I’ve been reading a bit lately about some interesting ecosystems where the primary food source for adult members of a species is young members of the same species.
At first, this might seem like it defies the laws of physics or something, but it doesn’t - it works like this:
-You keep producing offspring in huge numbers
-They go off and eat stuff - perhaps stuff you, as the adult, are not equipped to eat
-Once they’ve converted some of that food into energy reserves and bodily growth, you eat some of them
-But not all of them - a few escape to become the next generation of adults
I understand there are species of frog that do this (the tadpoles eat algae - the adults eat tadpoles), and that some species of crustacean do something similar (crayfish, for example - although adults and juveniles eat everything, cannibalism appears to be part of their survival strategy - saturate the system with food-gatherers of your own species, then eat them)
Are there any other examples - perhaps from other taxonomic classes?
The Matrix universe presents this concept - the machines dissolve human corpses to feed other humans with them. The same goes for the 1970s science fiction flick Soylent Green (starring Charlton Heston), featuring an overpopulated world in which the victims of the regular hunger riots disappear and are processed into food for the masses.
It’s obviously an interesting concept for sci-fi writers, because of the thrill and disgust associated with cannibalism, but I can’t help you with real-life examples of such behavior.
I would question your basic assumption that the purpose of producing mass quantities of offspring is solely in order to feed the adult population. Animals that produce mass quantities of offspring do so because they inhabit an ecological niche that has mass quantities of predators, and the individuals that succeeded in producing the most progeny would be the ones who would pass on their genes, and those would include genes for producing mass quantities of offspring. The salmon that only produced 15 eggs would statistically not have as good a chance of passing on her genes as a salmon that produced 15,000 eggs. So each one of the eggs that survived from the salmon who produced 15,000 eggs would have genes for “producing 15,000 eggs”.
So the reason they produce mass quantities of offspring is because that’s how you win the DNA race in a world filled with predators. The fact that adult salmon will chow down on their own offspring is purely coincidental, and is a subsidiary issue. The adult salmon population forms only one set of predators in an ocean full of predators.
Virtually all the species that I can think of that produce mass quantities of offspring, as in spawning species, will also eat the products of conception if they get a chance. So the answer to the OP’s question, “Examples of ecosystems where a species is its own predator/prey” would be, “Planet Earth”.
Plenty of animals will eat their young (as anybody who’s ever had a fish tank will attest to) - but are you looking for that, or for animals who mostly eat their young?
1.) When I first got my aquarium, I thought it was awful that guppies ate their own young. After I had it a while, and the guppies had outbred and displaced my mollies and tetras, I decided that the problem was that guppies don’t eat Enough of their young.
2.) The Larry Niven/Jerry Pournelle/Stephen Barnes novel The Legacy of Heorot features an ecosystem with adult creatures that prey on their own young as a major food source. The creature was based on work by consultant Jack Cohen, whop said it’s very similar to the life cycle of some sort of frog. I don’t recall the exact real creature (if I was ever told), but that would seem to be something that meets the OP’s criteria.
Spoiler for novel - don’t read if you plan to read the book:
Grendels are the adults, samlon are the juveniles. It took the colonists quite a bit of time and grief to discover (a) that Grendels actually exist, and (b), by killing off the Grendels, the samlon population exploded, which led to lots and lots and lots of immature, but still incredibly deadly, Grendels.
I was looking for examples of species where the primary food source of the adults is the young of the same species - and the adults are the primary predator of the young. I would imagine this sort of thing would be more likely to happen in restricted ecosystems - such as enclosed caves or plateaux.
It occurs to me that it’s not likely that creatures would try to saturate an area and eat only their own kind. They’d be opportunistic creatures, eating what they could find and catch. Those that would tend to survive are those that DON’T eat their own preferentially, but leave their own kind to survive and breed. Better to eat other, unrelated creatures. So you wouldn’t expect to find creatures of this exact description in nature*.
instead, you’d expect to find creatures that will eat other things, and only their own when forced to, or whemn it’s easy. I’ve raised tadpoles of various kinds, and none of my frogs ate their tadpoles, as long as they were well-fed. But tadpoles would eat dead tadpoles when they found them.
*Unless in some quirky circumstance, like in some pocket or island a local set of effects conspired to let your hypothetical creature eat everything else. Then, with only themselves to eat, they’d continue to develop so that they could survive best under those circumstances, and ate off any newly-introduced species before they had a chance to gain a foothold. That’d fit in well with the ecology of Legacy of Heorot, but wouldn’t explain the non-island grenddels having the same behavior on the mainland in “Beowulf’s Children”
A bit of a spoilery comment on one of my favorite books here:
In “Beowulf’s Children” the non-island grendels have the same essential behavior. In the water they’ve removed all competition, if I remember correctly.
Based on fossil evidence of ice age & near timeline burials, some anthropologists have concluded that until relatively recently (in epoch scales) , in times of food stress humans provided a good chunk of the protein supply for other humans.
In the real worl, how about spiders? Not only do females tend to eat the males (if the latter aren’t careful), but in some species the males sacrifice themselves to ensure succssful fertilisation.
There is some evidence that the Anasazi Indians of the Southwestern U.S. might have routinely practiced cannibalism.
Many large predators, such as lions and grizzly bears, will kill young members of their own species that are not related to them, but this is a function of reducing competition, not sustenance – most of the time, the animals don’t bother eating their bodies.
The trouble with this charming thought process is that it would result in the conclusion that all species which are potential prey would have 1,000s of offspring at a time. We know this is not true. Ergo, some part of your reasoning process is flawed.
Indeed, we can see that, in the examples the OP offers, part of why the species in question produce large amounts of offspring is that they prey on themselves, increasing the need for increased production of children. So, while you are right to point out that this is not the ONLY reason for the massive offspring, it certainly is part of the process.
Ya know, that’s a chilling thought. I thought that S G was made from people harvested from those suicide centers or just died naturally, but we didn’t see what happened to those scooped up by the riot trucks, did we.
This is kind of a spoiler … the “Samlon are immature Grendels” link is critical information you (and the colonists) don’t discover until near the climax of the book.
I figured that I was sufficiently vague that people who hadn;'t read it wouldn’t realize – I never said this was the main part of the story, only that it appears in the book. (And I deliberately didn’t mention grendels or samlon.)
Of course, now everyone knows, for the comments others have made.
