Global insect declines: Why aren't we all dead yet?

One study on a German nature reserve found insect biomass (e.g., kilograms of insects you’d catch in a net) has declined 75% over the last 27 years. Here’s a good summary that answered some questions I had about the study itself.

Another review study found that, globally, invertebrate (mostly insect) abundance has declined 35% over the last 40 years.

Insects are important, as I’ve been told repeatedly (and written about myself). So this news begs a very important and urgent question:

Why aren’t we all dead yet?

This is an honest question, and I want an answer. Insects are among the most numerous animals on earth and central to our ecosystems, food chains, etcetera. 35%+ lower populations are the kind of thing where, if you’d asked me to guess the result in advance, I would have expected marked effects on ecosystems. By 75% declines – if the German study reflects the rest of the world to any degree – I would have predicted literal global catastrophe.

Yet these declines have been going on for apparently decades apparently consistently, and the biosphere, while not exactly doing great, hasn’t literally exploded.

So what’s the deal? Any ideas?

Speculation/answers welcome. Try to convey how confident you are and what your sources are, if you refer to any.

(If your answer is “the biosphere has exploded already”, can you explain how, and why that hasn’t changed trends in things like global crop production or human population growth? I believe, and think most others will agree, that various parts of ecosystems worldwide are obviously being degraded, but not to the degree that I would expect by drastic global declines in insect numbers (especially compared to other well-understood factors like carbon dioxide emissions or deforestation.) If you have reason to think otherwise, let me know.)


Crossposted from my personal blog. Also see Brian Tomasik's comment on the original post, which I haven't assessed in detail but which seems important.

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(Epistemic status: not much domain knowledge here, working off of first principles and off the top of my head.)

I had a conversation about roughly this topic (specifically it was about the ecological effects of killing all the mosquitoes with CRISPR) with Critch once and I quite liked his reply, which was, roughly and as best as I can recall: ecosystems have many negative feedback loops in them making them much more resilient to this sort of shock than a naive analysis would suggest. If a bunch of insects die, whatever food sources they were relying on can proliferate, which means other organisms (insects or otherwise) have an easier time eating them, and can in turn can be eaten by whatever predators were preying on the insects earlier.

There's also a big difference between the effect of suddenly killing off 75% of the insects and the update you make after observing that 75% of the insects have died. The latter could just be a new equilibrium ecosystems have settled into in response to a more fundamental shock, e.g. global warming or pesticides or whatever else.

ecosystems have many negative feedback loops in them making them much more resilient to this sort of shock than a naive analysis would suggest. If a bunch of insects die, whatever food sources they were relying on can proliferate, which means other organisms (insects or otherwise) have an easier time eating them, and can in turn can be eaten by whatever predators were preying on the insects earlier.

This would make more sense to me if we were talking about the number of insect species rather than the total mass of insects. A bunch of insect species die out (killed by climate change, pesticides, urbanization, or what-have-you), but the few that survive proliferate and creatures at higher trophic levels don't care too much because the overall mass of insects remains the same even if the variety of insects declines.

However, the thing that Eukaryote is talking about is a decline in insect biomass, rather than biodiversity. In fact, what I've read suggests that insect biodiversity has declined less than insect biomass. We have mostly the same kinds of insects, but a whole lot fewer of them.

While it is plausible that other organisms have proliferated in place of insects, I would have expected that proliferation to be noticed and reported on. Moreover, insects occupy a pretty low trophic level. Naively, I would expect a 75% decline at this level to send shockwaves up the food chain, resulting in much smaller populations (and perhaps even extinctions) as insectivorous species (and species that prey on insectivorous species, etc) find their food supplies restricted.

My speculation is that we are seeing "trophic compression". That is, food chains today probably have fewer links in them than they did in the past. Instead of seeing insectivorous mammals, birds, reptiles, etc. eating predatory insects, we might be seeing them eating herbivorous insects instead.

Edit: A way to test this hypothesis would be to try to measure the biomass of predatory insects vs. the biomass of herbivorous insects. My prediction would be that we would see much greater declines in the biomass of predatory insects.

