Ecosystems and the 10x Rule
Guest post by Jerry C. Hinn, MS Zoology
So far you’ve populated your world with the apex species. I’m sure they’re regal examples of evolution gone right. Got a guess as to how many of that species there are? Good. Now, for every one of your species, we’ll need 10 times as many prey species that they eat, 100x as many fodder species those prey eat, 1000x plants that sustain the fodder, and lastly a countless swarm of bacteria, microscopic soil organisms, and decomposers to provide a healthy microclimate for the plants to thrive in.
Think about it. You eat multiple times a day. Even if you are vegan, you still consume far more plants, in far greater number of types, then there are of you (and that’s good because who wants to eat navy beans for every meal?). In order to sustain you with enough calories, you have to mow through a grocery cart full of diverse species, who had to come up out of the ground somewhere. If you have a pork chop along with your navy beans, then you also have to count the pig, who very likely eats plant species you wouldn’t, such as tree nuts like acorns and hickory that are largely inedible to humans but do fatten up pigs. If instead you have a nice filet of fish... well, you get the idea. In order to sustain a species, there have to be other organisms in the food chain for them to feed on.
The Trophic Pyramid
There’s two concepts I want to introduce. One is the trophic pyramid. “Troph” is Latin for “nourish” and pyramid is Latin for “pyramid.” The apex predators are at the top of the pyramid, and for good reason: The top is small (and also pointy, but that’s not as relevant as the size). There simply cannot be very many apex predators feeding, or the level below it will collapse. And that does actually happen. If the top predators starve, then the level they snacked upon will breed wildly. Which then overdoes it on the level below them, and those guys collapse. A real life example of that is the Lynx-Snowshoe Hare cycle. Snowshoe hares, being rabbits, eat a lot of plants and make a lot more rabbits. This in turn feeds lynx, and their population begins to bloom. Which is a bit of a problem as the hares are also hungry, and will graze on plants beyond their capacity to support that many mouths. The rabbits begin to starve just as the new generation of lynx was beginning to wean and hunt on their own, which in turn leads them to starvation. Any fluctuation in one level of the pyramid has a ripple effect, and these can, in a situation like the lynx who depend heavily on snowshoe hares, threaten them with extinction. Most trophic pyramids aren’t this tightly dependent on one prey species, but it is a good example of how they can topple.
The 10% Rule
The other is the 10% rule. Every level of the pyramid scales down by a factor of 10. This is because of the second law of thermodynamics. When energy is transferred, some is wasted and lost. When you eat food, you don’t convert all of it into muscle, fat and bone. A large part of your calories are spent keeping you warm and keeping your heart beating, neurons firing and kidneys filtering. So any predator that ate you isn’t getting 100% of the value of the food you had eaten; they’re getting about 10% of the biomass you ate in your lifetime through you. That’s why you “need” to invent 10 plants for every cow, and 10 kinds of cow for every burger fed to your apex predator.
Do you really need all that diversity? From a literary stand point, of course not. Unless your protagonist is an ecologist who constantly blurts out species names when they speak, it won’t come up in any amount of detail requiring that much homework. But if your protagonist takes a hike through the woods, or woods-equivalent, come up with some trees and bugs. They outnumber your character a thousand to one, and it looks like meat’s back on the menu.
Title image by Nihongraphy from Pexels and used with their kind permission.