- New research from Brazil shows terrestrial insects there are declining both in abundance and diversity, while aquatic insects are largely staying steady.
- Given a dearth of long-term data on tropical insects, the scientists took creative means to collect data, including contacting 150 experts for their unpublished data.
- Scientists believe the usual global suspects are behind Brazil’s insect decline: habitat destruction, pesticide use, and climate change.
- Experts say tropical countries need more resources, including long-term funding, to discover with greater certainty what’s happening to insects there. Large-scale insect loss threatens many of Earth’s ecological services, including waste recycling, helping to build fertile soils, pollinating plants, and providing prey for numerous other species.
In 1978, when he moved to his current home on the rural outskirts of Campinas, Brazil, Thomas Lewinsohn, a professor of ecology at Campinas University, routinely encountered a “rewarding assortment of insects” at night, he writes in a new paper in Biology Letters.
Today, Campinas, 100 kilometers (60 miles) from São Paulo, is a booming urban and suburban agglomeration of more than 3 million people. Conversely, in the 40-plus years since his arrival, Lewinsohn reports insect populations have “plummeted.” The scientist has failed to see many species, including plume moths and flat-faced longhorn beetles, in 15 years.
Lewinsohn’s story isn’t unique; it echoes anecdotal evidence offered up by entomologists, ecologists and nature lovers around the world. But the story of the great insect dying, which burst into media headlines in 2018 with apocalyptic screams and backlash claims of overhyping, has also seen the publication of numerous studies worldwide. However, that attention has always suffered from an obvious gap: a dearth of news and research from the tropics.
Now, Lewinsohn and his colleagues have released a paper that sheds some light on the status of insects in the Brazilian tropics: the findings are neither surprising nor uplifting, but they add critical data to otherwise blank spots on the global insect map.
“We found evidence of insect decline in terrestrial, but not in aquatic, insects,” Lewinsohn said.
Terrestrial vs. aquatic insect findings
Lewinsohn and his colleagues examined 75 projects tracking insects in Brazil. In all, 17 studies showed declines in terrestrial insect abundance, while only three showed a population rise. Eleven studies found declines in diversity, while one showed rising insect diversity. On average, these terrestrial studies covered 22 years of tracking.
“Overall, global studies are seeing a decline in insects, and the study we’ve done is further evidence of the trend,” says co-author Kayna Agostini from the Federal University of São Carlos.
The results for aquatic insects were far different. Just two studies showed declining abundance, with two showing rising abundance. Three studies found declining diversity against four showing rising diversity. Most of the studies evaluated, however, found a stable trend for aquatic insects in abundance and diversity.
The reason for the terrestrial and aquatic divergence is uncertain. The paper suggests the different findings may be due to far fewer studies being conducted of aquatic insects, and those generally being over a shorter time frames (11-year average time span versus 22 years for terrestrial studies). Also in some cases, aquatic insects may have seen recovery after polluted wetlands were cleaned up.
“[We] do not take heart from the observation that aquatic insect density and species richness seems to be holding. It is a false illusion,” Daniel Janzen and Winnie Hallwachs wrote to Mongabay after examining the study in which neither participated. Janzen, an ecologist with the University of Pennsylvania, has been warning of tropical insect declines for decades. Hallwachs is a tropical ecologist. Both have done entomology research for decades in the Guanacaste Conservation Area in Costa Rica, a UNESCO World Heritage Site.
“What [we] HAVE seen since the 1950’s is the huge declines of aquatic dispersal phases (adults that fly),” Janzen and Hallwachs wrote.
Still, the Brazilian findings on aquatic insects reflect other research, most notably a massive 2020 study in Science that found that insect populations on land are falling by nearly 1% per year, but are rising in water bodies, likely due to efforts around the world to clean up freshwater ecosystems. That particular study, however, was largely focused on Europe and North America.
Tropical gaps: ‘Many species are being lost before we even know them’
Published data on tropical insect population trends globally and in Brazil are still scant. That being true, Lewinsohn and his team didn’t just search published papers, but also reached out to more than 150 researchers across Brazil for additional data. Ultimately, they evaluated insect trends in published and unpublished studies, gray literature (technical reports and theses), and in journals so small they don’t show up in scientific literature searches.