Trophic compression seems likely. A further consideration, along similar lines, is what the food chain actually looks like when you turn it from an unweighted graph to a weighted graph. One could say that there are other species that prey on humans, but basically all humans die of non-predatory causes. Similarly, how many ants die because they're eaten by ant-eaters, rather than old age or starvation or so on?

What we could be seeing is something like a reduction of the fraction of insects that die for 'irrelevant' reasons from the food chain's point of view--smashed on a windshield, old age, etc.--in a way that means the biomass is lower but doesn't significant impact the amount of biomass available for consumption to the next trophic level up.

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I once came across an argument, that I can no longer find, whose name was something like "predators get too much credit," and pointed out that there are some parasite-predator mutualisms where the parasite gets the prey species to behave in a way that allows them to be caught, and then reproduces in the digestive tract of the predator. (The obvious example here is toxoplasmosis in mice/cats.) The argument claimed that, in some predator species, basically all of their prey were infected by those parasites.

This also seems potentially relevant; in a world where cats are entirely supported on infected mice, a reduction in the total mouse population is irrelevant if the population of infected mice stays the same.

Sure, we can build all kinds of explanations why decrease in insect populations wouldn't lead to decreases in insectivore populations. But that only makes sense if insectivore populations have in fact, not decreased. Haven't they? Do you have data on it? Surely human activity has damaged the populations of many species, both over the last few decades and the last few millennia.

One one hand, I'm suggesting that insectivore populations are really shrinking in response to insect loss, but it's hard to separate that from all the other causes they might be shrinking.

And on the other hand, I'm suggesting that in the past there might have been a surplus of insects (assuming they were more resilient to some human actions, or at least not as tasty). And that we're now returning to a more normal insect/insectivore ratio.

But that only makes sense if insectivore populations have in fact, not decreased. Haven't they? Do you have data on it? Surely human activity has damaged the populations of many species, both over the last few decades and the last few millennia.

My understanding is that they have declined, but in ways that weren't obviously related to lack of prey species. For example, amphibians have been declining seriously since the 80s, but a lot of that seems to be because herpetologists studying the decline were an infection vector for a fungus that decimated populations. Bat populations are also being decimated by a fungus. Ocean fish populations have halved since the 80s, and freshwater fish populations are down 80%. (One of many culprits here is also a fungus.)

[edit]See another comment, where a decline in bird populations is explicitly linked to a collapse in prey insects.

The Wikipedia link on amphibian decline mentioned the effects of artificial lighting on the behavior of insect prey species as a possible contributor. I suppose it’s possible that that’s a factor in the observations from the German study as well, particularly since they only looked at flying insects. But the observations were apparently made in nature preserves, so one would think that artificial lighting wouldn’t be that common in those habitats. There could still be indirect effects, though.

On a related vein of thought, one can almost model any currently-visible adults of a species with a high potential number of offspring-per-adult as a "surplus" that doesn't necessarily have a large effect on trophic "throughput". Phytoplankton come to mind as the extreme case; they sequester a huge quantity of carbon dioxide, but the oceans aren't green (usually...) because huge quantities of them are constantly sinking to the ocean floor or being eaten. That small surviving fraction still reproduces at a high enough level to maintain themselves. (Land plants seem to have a very different equilibrium, which probably has something to do with... better herbivore control by predators, and maybe also counter-herbivore adaptations and the necessity of infrastructure-deployment to handle water scarcity? Not especially confident on this.)

Insects don't have the reproductive rate of phytoplankton, though. And from the other comments, it sounds like this really is starving out some members of higher tropic levels.

Random speculative ideas:

I think my general answer is that we are seeing really massive changes to the ecosystem, it's just that humans don't really need to worry that much about the health of the ecosystem. By and large, and taking into account human industrial scale efforts and ingenuity, the only thing that we really need is fresh water and oxygen, and as long as those exist we can probably adjust our crop production somehow to make things work. There is a good chance we can colonize mars, which doesn't have a healthy ecosystem at all, because we have such a massive ability to reshape our environments to our needs.

What kind of observations would you expect to see in a world in which the biosphere has exploded and humans were able to reshape their environments as much as I think they can?

I would think that, even if we could adapt to things long-term, massive damage to the biosphere should still be more visible while we are adapting to the changed conditions.

