The questions that kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places that adults forget to explore. That is what inspired our seriesScience Question From A Toddler, which uses kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. The answers are for adults, but they wouldn’t be possible without the wonder that only a child can bring. I want the toddlers in your life to be a part of it! Send me their science questions, and they may serve as the inspiration for a column. And now, our toddler …
Q: What weighs more: all of the people or all of the bugs? — Carson S., age 4
This answer is maybe best illustrated by two unrelated studies whose authors probably never guessed they’d be used together. In 2012, scientists estimated the global human biomass (i.e., how much we all weigh) at 287 million metric tons. Five years later, a different group of scientists set out to estimate how much the world’s spiders were eating. They came up with a horrifying (if somewhat inexact) estimate of 400 million to 800 million metric tons’ worth of prey each year. In other words, just the subset of bugs eaten by spiders last year probably outweighs all the humans on Earth. Even if the humans are, generally speaking, a touch better off in the end.
So all of the bugs definitely weigh more than all of the humans. But as you hug your knees and gently rock, trying not to touch any of the filthy, bug-covered surfaces that surround you at all times, you should know that this apparent win for bugkind masks some serious problems for the bugs and, as a result, for us. Turns out, there are fewer bugs than there used to be — both in total weight and in terms of species diversity. And we humans are to blame.
But first, let’s back up a minute and talk a bit about why it’s possible for bugs to outweigh humans. The sheer number of bugs in the world is a little difficult to fathom. “There’s an estimated 10 quintillion insects on the globe,” said Julie Peterson, professor of entomology at the University of Nebraska-Lincoln. “That’s 10 with 18 zeros after it, and that’s just insects. That’s not counting other arthropods like spiders and mites.” Insects — along with ticks, centipedes, spiders and all the other land-dwelling creepy-crawlies that we colloquially call “bugs” — probably represent as much as 80 percent of the species on this planet. In contrast, humans are a single species, made up of (as of this writing) 7,386,922,190 individuals.
To make those enormous numbers easier to grasp, let’s turn to an anecdote: Elizabeth Borer, a biology professor at the University of Minnesota, told me about a 1982 study in which an entomologist named Terry Erwin went to Panama and started taking samples of the beetles he found in one type of local tree. To do this, researchers fog a tree with pesticide the way an exterminator might fog a house, and then they count and categorize the unfortunate bugs that fall out. Erwin found more than 955 species of beetles in just 19 trees. Not 955 individual beetles. Species. Based on what he knew about the prevalence of this type of tree in the Panamanian forest and the prevalence of beetles compared to other kinds of insects, Erwin came up with a back-of-the-envelope calculation that every hectare of Panamanian forest could be home to as many as 41,000 species of insects — millions, maybe hundreds of millions, of individuals living in an area not much larger than a couple of soccer fields.
And this is why bugs, as a whole, beat humans in a pound-for-pound weigh-off. Individual bugs may be small — the largest species, such as New Zealand’s grasshopper-like giant weta, top out around 70 grams, Peterson said — that’s about the size of a jumbo chicken egg. But even if you take what Peterson thinks is likely an underestimate of the average bug size — 0.05 milligrams, say — it still adds up by the time you multiply it by 10,000,000,000,000,000,000. This is a numbers game, and the bugs are very much ahead.
Granted, all of these numbers come from extrapolation and estimation. Scientists frequently do studies like Erwin’s, going to a region and taking samples that tell them how many bugs live in a tree, or on one square meter of ground. Take enough samples like that, and you start to get an idea of what’s normal for a particular kind of ecosystem. Then it’s just a matter of figuring out how much of that ecosystem covers the Earth and doing the math, Peterson said. The estimates also assume that there are a lot of insect species we don’t yet know about. One million species of insects have been named and documented, their type specimens sealed in jars or illustrated in books. There may be more than 4 million species yet to be catalogued.
