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.
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.
---
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.
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.