As an imperfect analogy, consider global warming and sea levels rising. If sea levels were to rise so that most of what are currently coastal cities would go under water, then we could certainly adapt to that by building new cities inland and moving everybody there. But it would also be very obvious to everyone that this is what's happening.

Similarly, if the biosphere exploded, I would expect there to be obvious signs of it, in the form of e.g. news articles about how crop production is failing and how new technologies are being developed to compensate. I have seen, for a long time, news articles about e.g. reductions in the amount of potential cropland, possible rises in sea levels due to climate change, changes in the price of oil causing famine in poorer countries, and the need to develop renewable energy sources due to fossil fuels declining. But with maybe one exception, I don't recall running into much news about how agriculture is reacting and adapting to an ongoing ecosystem collapse.

I would also weakly expect changes in my ordinary surroundings that were obvious to me even as a non-expert, e.g. my local forest getting noticeably less healthy.

The changes to American national forests upon the reintroduction of wolves (and thus a reduction in the deer population) were supposedly obvious to non-experts; similarly, changes based on the collapse of friendly species (such as chestnuts being destroyed by blight) were also somewhat obvious (but perhaps less so, if you didn't know what chestnut trees looked like).

I do recall another 'friendly' species decline that was of economic importance besides bees--vultures in India have been mostly wiped out by a cattle drug, which means huge increases in the amount of feral dogs, which means an increase in rabies (for humans as well). In California, condors were mostly wiped out through a similar process (lead poisoning from eating animals that were shot with lead bullets is a major cause of death now) but it's not obvious that the reduction of condors has been hugely impactful (probably because we have better systems for dealing with roadkill / animal carcasses that the condors had been dealing with for us before). With bees, it seems like we both successfully prevented their complete decline and found out that it wasn't too costly to replace their pollination services--either with robots, as linked, or with humans, as was done in China.

Some speculation

The original article mentions few reasons what may be going on in Germany - industrial scale use of pesticides or the increase in farms dedicated to a single crop. The same factors plus better crops and fertilizers allowed for 10x increase in crop productivity (check this truly amazing graph https://www.agry.purdue.edu/ext/corn/news/timeless/images/CornYieldTrend_US.gif)

Industrial farming is progressively more decoupled from ecosystems and does not need rich ecosystems (imagine hydroponic indoor farms growing vegetables just from water, artificial nutrients, and under LED light)

Also my guess would be you are overestimating importance of insects (copared eg to bacteria)

[Epistemic status: high confidence grounded in empirical observables, no domain expertise]

In fact, those shockwaves up the trophic chain are already being observed: https://www.theguardian.com/world/2018/mar/21/catastrophe-as-frances-bird-population-collapses-due-to-pesticides

The reason we aren't all dead is that humans exist atop a mostly separate food chain from wild animals. If all wild insects die, there will be collapse of wild ecosystems, but crop yields will be the same, if not better due to the lack of pests. Most pollination of commercial scale crops which need active pollination is done with rented beehives, these days. See https://en.wikipedia.org/wiki/Pollination_management

From the link:

The study is the result of a collaboration with the Entomological Society of Krefeld – essentially made up of members of the public

This decreases my confidence in the robustness of the results of this particular study. Unfortunately I'm fairly unversed this area so I don't know whether this is a widely repeated result or an outlier.

For the former (German study with the biomass numbers), perhaps? For the review study (which reported on worldwide species numbers and Britain range distributions), I flipped through the supplement and it looks like they actually did try very hard to adjust for the fact that they were working off of volunteer data (Filtered for surveys that had identified certain other similar species, and which had lasted for over 1hr. Also, excluded some species that they expected to have "gone into hiding" more). Something interesting might be going on with the Beetles, but it seemed suggest that Butterfly/Moth and Wasp/Bee/Ant species are almost all either holding steady, or dropping. I think significant reductions in flying insect catches might actually be plausible.

Non-expert, but high confidence:

Almost all claims exaggerate the importance and sensitivity of ecosystems and the biosphere. Professors of entomology have clear incentive to exaggerate the importance of their field. Other people hold nature on a pedestal and will happily believe strong claims uncritically. They'll read how the extinction of cockroaches might mean that red-cockaded woodpeckers lose 50% of their natural diet, and think this is a global catastrophe.

