the study by Hallmann et al.). While it is great to know that the abundance of stream insects has increased from 1984 to 1998 and the abundance of bark beetles fell from 1975 to 2012, it is not very useful to combine both data sets and conclude that there are no trends.

“The decline of insects is well documented by numerous studies and the Red Lists. While the term “insect apocalypse” is probably too dramatic (and has been introduced by the media rather than by scientists), the decline of many insect species is undoubted. However, the study shows that the message is much more complex than a simple “insect apocalypse”. There are winners and losers of global change. Some species have expanded their range in response to global warming or human transport, while others are losing valuable habitats. Some species which were formerly rare, like the Large Tortoiseshell (Nymphalis polychloros), have increased again, while other formerly common species are now in decline, like the Small Tortoiseshell (Aglais urticae). It is difficult to combine all these very diverse trends in a single number or trend, particularly as data is completely lacking in many developing countries. Virtually all studies are from northwestern Europe and North America and even those are not based upon a standardized monitoring, but a combination of numerous (very different) single studies. Therefore, I agree with the authors that standardized monitoring is crucial. Only if we understand the drivers of insect declines, we can take appropriate measures to stop the global loss of biodiversity.”

Dr Lynn Dicks, Lecturer in Animal Ecology at the University of Cambridge, said:

“Crossley and colleagues present a thorough and useful new analysis of arthropod abundance and species richness datasets from 13 sites or landscapes in North America, where there has been long term ecological research since the early 1980s. They find no overall trend, with some sites and species decreasing while others are increasing. This is quite different from findings of other recent studies showing long-term, in some cases dramatic and recent declines, at least for arthropods on land.

“The global community of biodiversity data scientists is now looking intently at available data on status and trends in insect populations. As always in the environmental sciences, the available data are extremely incomplete, and there are many ways to analyse them, so opinions about overall patterns must be subject to open scientific debate. Here, the data are cut to look at trends in individual species where possible, which means that trends in one or two abundant species have less influence on overall results. Crossley’s datasets include aphids on farmland in the Midwest (40% of all the time series), and samples in urban habitats in Phoenix, Arizona (24% of all time series), which on balance were unchanged or slightly increasing.

“There are estimated to be 5.5 million insect species on Earth, and 7 million arthropod¹ species. The 5,375 time series in this new analysis represent trends in a tiny fraction of these species (<0.1% if each time series was an individual species), each observed in an unknown fraction of its total range. It is like looking at one piece of a 1,000-piece jigsaw and expecting to see the whole picture. You could, in fact, see a very biased view, depending on what happened to be on your single piece.

“If there were consistent, rapid declines in all arthropod species everywhere, then this analysis would also show that, and humanity should be very worried indeed. But no biodiversity scientists would ever expect that. Life on Earth is intricately diverse and varied, and there are always winners and losers. Even in the most degraded or extreme environments, a few species survive and thrive. There is a consensus that some important species and groups of insects are declining in some places, such as pollinators in North America. The important questions are: what species are we losing, why is this happening, does it matter to the integrity of ecosystems or the long-term stability of our environment and what can be done about it? Crossley and team have helped to answer these questions, but their analysis also reveals why long-term monitoring of all kinds of small animals, coupled with painstakingly detailed data analysis, are so important.”

¹ Arthropods are a broader group of invertebrates with segmented bodies, legs and a hard exoskeleton, which includes insects, but also spiders, ticks, crabs, and crayfish for example. Several of Crossley’s datasets are arthropods, not just insects. Estimates of total numbers from Stork, N. E. (2018). How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth? Annual Review of Entomology 63:31-45. doi: 10.1146/annurev-ento-020117-043348.

‘No net insect abundance and diversity declines across US Long Term Ecological Research sites’ by Michael Crossley et al. was published in Nature Ecology & Evolution at 4pm UK time on Monday 10 August 2020t.

DOI: 10.1038/s41559-020-1269-4

Declared interests

Prof Bonsall: ‘I have no conflicts of interest with this study.’

Dr Dicks: ‘I have research projects on insect conservation and management of pollinators, co-funded by the Bumblebee Conservation Trust, Hutchinson’s Ltd and Berry World. None of these are specifically related to the issues commented on here. I work in partnership with Primafruit Ltd and the Cool Farm Alliance, and I advise Defra on implementation of the National Pollinators Strategy.’

No others received.