“We tapped into this information by querying a large number of experienced scientists. Scientists that have worked for many decades on a particular insect group, or in certain localities — [researchers who] will notice changes and trends, even if they have not conducted standardized sampling,” Lewinsohn explained.
While the team cast a wide investigative net, there are still a number of limitations to the study. Most of the gathered research focused on the Atlantic Forest biome, with both the Cerrado grasslands and the Amazon rainforest also represented, although in fewer studies. No studies were found for two of Brazil’s largest ecosystems: the Caatinga dry forests and the Pantanal wetlands — proof, once again, of the scarcity of tropical insect research.
The other limitation: most of the terrestrial research evaluated looked at particular insect groups, and not necessarily insects as a whole, with most focusing on butterflies, bees and scarab beetles.
Lewinsohn says there “are still too few [tropical studies] to establish any general trends, or lack of trends.”
Most of the total available research tracking the decline of insect abundance and diversity has been conducted in Europe and North America, with the most stunning detected declines seen in Germany. This fact points to a troubling lapse: Researchers remain mostly focused on temperate species even though most of the world’s insects, by far, are found in the tropics. In fact, tropical arthropods (insects, arachnids, millipedes and centipedes) are arguably the world’s most biodiverse grouping of life on our planet, and the majority of tropical insect species remain undescribed by science.
The reason for this lapse is clear: Most tropical countries lack the scientific resources and funding for long-term insect monitoring in either diversity or abundance.
Still, wherever scientists have looked in the tropics, they’ve generally documented insect declines. For example, a study in 2018 found shocking declines in insect abundance in Puerto Rico (a sixtyfold decline) and Mexico (an eightfold decline). Another study in Costa Rica found a 40% decline in common caterpillars in the Guanacaste Conservation Area, where Janzen and Hallwachs work.
Not all tropical research shows a decline: A recent paper in Biology Letters found that tiger moth abundance had actually risen in the last 12 years on Panama’s Barro Colorado Island. However, that study remains an outlier.
“Many species are being lost before we even know them,” said Filipe França, Lecturer at the School of Biological Sciences in the University of Bristol. França was not involved with the Brazilian research, but has studied Amazonian dung beetles.
The triumvirate of causes behind insect decline
The reasons for Brazil’s dwindling insect abundance and species populations are likely the same as everywhere else: habitat destruction, climate change, and pesticides.
Brazil has one of the highest rates of deforestation in the world. The destruction of the Amazon rainforest there has risen under the current president Jair Bolsonaro, who has dismantled environmental regulations, gutted environmental agencies, and encouraged a sprawling network of new roads penetrating the forest.
Lewinsohn says habitat destruction “has accelerated violently in the last decades” in critical Brazilian ecosystems like the Pantanal wetlands, the Cerrado savanna and, of course, the Amazon rainforest, which alone has seen nearly 20% of its biome in Brazil destroyed by agribusiness.
Brazil is also one of the largest users of pesticides worldwide. The country has a reputation for its use of highly hazardous pesticides, and in 2019 the Bolsonaro government approved a staggering 474 new pesticides — some of which are banned elsewhere; in 2018, Brazil used more than 60,000 metric tons of hazardous pesticides banned for use in the European Union — though not banned there for manufacture and export.
“Many conservation areas get spill-off from aerial spraying of adjacent crop fields,” said Lewisohn, adding this likely causes undocumented insect declines. Moreover, pesticides can be spread by the insects themselves. Agostini says adult insects may carry pesticides with them back to their nests, effectively wiping out the next generation.
Climate change has also taken a massive toll on insect populations, according to entomologists. Significant research shows that higher temperatures damage insect sperm. In addition, a 2020 study led by França found that a climate change-intensified 2015-16 El Niño resulted in a year of severe drought and fire that took a heavy toll on dung beetles in the Brazilian Amazon: 64% of dung beetles vanished after a fire and 20% after a drought. Such losses don’t only impact insects — the decline in Amazon dung beetles, which are responsible for spreading plant seeds, could diminish tropical forest flora diversity as well.
Global warming-induced extreme weather also has the potential to disturb basic insect needs.