The real worst case scenarios of insect extinction and other ecosystem disturbances, are probably more like scenarios where cucumbers get really expensive, rather than anything close to global catastrophe. This is evidence of that. If a 75% reduction in insects doesn't have almost any effect, I'm not sure what would be needed for catastrophic results.

Entomology bacchelor here. My naiive model would have been that termites and earthworms, followed maybe by "the entirety of all pollinators", would have major effects on ecology at the macro-scale. But my default model also has comments along the lines of "there are species that will trigger black swan effects, and you can't always predict which ones they are." And that later part of me is very confused.

The model is less that every single species matters, and more that there are "keystone species": the often-highly-specialized regulator (a predator, parasitoid, or lethal disease) of a toxic/high-reproductive-rate/invasive-like species, where if that invasive species were to be left unchecked, it would dominate the environment in a manner detrimental to just about everything else living there. See: Otters that kill sea urchins which would otherwise detatch kelp from the sea floor, or... in a case closer to what I'd expect here, things like small wasps that specifically kill a beetle that would otherwise kill large numbers of trees.

Part of why this is so confusing to me is that if you take some of these invasive-like species, and introduce them to new habitats, they DO cause huge problems (see: Kudzu, certain species of mussel... I could go on and on). And due to combinations of things like "toxins" or "low nutritional content" or various really good counter-herbivore adaptations, I actually wouldn't be that surprised if only 1 or 2 things in their native environment actually subsist predominantly off of the invader, or at least eat it at such a level that it would kill large numbers of them.

This suggests that nature isn't a Rube Goldberg machine that some imagine. It seems that most of the commenters are also in the "life finds a way" camp, so I don't have much to add.

The interesting questions instead need to be answered by you. If I had told you 27 years ago, that insect populations would fall by 75%, what specific effects would you have predicted? I myself would have guessed that some insect eating animal populations would shrink (presumably they have), maybe get outcompeted by plant eating populations. And maybe that some plants pollinated by insects would be outcompeted by other more independent species. But I don't really see what should lead to catastrophe, especially not the kind that would hurt trees themselves. On the other hand, I could even imagine some forests becoming healthier due to reduction in parasites.

Human male sperm count failed 60 per cent since 1970s, so the effect is almost the same. Something is killing small things! Seriously, may be there is unidentified toxin which slowly affects both sperm and insects?

https://www.researchgate.net/blog/post/sperm-counts-in-the-western-world-have-declined-nearly-60-percent-since-the-1970s

And, like with sperm, it may be that there were many more insects than needed to fulfill their role? Like, if 20 sperm reach an egg, you can lose 95% of them and end up just as pregnant.

Not entirely true; low sperm counts are associated with low male fertility in part because sperm carry enzymes which clear the way for other sperm - so a single sperm isn't going to get very far.

95% of the sperm reaching the endpoint, then, if they're not independent.

Not an expert; I think a better question would be not "why aren't we dying" but "what we can actually see if...". We're not dying on "this" scale due to very many reasons. Here's just some thoughts I had, all "weak hypotheses".

So, maybe:

1) we need better studies, and simply cannot say anything from "undifferentiated insect biomass" (seriously, what part of it was pollinators, what part - predators, etc?)

2) due to some weird thing, most of the effect is happening in the sea, not on land (how much organic matter is washed out now, and how it compares to 50 years before?)

3) it's not extinction we should be looking out for, but rapid evolution of other organisms that are going to occupy the freed niches (since the one rule of biology says "it will be consumed" :); an especially interesting problem here is evolution of communities, not of species. Think drier plains & Artemisia, Atriplex, etc -based herbaceous layer instead of the usual "full-blooded Poaceae-Fabaceae seesaw"? Dunno when desertification will catch up, but if Ambrosia is going to be typical of the new coenoses, pollinators are gonna sell out.

(that's just off the top of my head)

just an idea - most of the biosphere is in the ocean and ocean ecosystems are in pretty bad shape, but we don't "see" this happening so it doesn't seem like as big a deal as it actually is.