Tropical Insect Populations Crashing?
Early in the 1980s, a scientist named Terry Erwin decided to count the number of Coleoptera species — the insect order containing beetles and weevils — in the rainforest canopy. He was trying to answer a really big question: how many species live on Earth? And he figured beetles, one of the planet’s super-families, was the place to start.
So he packed off to Panama and picked 19 trees of a single species, Luehea seemannii. Then, he bombed them with pesticide — a method dubbed “fogging,” well known to entomologists and exterminators. Counting the dead, Erwin identified 988 distinct beetle species.
Then he did some back-of-napkin-calculations, estimating how many of those beetles might be endemic to Luehea seemannii, and how many rainforest tree species there might be in the world, etc., etc., until he concluded there could be 30 million tropical arthropod species on the planet.
Most scientists today have considerably curbed that estimate, putting the total count for all species of plant and animal at around 8-plus million. But even though that total is smaller, Erwin’s work proved vital in two ways: it showed that insect diversity was far greater than anyone had then imagined in tropical rainforests, and that rainforests were among the most biodiverse places on the planet. In other words, when it comes to the insect kingdom, the tropics are king.
Fast forward to 2012 when scientists announced the findings of a survey on a single acre of rainforest in Panama. They collected 130,000 individual arthropods (insects plus spiders, millipedes, centipedes and scorpions) totaling a stunning 6,144 species.
Let’s put that into context: there are around 6,500 identified mammal species in the world. That’s including everything from the freaky aye-aye to the blue whale to the elegant saola, plus species-diverse families like rodents and bats. However, just one acre of rainforest held nearly an equal number of unique arthropod species.
“There are more species of insects within 25 kilometers of my house in [Costa Rica’s Guanacase Conservation Area] than in all of Europe,” says University of Pennsylvania entomologist Dr. Daniel Janzen, who’s been studying insects in the small Latin American nation since the 1960s. “Costa Rica has as many species as all of North America.”
Those staggering numbers suggest a disturbing question: if insects are plunging in abundance globally, as recently trumpeted in the mainstream media — then what’s happening in the mega-diverse rainforests now being impacted by climate change and invaded by oil palm plantations and cattle ranches?
The answer: we don’t know yet. Little research on insect abundance has been done in the region where nearly all insect species live: the tropics.
To assess the situation, Mongabay, a nonprofit provider of conservation and environmental science news, interviewed over a dozen entomologists who have worked in the tropics or subtropics, including Oceania, Southeast Asia, Latin America and Africa to gain a sense of the state of insects in their most glorious kingdom.
Red Flag in Puerto Rico
While some recent studies have looked at insect abundance declines in temperate regions, to date, only one paper has investigated this question in depth in the tropics — and it was an eyeopener.
In 2013, Bradford Lister and Andres Garcia traveled to Puerto Rico’s Luquillo rainforest to repeat a 1976 study which measured the total biomass of the forest’s arthropods — the total weight of captured insects and other arthropods. They found that in the intervening decades' populations had collapsed. Sticky traps caught up to 60-fold fewer insects than 37 years prior. Netting on the ground caught eight times fewer insects than in 1976.
When published in 2018, the paper hit the science world like a bomb — especially coming after a 2017 study identifying a similarly catastrophic multi-decade decline in flying insects in German nature reserves.
Lister and Garcia’s paper also looked at insect abundance in another tropical forest, this one in Western Mexico. There, biomass abundance fell eightfold in sticky traps from 1981 to 2014, not as dramatic as Puerto Rico (the timescale was shorter), but still disturbing and evidence that Puerto Rico wasn’t a one-off, and potentially an Earth-sized problem.
“Puerto Rico might be more sensitive than other regions due to its small and isolated landmass,” noted Michelle D’Souza, a postdoctoral researcher currently leading an insect sampling project in South Africa. But even that wouldn’t explain the decline in Mexico.
Lister and Garcia argue that climate change — specifically extreme, global warming-fueled heat waves — is wiping out insects en-masse in the tropics, even in protected areas. Though other entomologists pin the blame more on the destruction of ecosystems.
While the Puerto Rico study was shocking, the data pointed to something Janzen says he’s been watching happen over decades. Janzen and fellow-entomologist and partner, Dr. Winnie Hallwachs, recently wrote a paper in Biological Conservation, that reads like a scientific eulogy to a lost world.
In heart-breaking detail, their paper — at times thick with scientific jargon, and at others uncommonly poetic — describes how the increasing unpredictability of rainy seasons, increasingly extreme heat and desiccation, all due to global warming, have wiped out the insect abundance that characterized the Guanacase Conservation Area in Costa Rica through the 1980s.
