Why land insects are in decline in Brazil
New Brazilian research shows that terrestrial insects are decreasing in both abundance and diversity; aquatic insect populations remain stable.
Given the lack of long-term data on tropical insects, scientists have adopted creative means to collect data, including contacting 150 experts for as-yet-unpublished data.
Scientists believe that the main factors responsible for the decline of insects are habitat destruction, pesticide use and climate change.
Experts say tropical countries need more resources to find out with greater certainty what is happening to insects in their territories. Large-scale losses threaten many ecological services of insects, such as recycling waste, forming fertile soils, pollinating plants and also as a food source for other species.
In 1978, when he moved to his current home in rural Campinas, Thomas Lewinsohn, professor of Ecology at the University of Campinas, always found a “satisfactory variety of insects” at night . Today Campinas, which is 100 kilometers from São Paulo, is an expanding urban and suburban agglomeration, with more than 3 million inhabitants. And, as Lewinsohn reports in his new paper , insect populations “dropped precipitously” within 40 years of their arrival. Species such as feather moths and longhorn beetles have disappeared.
Lewinsohn's story is not unique; it reflects anecdotal evidence from entomologists, ecologists, and nature lovers worldwide. But the story of the great insect death , which broke the media headlines in 2018 with apocalyptic warnings and backlash that accused them of exaggeration , also saw the publication of countless studies around the world. However, this attention has always had an obvious flaw: the lack of news and research in the tropics .
Now, Lewinsohn and his colleagues have published a paper that sheds light on the situation of insects in the Brazilian tropics: the findings are neither surprising nor encouraging, but they add essential data to gaps in the global insect map.
Discoveries about terrestrial and aquatic insects
Lewinsohn and his colleagues examined 75 projects that tracked insects in Brazil. In all, 17 studies show a decline in the abundance of terrestrial insects, while only three showed a population increase. Eleven studies found a reduction in diversity, while one showed an increase in it. On average, these land studies span 22 years of follow-up.
“Overall, studies around the world are seeing insect declines, and the study we did is further evidence of this trend,” says co-author Kayna Agostini of the Federal University of São Carlos.
The results for aquatic insects were quite different. Only two studies showed a decline in abundance, and two showed the opposite. Three studies found a decrease in diversity, while four showed an increase in it. Most of the evaluated studies, however, found a stable trend for aquatic insects in terms of abundance and diversity.
The reason for the divergence between terrestrial and aquatic insects is uncertain. The article suggests that the different findings may be due to fewer studies being carried out on aquatic insects, and that these are generally done over shorter periods of time (11 years, on average, versus 22 years in terrestrial studies). Also, in some cases, aquatic insects may have recovered after polluted areas were cleaned up.
“We were not convinced by the observation that the density and species richness of aquatic insects seem to be maintaining themselves. It's a false illusion," wrote Daniel Janzen and Winnie Hallwachs to Mongabay after reviewing the study, which they did not participate in. Janzen, an ecologist at the University of Pennsylvania, has been warning about the decline of tropical insects for decades. Hallwachs is a tropical ecologist. Both have carried out entomological research for decades in the Guanacaste Conservation Area, in Costa Rica, declared a World Heritage Site by Unesco.
"What we've seen since the 1950s is a huge decline in aquatic dispersal stages (of adult flying insects)," wrote Janzen and Hallwachs.
Still, the findings of Brazilian scientists on aquatic insects reflect the conclusions of other research, especially a large study from 2020 , published in the journal Science, which revealed that populations of terrestrial insects are falling by almost 1% a year, but are increasing in water bodies, likely due to worldwide efforts to restore freshwater ecosystems. However, this particular study focused primarily on Europe and North America.
Losing species before you even know them
Published data on insect population trends in the world and in Brazil remain scarce. Therefore, Lewinsohn and his team not only searched for published articles, but also contacted more than 150 researchers across Brazil to obtain additional data. Finally, they assessed insect trends in published and unpublished studies, in the gray literature (technical reports and theses), and in journals so small that they don't even appear in scientific literature searches.
“We arrived at this information by consulting a large number of experienced scientists. Scientists who have been working for many decades with a certain group of insects, or in certain localities – [scientists who] notice changes and trends, even if they are not taking standardized samples,” explained Lewinsohn.
Although the team has cast a wide net of investigation, there are still numerous limitations to the study. Most of the research gathered focuses on the Atlantic Forest biome, while the Cerrado and the Amazon are represented in a smaller number of studies. No studies were found for two major Brazilian biomes: the Caatinga and the Pantanal – proof, once again, of the lack of research on tropical insects.
The other limitation: most of the terrestrial surveys evaluated look at specific groups of insects, and not necessarily insects as a whole, with most of the work focusing on butterflies, bees and beetles.
Lewinsohn says that “there are still too few [studies in the tropics] to establish any general trends, or lack of trends.”
Most of the research tracking declines in insect abundance and diversity has been conducted in Europe and North America, with the most striking declines recorded in Germany . This fact points to a problematic flaw: researchers continue to focus primarily on temperate species, even though most of the world's insects, so far, are found in the tropics. In fact, tropical arthropods (insects, arachnids, myriapods and centipedes) are undoubtedly the most biodiverse group of living beings on our planet, and most species of tropical insects have yet to be described by science.