“The rainy season starts two weeks early, but did your [insect] ovaries and your mate-seeking behavior as well?” Janzen and Hallwachs asked. “If your species-specific food plant pops its new leaves … but your dormancy does not break until cued by the first rains two weeks later, your kids starve to death … Now, multiply that across tens of thousands of species of insects and their interaction networks in any hectare of tropical forest.”
Climate change and deforestation for crops, says Lewinsohn, could also create winners and losers among insects.
“Some insect taxa may benefit from [human-induced] environmental changes — higher temperatures may extend growth or reproductive seasons in mountains, and crop-feeding herbivores have oceans of [agricultural] resources at their disposal … Some will suffer from catastrophic dry seasons, whereas others [will thrive when they] experience fewer spells of harsh cold weather.”
Given the limitations of the recent Brazilian study, and the many ongoing unanswered insect questions throughout the tropics, Lewinsohn says its vital for more resources to focus on this conservation issue there.
“We need continued and reliable support for personnel, fieldwork, processing samples and maintaining reference collections,” he said, noting that long-term funding is required.
França also emphasized the need for integration across insect studies, so that research across locales can be more easily compared, noting that “projects adopt distinct approaches to collecting insect data.”
Janzen and Hallwachs write that Lewinsohn is “certainly capable of conducting and guiding a major study” of Brazilian insects “if someone [offers] tens of millions of dollars to him.”
In the meantime, Lewinsohn calls for better enforcement of laws already on the books to provide stronger protections for Brazilian ecosystems.
“Unfortunately, the current government has been actively undermining environmental law enforcement. So, the first step is clearly to abide by the existing legislation and rebuild the agencies and personnel responsible for it. At the same time, agricultural owners and entrepreneurs need to be convinced of short- and mid-term gains by shifting practices towards reducing and reversing environmental impacts.”
A lot is at stake. Brazil is one of the most biodiverse countries in the world. And insects, in their innumerability, underpin every ecological service — from recycling waste to building fertile soil, to pollinating plants to providing prey for numerous other species.
As famed entomologist E.O. Wilson wrote in 1987, it’s “the little things that run the world.”
Banner image: A Charonias theano butterfly in Brazil. Image by André Lucci Freitas.
Lewinsohn, T. M., Agostini, K., Lucci Freitas, A. V., & Melo, A. S. (2022). Insect decline in Brazil: An appraisal of current evidence. Biology Letters, 18(8). doi:10.1098/rsbl.2022.0219
Van Klink, R., Bowler, D. E., Gongalsky, K. B., Swengel, A. B., Gentile, A., & Chase, J. M. (2020). Meta-analysis reveals declines in terrestrial but increases in freshwater insect abundances. Science, 368(6489), 417-420. doi:10.1126/science.aax9931
Hallmann, C. A., Sorg, M., Jongejans, E., Siepel, H., Hofland, N., Schwan, H., … De Kroon, H. (2017). More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLOS ONE, 12(10), e0185809. doi:10.1371/journal.pone.0185809
Lister, B. C., & Garcia, A. (2018). Climate-driven declines in arthropod abundance restructure a rainforest food web. Proceedings of the National Academy of Sciences, 115(44). doi:10.1073/pnas.1722477115
Salcido, D. M., Forister, M. L., Garcia Lopez, H., & Dyer, L. A. (2020). Loss of dominant caterpillar genera in a protected tropical forest. Scientific Reports, 10(1). doi:10.1038/s41598-019-57226-9
Lamarre, G. P., Pardikes, N. A., Segar, S., Hackforth, C. N., Laguerre, M., Vincent, B., … Basset, Y. (2022). More winners than losers over 12 years of monitoring tiger moths (Erebidae: Arctiinae) on Barro Colorado Island, Panama. Biology Letters, 18(4). doi:10.1098/rsbl.2021.0519
França, F. M., Ferreira, J., Vaz‐de‐Mello, F. Z., Maia, L. F., Berenguer, E., Ferraz Palmeira, A., … Barlow, J. (2020). El Niño impacts on human‐modified tropical forests: Consequences for dung beetle diversity and associated ecological processes. Biotropica, 52(2), 252-262. doi:10.1111/btp.12756
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