“Gone are the spiderwebs that decades back entangled those leaves. Gone is the nighttime sparkle in the leaves reflected from thousands of lycosid spider eyes,” they write. Indeed, from 1992-2014 the number of caterpillars collected annually by taxonomists at their study site dropped by around half — the numbers are still declining.
Surveying insects at night in Costa Rica’s Guanacase Conservation Area. Costa Rica has as many insect species as all of North America. Image by Daniel Janzen.
Counting insect declines in the dark
When asked how insects are faring in their study regions, nearly all the entomologists working in tropical and subtropical regions answered with the same refrain: we don’t really know, but anecdotally it’s not good.
“We have very little information on trends in insect populations in [Southeast Asia], something that is true for most parts of the planet,” says Dr. Owen Lewis, an entomologist and ecologist at the University of Oxford, who’s done work in Borneo. “Ideally, to get this information we would have regular, repeated sampling of the same locations using the same methods, over periods of years or decades.” But those sorts of studies haven’t been done — a baseline is lacking nearly everywhere.
Research on insects, both tropical and temperate, has focused largely on discoveries — naming new species, describing natural histories and ecology. But, lacking data, most tropical scientists agree: it’s obvious something serious is happening.
“It appears that [tropical] insects are being affected even in regions that have been less impacted by agriculture,” said D’Souza, pointing to South Africa, where she’s heard from other researchers that abundance seems to be plummeting. “What we hope to be a situation isolated to a few regions is appearing to be a very widespread problem.”
Researchers say that it’s reasonable to expect huge losses in tropical insect abundance and diversity for two reasons: forest destruction and climate change. Other threats — pesticides and invasive species — are also mentioned, but less frequently and emphatically.
“Because the climate is warming in the tropics, and tropical forests are experiencing the greatest deforestation rates, it’s probable that insects will not fare well,” notes Danielle Salcido, a University of Nevada graduate student studying Neotropical insects.
Habitat loss is the first concern for most entomologists questioned, and that’s hardly surprising: roughly a quarter of the Amazon rainforest, the world’s largest and most diverse, is gone. The Congo, the globe’s second largest rainforest, saw an area erased nearly the size of Florida from 2000-2014. And Borneo, the Earth’s third largest island, has lost half its forest cover in just 60 years.
In fact, nowhere have forests been falling faster in recent decades than in Southeast Asia, where Indonesia is now the leader of global forest destruction, even beating Brazil. “We can be confident that populations of many [insect] species in Southeast Asia will be doing poorly overall,” says Lewis, despite sparse data. He adds that forest conversion to agribusiness “especially oil palm plantations” means that “forest-dependent insect species will have crashed.”
Some species may see rapidly rising populations — those that do well in disturbed environments — but others will suffer. “A few winners, but many losers,” Lewis concludes.
African entomologists likewise cite habitat destruction as the most pressing issue. University of Pretoria entomologist Mervyn Mansel reports that the insect species that he studies, lacewings — many endemic to South Africa — are imperiled by habitat loss. Still, he adds, it’s impossible to extrapolate onto other insect families.
“This [problem] is extremely difficult to assess on a general scale. Some insect groups may be disappearing due to excessive urbanization [and] habitat destruction… On the other hand, some insect groups are flourishing due to favorable conditions created by humans,” he says.
Mpala Research Center Executive Director and entomologist Dr. Dino Martins, says he’s anecdotally noticed a decline in insects in his home country of Kenya, but there’s no hard data yet. Still, he thinks habitat loss is the biggest factor at work in East Africa — especially due to the loss of particular plants: “If you lose endemic plants, it’s obvious that the endemic insect species that live on them, pollinate them, and parasitize them, would disappear.”
Still, not all tropical nations may be seeing uniform rates of decline. Dr. Vojtech Novotny, a University of South Bohemia entomologist who studies insects in Papua New Guinea, says he hasn’t noticed losses. “If I were to speculate, I would not expect a serious decline taking place here. I have not noticed it over the past 25 years.” This may be because, unlike many tropical countries, Papua New Guinea still has significant primary forest cover. Though that may be changing, as the nation undergoes an epidemic of illegal logging.
“Insect species and population densities have been gradually declining in the Neotropics for at least a century, anywhere that humans have begun their usual overpopulation, clearing of natural ecosystems for human purposes, and massive uses of pesticides,” Janzen explains. He adds that climate change is now an “impact that blankets over it all, rather than being in pockets and regions.”
Climate change: the heat is on
While most entomologists blame habitat loss as being key to tropical insect declines to date, they see global warming as an up and coming ecosystem-wide exterminator.
“Climate change is a more insidious change which could cause just as many extinctions in the long term,” Lewis says. And it could hit insects in multiple ways: via intensifying extreme weather — more and longer heat waves and sudden deluges. At the same time, rainforests and cloud forests that once saw near-constant moisture, are drying out, due to the one-two punch of escalating heat and longer droughts, something Janzen and Hallwachs described in their study sites.
