I’ve talked a lot about various things related to ecology, and even put it in the website name, but I haven’t gotten too much into the nitty-gritty of the science itself on this blog yet.
Well, dear reader, you’re in luck! Today I’m going to talk about Trophic Levels.
“What are trophic levels?” you ask, because you didn’t bother to click the link. I’ll break it down for you.
Ecologies, from the deepest ocean depths to the highest mountain treelines, and everywhere in between, are all based on energy. Energy harnessed into the ecology, how it makes its’ way around the food web/chain and environment, and ultimately how it leaves the ecology.
Trophic levels are the different basic levels of the food chain, based on the flow of that energy into the system. For the purposes of building ecologies for your campaign setting, you can think of them as similar to a wizard’s spell levels, with each level having “slots” for various organisms.
Now, these are not hard and fast levels or slots when it comes to the living things you put in there. (Deer have been known to eat birds, for example) But they’re certainly good enough for our purposes of building a world.
So, let’s take a look at the different levels.
Primary producers (at Trophic Level 1) are the forms of life that bring energy INTO a system. The easiest example would be plants, that bring in food from sunlight, but it can also be from anywhere you could feasibly reliably draw energy from: low-level radioactive ore, the heat and chemical energy from thermal vents along the ocean floor (each of which is so separate they often have their own unique little ecosystems and whole species who can live just around that one set of vents), etc.
It doesn’t even have to be plants. Remember, this is a guide for where ORGANISMS fit in the ecosystem, and how energy flows through it. It doesn’t matter what those organisms or energy sources ARE.
Basically: If it turns non-organic stuff into food, it’s a Primary producer.
Primary consumers (at Trophic Level 2) are the forms of life that eat the primary producers. Bet you never thought of a cow as a predator before, huh?
They take the energy from the producers and use it themselves, insert Hollywood joke here, moving it further through the system. They can’t make all the complex chemicals that the producers can, because they simply just take those chemicals from the producers themselves.
Secondary Consumers are to Primary Consumers what the Primary Consumers are to the Primary Producers. They eat them.
One thing to keep in mind is that this doesn’t necessarily mean they kill them. Biting flies and the like would be one simple way you could have a secondary consumer while leaving the primary consumer alive.
Same deal, but to the Secondary Consumers. And so it goes:
- Level 1: Plants and algae make their own food and are called primary producers.
- Level 2: Herbivores eat plants and are called primary consumers.
- Level 3: Carnivores that eat herbivores are called secondary consumers.
- Level 4: Carnivores that eat other carnivores are called tertiary consumers.
- Level 5: Apex predators that have no predators are at the top of the food chain.
However, around here we start to reach an interesting phenomena. Actually, a couple. See, while each level gets its energy from the level directly below it, some of that energy is lost due to simply living, or the process of digesting, or what-have-you. Ultimately, only about 10% makes it up each level.
And that’s for each level. Primary Consumers live off of about 10% of the energy that was in the Primary Producers. Secondary Consumers live off of about 10% of the energy that was in the Primary Consumers. Etc.
It’s not a perfectly efficient system.
This means that you need about 10x the amount of biomass of level 2 within level 1, because level 2 needs to eat 10x more to sustain itself. Same with the other levels. Each level you go up, you divide the total biomass in the level below by 10 to find out how much biomass you have to work with that can actually be sustained by the system.
If, for example, every species had the exact same amount of biomass, you’d need 10 plant species before you could have a single herbivore.
Fortunately for the sake of spending our creative energy efficiently, not all species DO have the same amount of biomass. A few plant species can be so successful that they can support many other animals all on their own. But we need to keep those numbers in mind when we’re describing the world. The wild plantlife is like the farmer’s crops: it takes whole fields to feed people. A world barren of producers will be barren of consumers as well. That’s why a lot of deserts are actually filled with plants, despite the common idea.
The other problem we run into with the low efficiency between trophic levels is one of poisons.
Those few substances in regular abundance that bodies can’t naturally process out, or at least do so very slowly, wind up being concentrated more and more the higher up the chain they go, since each level winds up eating (over their lifetimes) quite a few times their own bodyweight in the level below.
The classic example to use is Mercury. Fish on the higher trophic levels wind up having more and more mercury in their systems, and as a result if you eat too much fish from a high trophic level in too short a time, you can wind up giving yourself mercury poisoning. (This is why there are advisories for consumption on certain predatory fish. Don’t eat more frequently than those say and you’ll be fine.)
That covered, let’s go back a bit.
There was one group which I left out in my earlier description of Trophic levels.
The Decomposers, the organisms that break down other dead or decaying organisms. Without them, energy would simply shoot straight through the system and out the other end, resulting in a lot of wasted potential.
Instead, the decomposers help recycle the energy that would otherwise be lost in dead organisms back into the system. They’re also the reason we’re not up to our neck in dead things. All that matter has to go somewhere, may as well go into the soil or back into the system.
Tying it all together
With all of that in mind, let’s look at how you can use this to make a rough food chain for your dungeon, forest, city, etc.
Since each level has to have about 10x the energy (and thus roughly 10x the biomass) of the one above it, we can reflect that by having each level have more “slots” for organisms than the level above it. (You can also reflect this in-game by having only 10% of the animals players run into be a predator, the rest being either nice scenic descriptions, territorial herbivores, puzzle encounters involving animals, etc.)
Let’s put, for very rough starters, 5 slots for the Primary Producers, 3 for the Primary Consumers, and then leave 1 slot open for a predator as a Secondary Consumer. After that, include 1 or 2 Decomposers (and perhaps a carrion-eater or scavenger if you like), to recycle that energy back into the system in some way.
This should mean that (if I’m doing math right, that is) each Primary Producer species needs to have on average 6 times more biomass, species-wide, than each Primary Consumer. Primary Consumers, on the other hand, need to have on average 3 times more biomass species-wide than the single Secondary Consumer.
I mostly do this because I know that most people don’t like to spend time coming up with 100 plants to be able to do 1 predator. And do keep in mind that I’m talking species-wide here. You can have a giant super-predator. There just has to be VERY FEW of them in an ecosystem LARGE enough that the level just below them can support their massive bulk. If they grow too large in size or number, or the ecosystem grows too small, they’ll face trouble.
So, there you go! A simple little “fill in the blank” guideline for building your own little mini-ecology!
It’s not perfectly accurate, but hey, it’s better than nothing! And for those balking at coming up with 5 producer species, don’t worry about it, you can have them still be useful to the party by giving them either poisonous or medicinal properties. Or have them be stuff the players themselves can eat as well.
Just remember to keep in mind how energy is flowing into the system, any ways that energy can flow out of the system, and how each of the levels relates to one another. Don’t be afraid to add in more species, just keep the number higher towards the bottom of the pyramid.
And don’t throw random giant dragons into a desert without thinking about where all their food comes from.