The reason for this lack of data is clear: most tropical countries lack the scientific resources and funding to monitor insects over the long term to assess their diversity and abundance.
Yet wherever scientists have studied, they have generally documented declines in tropical insect populations. For example, a 2018 study found severe reductions in insect abundance in Puerto Rico (by six times) and Mexico (by eight times). Another study in Costa Rica found a 40% drop in common caterpillars in the Guanacaste Conservation Area, where Janzen and Hallwachs work.
Not all tropical surveys show declines: a recent article in the journal Biology Letters revealed that tiger moth abundance had actually increased over the past 12 years on Barro Colorado Island in Panama. However, this study deviates from the curve.
“We are losing many species even before we know about them”, says Filipe França, professor at the School of Biological Sciences at the University of Bristol. França was not involved in the Brazilian research, but he studied the beetles popularly known as dung beetles in the Amazon.
The main causes of insect decline
The reasons why the abundance and populations of insect species are decreasing in Brazil are probably the same as in the rest of the world : habitat destruction, climate change and pesticides.
Brazil has one of the highest deforestation rates in the world. Destruction of the Amazon rainforest increased during the tenure of President Jair Bosonaro, who dismantled regulations, scrapped environmental agencies and encouraged the expansion of a network of new roads that penetrate the forest.
Lewinsohn says that habitat destruction has "accelerated violently in recent decades" in key biomes like the Pantanal, the Cerrado and, of course, the Amazon, which alone has seen about 20% of the original vegetation destroyed by human action in Brazil .
Brazil is also one of the largest users of pesticides in the world. The country has a reputation for using highly hazardous pesticides , and in 2019 the Bolsonaro government approved no less than 474 new pesticides – some of which are banned in other countries; in 2018, Brazil used more than 60,000 metric tons of hazardous pesticides banned for use in the European Union – which, however, did not ban their manufacture and export.
“Many conservation areas are affected by aerial spraying on adjacent crops,” says Lewisohn, adding that this likely causes an undocumented decline in insects. In addition, pesticides can be spread by the insects themselves. Agostini says that adult insects can carry pesticides back to their nests, ending the lives of the next generation.
Climate change is also having a major impact on insect populations, according to entomologists. Significant research shows that higher temperatures damage insect sperm. Furthermore, a 2020 study led by France revealed that climate change intensified by the 2015-16 El Ni̱o resulted in a year of severe drought and wildfires that heavily impacted dung beetles in the Brazilian Amazon: 64% of these animals disappeared after a fire and 20% after a period of drought. Such losses do not only impact insects Рthe decline of dung beetles in the Amazon, which are responsible for spreading plant seeds, may also reduce the diversity of flora in the forest.
Extreme weather caused by global warming also has the potential to affect insects' basic needs.
“If the rainy season starts two weeks earlier, will the insects' mate-seeking behavior also?”, ask Janzen and Hallwachs. “If the specific plant that feeds a certain species releases new leaves, but the dormancy period [of the insect] only ends when the first rains arrive, two weeks later, the offspring die of hunger. Now multiply that by the tens of thousands of insect species and their interaction networks in any given hectare of rainforest.”
Climate change and deforestation for planting can also create winners and losers among insects, says Lewinsohn.
“Some insect taxa can benefit from environmental changes – high temperatures can extend growing and breeding seasons in the mountains, and plant-eating herbivores have oceans of [agricultural] resources at their disposal. Some will suffer from catastrophic dry seasons, while others [will do better] by experiencing fewer waves of cold weather.”
Prognosis for insects
Given the limitations of the Brazilian study, and the many unanswered questions about insects in the tropics, Lewinsohn says it's essential to allocate more resources to this conservation issue.
“We need ongoing, reliable support for staffing, fieldwork, sample processing and maintenance of reference collections,” he says, noting that long-term funding is needed.
França also emphasized the need for integration between insect studies so that surveys from different sites can be more easily compared, noting that “projects take different approaches to collecting insect data.”
Janzen and Hallwachs write that Lewinsohn “is certainly capable of conducting and guiding a large study” of Brazilian insects “if someone [offers] tens of thousands of dollars for him.”
Meanwhile, Lewinsohn calls for better enforcement of existing laws to increase protection for Brazilian ecosystems.
“Unfortunately, the current government has acted to undermine environmental enforcement. So, the first step is clearly to comply with existing legislation and rebuild the [environmental] bodies and teams responsible. At the same time, agricultural landowners and entrepreneurs need to be convinced of the short and medium term gains they will have by changing practices with the aim of reducing and reversing environmental impacts.”
There's a lot at stake. Brazil is one of the most biodiverse countries in the world. And insects, in their multitude, sustain all ecological services – from recycling waste to building fertile soil, to pollinating plants and serving as prey for countless other species. As famed entomologist EO Wilson wrote in 1987, it's "the little things that make the world go round."
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