“It’s important to remember that small increases in temperature have large negative effects on tropical insects,” notes Salcido.
In Kenya, Martins expresses another concern: that climate change could knock insects and plants “out of sync.”
“We are disturbing the [climate] system to the point where the productivity and the interactions between species are being impacted,” he says. Flowers bloom early or late, and there’s no one there to pollinate them, or insects emerge looking for their food sources, but flowers have yet to bloom.
Because we know so little about tropical insect lifecycles, we know even less about how families or species might respond to climate change. But some research suggests the news won’t be good: a 2018 study found that heatwaves created under laboratory conditions (5-7 degrees Celsius above optimal temperatures for five days) decimated the sperm of male flour beetles. The authors even found that “successive heatwaves almost sterilize males.” If this study carries over widely to the field, climate change may turn whole populations of male insects into eunuchs.
The pesticide problem
Pesticides, though not at the top of every tropical scientist’s list, are a chronic concern.
Francisco Sanchez-Bayo, an ecologist with a specialty in pesticides at the University of Sydney, says toxic chemicals may play an even bigger role in die-offs in tropical developing countries than in the industrialized world.
“Pesticide usage has been increasing worldwide at an alarming rate, mainly because developing countries are buying tons and tons of chemicals without much control,” he says.
Pesticides are typically less regulated, and use is more haphazard in developing countries, where farmers are often untrained and unprotected. Brazil, the world’s largest pesticide consumer, provides a good example: Jair Bolsonaro’s rightist government is now rushing hundreds of largely untested, potentially toxic, pesticides to market, with arguably predictable results: a die-off of half a billion butterflies over a three month period was reported in the Latin American nation in March of this year.
Salcido says that agribusiness-driven deforestation, combined with pesticides, are like twin “tsunamis wiping out entire communities of organisms.”
“Things are becoming more homogenized,” she notes. “For me that is like limiting the colors or media an artist has at their disposal to create a work of art. You start taking away colors of paint, the artist can still create a piece of art, but whether it was as beautiful as it could have been is up for debate.”
Lack of capacity
A lack of scientific knowledge and research capacity further imperils tropical insects. Manu Saunders, an entomologist and ecologist at Australia’s University of New England, notes that Australia’s insects are little known or studied — scientists have only described 20-30 percent of the continent’s insects. That’s about the average worldwide, but surprising given that Australia is an industrialized nation with first-class universities.
Saunders blames this lack of data on a “perfect storm”: little public interest, little money, lack of college-level coursework, and few young students choosing this career path, “often because of lack of funding and therefore jobs.” She says there is no research on population trends in Australia.
Entomologist Helen Spafford, formerly at the University of Hawaii at Manoa, says Oceania suffers from a lack of funding for insect research, but she believes populations are dropping there “due to habitat destruction, changing environments and the presence of invasive species.” Invasive insect species pose a similar problem as invasive mammals do to small island nations — invasives have a tendency to overrun islands at the expense of natives.
And yet, scientific “funding and capability” continue to decline in Oceania, Spafford adds, leaving much of the region in the dark about what’s happening to its arthropods. “The critical issue here is that the changes we are seeing are happening at such a rapid pace that at this point we are really just trying to catch up.”
Although it’s the planet’s second largest continent, Africa — with 54 countries and 1.2 billion people — is probably the least surveyed and studied for its insects. There are only a few entomological societies on the entire continent, and the researchers that exist there are based in more developed countries, like South Africa. In biodiverse places like the Congo basin, entomologists are rare.
D’Souza reports that around 100,000 insect species have been described in sub-Saharan Africa, but expects the real figure is at least six times higher, meaning entomologists may still be missing more than 85 percent of species.
Surveys in the making
Thanks partly to the recent alarm sounded over global insect abundance decline, more hard data may start trickling out of the tropics soon.
Indonesian Institute of Sciences spider expert Cahyo Rahmadi points to ongoing surveys in Gunung Halimun National Park on Java. Researchers first surveyed the park in 2000-2001 to determine seasonal insect patterns, but have now returned to do a follow up survey that may show how insects are faring 20 years later.
Salcido is also currently engaged in Central and South American surveys: “Based on preliminary results of some of our tropical sites, insects appear to have [suffered] similar fates,” as those reported in Puerto Rico. Still this doesn’t mean all insect families are declining everywhere: Salcido points to an Andes high-altitude study site with stable populations of Lepidoptera, a family that includes moths and butterflies.
