The Texas frosted elfin (Callophrys irus hadros), a small butterfly subspecies found only in Arkansas, Texas, Oklahoma and Louisiana, has lost most of its prairie habitat and is thought to have declined dramatically over the past century. Image by Matthew D. Moran / The conversation.
By Matthew D. Moran, Hendrix College
In recent years, the concept of an insect apocalypse has become a hot topic in the conservation science community and has captured the public’s attention. Scientists who warn that this catastrophe is missing claim that arthropods – a large category of invertebrates that include insects – are rapidly receding, often indicating a general invasion of ecosystems around the world.
Beginning around the year 2000, and more frequently since 2017, researchers have documented large population declines among moths, beetles, bees, butterflies and many other insect types. If controlled, this trend would be of serious concern, especially considering that insects are important animals in almost every terrestrial environment.
But in a newly published study I co-authored with 11 colleagues, we checked more than 5,000 sets of data on arthropods in North America, with thousands of species and dozens of habitats over decades. We found essentially no change in population size.
These results do not mean that insects are good. Indeed, I believe there is good evidence that some species of insects are in decay and in danger of extinction. But our findings indicate that, in general, the decline of large-scale insects remains an open question.
The debate
For most scientists, the idea of eradicating insects is a progressive prospect that would have detrimental consequences for all aspects of life on Earth, including human well-being.
But some scientists were skeptical about the reported insect apocalypse. A number of studies showing broad declines were geographically limited, focusing mainly on Europe. Typically, these studies analyzed only a few species as groups of species.
Insects have developed adaptations that allow them to live in an enormous range of environments around the world.
Some particular long-term observations showed that decline in the past 30 years occurred after periods when the relevant insect populations increased. Many insect populations are known to fluctuate naturally, sometimes dramatically.
Many scientists concluded that although the outlook was for massive insect loss, the jury was still out on what actually happened.
Spotlighting North America
Ecologist Bill Snyder and I thought the studies suggested that widespread injections produced an intriguing pattern with significant ramifications, but that the evidence was not yet strong enough to draw conclusions. We wanted to investigate what happened in North America, which has an enormously diverse landscape and, surprisingly for us, was not widely analyzed for insect decay.
For our study, we decided to use data from sites in the Long Term Ecological Research network, which is supported by the National Science Foundation. The network covers 28 sites in the US that have been studied in depth since the 1980s, and includes deserts, mountains, prairies and forests. With almost 40 years of data collection, we hoped that trends on these sites would be a good complement to European insect studies.
Monarch Butterfly (Danaus plexippus) larva prepared on puppet, or physically transformed into a butterfly. This highly migratory species has declined in recent years over parts of its North American range. Image by Matthew Moran / The conversation.
We put together a team of 12 people that included six undergraduate students, postdoctoral scientists Michael Scott Crossley and Amanda Meier, and colleagues from the U.S. Department of Agriculture. When we compiled our datasets, at least some of us expected broad-spectrum insect decay.
Instead, the results left us confused. Some species we considered were declining, while others were increasing. But by far the most common result for a species on a particular site was not a significant change. The vast majority of our species had stable numbers.
At first we thought we were missing something. We tried to compare different taxonomic groups, such as beetles and butterflies, and different types of food, such as herbivores and carnivores. We visited urban, agricultural and relatively troubled areas to compare. We tried to compare different habitats and different periods of time.
But the answer remained the same: no change. We had to conclude that on the pages we examined, there were no signs of an insect apocalypse and, in fact, no broad decay.
Students from Matthew Moran’s lab at Hendrix College sampling insects in a natural prairie in Arkansas using a suction machine. Studies like these help scientists collect long-term data on insect populations. Image by Matthew Moran / The conversation.
Explain continental differences
We are confident in our analysis and our conclusion, but a more important question is why our results are so different from those of other recent studies. I see two potential explanations: location and publication word.
As I have noticed, most insect repellent papers came from European data. Indeed, Europe has better and more comprehensive data in the long run than other parts of the world. It is also one of the most densely populated parts of the world – three times higher than North America.
Moreover, almost the whole country of Europe is adapted for human consumption. Agriculture is widespread and intensive, and cities and suburbs cover large areas of the landscape. So perhaps it is not surprising that Europe has also lost a greater proportion of its wildlife compared to North America.
Publication bias is not about dishonesty or false results. It refers to the idea that dramatic results are more publishable. Reviewers and magazines are more likely to be interested in species that disappear than in species that show no change over time.
Just rejected from a top magazine because our findings are not surprising enough. “Are there counterintuitive or unexpected findings that the authors can highlight more? If not, does the paper feel like a documentation of things we would already expect.” #publicationbias pic.twitter.com/4BuPlWBTaW
– Dr. Rebecca Kreitzer (@rebeccakreitzer) November 8, 2019
The result is that over time, declining species can be represented in the literature. Then, when scientists search for papers on animal populations, declines are primarily what they find.
We selected long-term ecological research sites for our analysis in part because they had “raw” data available that were not peer-reviewed for publication and were not collected pending delays. After all, scientists collect this data to monitor ecosystems and observe trends over time. In other words, it was indefinite data. And because the datasets were so varied, they covered a wide range of species and habitats.
The future of insects
Our study will not be the definitive answer. While the human population continues to grow and conserve an ever-increasing portion of the world land, water, space and biomass, other species can only retreat and survive with fewer resources. I have no doubt that every time a forest is cut down, a prairie is plowed when a field is hardened, the world loses some of its animal and plant life.
Quantifying this process will require more oversight, more conservation biologists working in the field, and more awareness of how human actions affect the Earth’s biodiversity. But it may be that insects, which have survived millions of years through many biological disasters, find a way to survive our presence as well.
Matthew D. Moran, Professor of Biology, Hendrix College
This article has been republished The conversation under a Creative Commons license. Read the original article.
Bottom line: Although recent reports of dramatic declines in insect populations have raised concerns about an ‘insect apocalypse.’ a new analysis of data from sites in North America suggests that the case has not been proven.
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