And if all of that isn’t blowing your mind, consider what those quintillion bugs mean to the world. For starters, they’re an important part of the food chain — bird health, in particular, depends on bug health. Bugs are also pollinators, and that’s not just bees. Wasps, ants, flies and beetles all get in on that hot plant reproductive system action. And while not all the food we eat relies on pollinators, some of the really good stuff — almonds, avocados, many fruits and nuts, and the alfalfa that feeds our meat animals — does. Then there’s the role bugs play in decomposition. Dung beetles save the U.S. cattle industry $380 million every year by breaking cow poop down into dirt, a service that also helps to put nitrogen — an important source of plant food — back into the soil.
Bugs matter, and if scientists know how many bugs are in a square meter or what those bugs weigh, they can get an idea of how capable the existing bug population is of doing all the jobs bugs do. This is what scientists mean when they talk about “biomass.” If you know how much material a single dung beetle is responsible for decomposing, then knowing how many dung beetles there are helps you understand how much can be decomposed. If you know how many pounds of bugs a single bird eats, then you know how many birds can live off the bugs in a Panamanian tree. And the answers to those questions are pretty important, because they tell you practical facts — like whether birds can survive in a given habitat, or whether the poop is going to start piling up on your farm.
That means biomass is both a measure of the health of an insect community and of nature as a whole. And this is where the wacky science of weighing bugs starts to overlap with the existentially stressful science of watching helplessly as ecosystems collapse. Invertebrates, a group that includes insects, are poorly studied by conservation biologists, at least in comparison to their numbers, and the health of their communities can vary a lot by location and species. But the research that does exist suggests that insects aren’t doing well. For instance, the International Union for the Conservation of Nature (the group whose research plays a big role in determining which species we consider endangered) tracks only 3,623 species of terrestrial invertebrates — bugs, basically, plus worms and some mollusks. But of those, 42 percent are threatened with extinction. “We’re probably losing species faster than we can give them names,” Peterson said.
And you can see this in specific groups of species, as well. A 2015 study took advantage of a long history of records documenting populations of moths and butterflies in a protected grassland in Germany. It found declines in the number of species recorded, from a high of 123 in the 1870s to 71 by the early 2010s. What’s more, habitat-specific species were more likely to have been lost. In the 1870s, 50 percent of the moth and butterfly species were generalists: animals that can happily live in many places. By the 2010s, 68 percent were generalists. The species that were particular to those German grasslands faded faster. Peterson said that this pattern is reflected in many other places. Resilient generalists survive, while the species that can’t adapt as easily flounder. And that’s bad for people, because the most resilient generalists are the species we consider pests — cockroaches, say, or mosquitoes. “As we lose insect diversity, we’re seeing an increase in pest species,” she said.
We’re losing pollinators. We’re losing the food other animals eat. We’re losing the bugs that bury poop and dead things and help return waste to the soil.
And the culprit, inconveniently, is us. The biggest threat to insect species is habitat loss caused by agriculture, logging and infrastructure development. And that makes stopping the loss of insects difficult, Peterson told me. Often the people who need the bugs the most — for example, the Nebraska farmers who rely on burying beetles to serve as undertakers for the dead frogs and mice that help make their farmland fertile — are also the people whose livelihoods depend on destroying those bugs’ habitat. This beetle was native to the prairie, Peterson said. There’s not much prairie left and, consequently, not many beetles. But the beetles helped make the rich prairie soil, which made their habitat a great place to convert into the farmland that wrecked the beetles’ own homes. If they die off, that’s not good for farmers. But farmers also can’t just stop farming, because a prairie can’t feed humans.
There are no easy answers. In Nebraska, Peterson said, the Environmental Protection Agency now requires farmers who want to use insecticides to first make sure that there aren’t burying beetles on their land. If they find these helpful bugs, farmers have to use a more expensive insecticide that can kill pests while protecting the beetles. Peterson sees this as a trade-off: Spend a little more money now on insecticide so you don’t have to spend as much on fertilizer later. But the world is full of millions of conflicts like this — as many as there are species of insects, probably. There’s almost no way to make everybody, insects and humans, happy. The bugs might weigh more than us — for now — but we might not really feel that weight until it’s gone.