Janzen is now focused on a mission to catalogue every species in Costa Rica and then employ that knowledge to educate the country’s citizens. If “you find Costa Rica $100 million to spend over 10 years to facilitate the self-emergence of a bio-literate tropical country, you will see what really could be done if people actually care about insects,” he states.
As part of that work, Janzen has proposed DNA barcoding of every species in every Central American country — using a method developed by Paul Hebert at the Centre for Biodiversity Genomics (CBG) — and then making that data publicly available for education and utilization.
Janzen believes that “bio-literacy” should be taught, just as “education normally teaches literacy, but to everyone, not just the anointed of society.”
The Arthropod Initiative and Global Malaise Trap Program
Entomologists cited time-and-again one global project as having the potential to shed light on what’s happening in the tropics: the Arthropod Initiative.
The brainchild of Yves Basset, the Arthropod Initiative is gathering data at sites around the world through the Smithsonian Institute, where Basset works. The program currently has monitoring sites in Panama and Thailand (running since 2008), Papua New Guinea, Ecuador and China, with hopes of expanding to Gabon, Singapore, and the Philippines soon.
“At these sites, we have data on abundance for each insect species,” reports Basset, while also taking exacting measurements to estimate each species’ biomass and overall biomass.
Researchers involved in the program agree that the initiative needs a few more years before trends are really apparent, but Basset says early data shows “some species are increasing, others are declining.”
Such trends, he says, depends on the taxa. “It is a bit as if you put together polar bears and elephants, and talk about the threats for their respective population dynamics.”
The Arthropod Initiative isn’t the only global tropical program moving ahead. Martins and D’Souza, in Kenya and South Africa respectively, are leading regional surveys with the Global Malaise Trap Program run out of the CBG.
D’Souza says her work in South Africa’s Kruger National Park is “one of the first projects that is sampling and delivering data on both [insect] species richness and abundance at [high] speed and resolution, made possible by using DNA-based identification systems.” To date, D’Souza’s team has detected 9,000 species after sampling 180,000 specimens in the park, which is a little smaller than Israel. She expects Kruger holds roughly 20,000 total insect species.
The Global Malaise Trap program collects samples caught in malaise traps, a tent-like structure used most often to catch flying insects (the same type deployed in the 2017 Germany abundance study). The program monitors 158 sites in 33 countries, and has collected two million samples since 2012, and aggregated its data via the online Barcode of Life Data system. While the program is focused on understanding insect biodiversity, future data could track population trends.
“The program has a lot of potential to investigate insect decline,” says Kate Perez, Field Operations and Collections Lead for CBG. But insect decline is not presently the priority: instead, the program is expanding to new regions to get a more complete picture of global insect diversity. Still, Perez notes that a few partners have re-sampled their sites, providing potential abundance trends.
Radboud University ecologist Dr. Hans de Kroon of the Netherlands, a co-author of the 2017 German study, says tropical entomologists should take a page from temperate research. The “Germans were just perfect in how they set up that monitoring program, documented everything in a very meticulous way, [with a] highly standardized way of doing measurements. That’s why we could use it in such a solid way in our analysis… [and] if you don’t have any historical data, [still] make interesting comparisons.”
Such non-historical insect abundance comparisons could, for example, include a type of research already done for birds and mammals: simultaneously comparing current insect species populations in heavily-degraded habitats, such as oil palm plantations or cattle ranches, versus nearby secondary and primary forest.
The answer may be in the freezer
Janzen and Hallwachs express great concern for the future of tropical insects in their 2019 paper: “We will lose much of the insect community that today is still in the cloud forests, being burned off the tops of tropical mountains, just as we lose the deep valley ecosystems, drowned by hydroelectric impoundments or plowed under by industrial farming.”
But even as the media finally sounds the alarm about insect decline, a grim truth is faced by all entomologists: funding is difficult to get and harder to keep. “We need to attract sponsors and patrons, and so far, we have failed to do that,” Basset admits of the Arthropod Initiative.
Few individual donors or foundations show much interest in insect surveys and preservation; most big conservation organizations and governments shy away too.
Even Dr. Bradford Lister, co-author of the blockbuster study in Puerto Rico, says he’s not sure he’ll have the funding to continue monitoring there and in Mexico. “We have to go back again, see what’s happening to the insects. And beyond that, we’d like to get other [data] points in the tropics as well,” he says. But above all “we have to get money.”
Lister hopes to secure a four- to five-year grant by this fall to keep surveying through 2021. But that won’t happen without the cash. Meanwhile, his team’s insect specimens sit awaiting study.
“I have the last five years of massive insect biomonitoring [collections] here, sitting in my freezers, while I search for the funds to do its analysis — at least 700,000 frozen insects of at least 200,000 species (at $3/insect),” wrote Janzen in an email. “Who gives a damn enough to be willing to fund that public service?”
Who gives a damn indeed?