Comparing biomass by ecological role across ecosystems, or something

Caveat: I am not a biologist. The below is the result of idle speculation only, and offered for consideration solely out of curiosity. If there’s nothing to it, please don’t make fun of me. Or at least make fun of me in an entertaining fashion.


I regularly watch nature documentaries on PBS, the Science Channel, National Geographic, and so on. One evening, it occurs to me that there are pretty consistent functional niches in each ecosystem. You’ve got the top predator (lions, sharks, crocodiles, etc.), big grazers (wildebeest, whale sharks), small “insectile” predators (spiders, certain shrimp), scavengers (houseflies, crabs), plant matter (grass, algae) and so on.

An ecologically distinct habitat is, at least in the large view, a system. If there aren’t enough deer, the wolves starve. If there isn’t enough green matter, the deer starve. Everything seeks an equilibrium; when a disruptive element is introduced (e.g., rabbits in Australia), the system flies out of whack for a while, but eventually it settles down: either the rabbits will eat everything and starve, freeing the landscape for recovery, or some other balance will be achieved, even if it takes a while.

So, sez my very curious but not well educated brain, it seems to me that generalizations could be made about the various niches occupied by organisms in those habitats, for purpose of comparing the habitats. In other words, what I’m asking is this: Is there any consistency to how much of the biomass each functional type occupies?

Take the African savannah, a lake, and the rain forest. In the first, the top predators are lions, followed by leopards (I’m being loosey-goosey here for purposes of illustration), followed by, say, baboons, and so on. There are also predatory niches by animal size, occupied by foxes, snakes, and on down to spiders. There are large herbivores – elephants, zebras, antelope, etc. – and on down the scale. In the lake, the top predator might actually be considered not “of the lake” – bears, fishing birds, what have you. However, you might find roughly corresponding niches to the savannah, through herbivores (algae eaters) to scavengers. Ditto the rain forest: Depending on where you are, you have either jaguars or tigers in the top slot, then, say, okapi (not many left) and the other herbivores, orangutans as omnivores, and ants and other insects as both predators and scavengers at the lowest level. And, of course, they all have plant matter of varying types underlying the whole food chain (or, more properly, food web).

Given that these function as systems, with each element relying on other elements for stability, is there any basis for comparison by, say, percentage of biomass? In other words, is there an operating theory that says the top predator(s) can’t represent more than 1% (or whatever) of the total, or the system doesn’t work properly? And that the plant matter has to be 10% (or whatever)? And so on, through the niches? And that if these are thrown out of whack, the system destabilizes?

It occurs to me as I write this that it’s sort of a numerical approach to the Gaia theory, which I think is useful as a metaphor but is otherwise New Agey woo-woo hand-waving. Even so, I have to wonder if this kind of measurement-based attack wouldn’t reveal some interesting things.

And at the same time, I recognize the extremely problematic nature of the question. The deep ocean, for example, is in many ways its own ecological niche, with its own organisms, but it relies on dead organic material floating down from above for a lot of its energy. How would that be divided? Similarly, the bears fishing the river by the aforementioned lake serve roles in two different ecosystems, though the functions are fairly dissimilar (eating fish vs. eating berries and fruit). How would their roles be distributed between the two systems? And beyond the simple question of definitions and boundaries, don’t many biologists specialize pretty heavily in one area or another (Bert Holldobler and his ants, for example)? Wouldn’t it require an uncommon breadth of knowledge to try to treat such an overarching idea with any degree of thoroughness and respectability?

I realize this notion lends itself to a lot of fuzzy thinking (the Gaia hypothesis, for example), which is why I ask the collective Straight Dope Community to bring me clarity. Can the problems I’ve detailed above be overcome, or are they pretty much deal-breakers? If this question has been studied and discarded, I’d be interested to know about it. If this hasn’t been studied but is considered pointless by the few people who have bothered to think about it, I’d like to know that too.

Thoughts from the board’s biologists and other thinkers?

It has been studied; try a Google search on ‘ecology biomass distribution’ or a similar set of terms.

I’m not a biologist either, but I would speculate that the distribution you mention would be pretty similar in almost all habitats. It takes a certain amount of vegetation to support a herbivore, and a certain mass of those to support a carnivore.

Your assumption of stability, on the other hand, mostly doesn’t happen. Populations are changing all the time, and even though they might have a steady average over the long term, they can go from boom to bust and back again relatively quickly. Look up (for example) ‘chaos theory population changes biology’.

Predator / prey ratios depend upon whether the predator is warm-blooded or not. According to one web page, mammals can have a predator - prey ratio of 1:100, while “soil litter communities” can have a ratio that is closer to 1:3.

Don’t think it is anything to do the blood warmth. What determines a stable ratio is the length of time a population takes to replenish itself and the nutritional value. E.g lets say it takes 2-3 years for a cow to go through the calf - adult - calf cycle. Lets say a wolf need 30 cows a year to survive. Then the minimum ratio is about 3 x 30 or 90:1 to keep the population going against wolf predation (in reality it may be a lot larger of course).

However, if a soil jumping critter can breed 30 eggs once every 30 days, and a carnivourous slimy thing needs 1 critter every 5 days to live, then one can obtain a much lower ratio, in this case 1:6.

Cerviase – This is the kind of thing (some) ecologists think about, so it is a good question. The answer, though is that conditions in ecosystems are different enough that there’s no general law about biomass distribution.

Most terrestrial ecosystems do have a pyramid shape – more biomass in primary producers (i.e. plants), less biomass in herbivores (e.g. deer) and even less biomass in higher carnivores (e.g. wolves). As noted, though the ratios change across different systems.

And in most ocean ecosystems, there’s a diamond, not pyramid shape: the biomass of the primary producers (algae, mostly) is actually less than that of the ‘herbivores’ (from one-celled paramecia up to barely visible crustaceans).

This is entirely possible, as long as the energy flowing through each level decreases as you move up the chain. To illustrate, **using totally made up numbers and a waayy too simple food chain ** let’s say there’s 2 kg of algae in a certain lump of seawater, with 6 kg of herbivorous plankton, and 0.5 kg of fish. The algae are pulling in tons of sunlight, and growing and reproducing like mad – enough to double every two days, if left unchecked. But they’re being eaten just as fast as they grow - 1 kg a day. The ‘herbivorous’ plankton eat this 1 kg a day but of course don’t use it all to grow, so they’re growing only at 0.5 kg/day (in other words, if there weren’t any fish eating them, they’d only double in population in 12 days). The fish are eating the plankton just as fast as they grow so they get 0.5 kg/day, which is just enough to make up for the fish that are dying, as the fish don’t use it all to grow with.

There is a general law that because every organism isn’t perfectly efficient at digestion, and uses up lots of energy just moving around and so forth, the energy (and mass, too) flowing through the levels has to decrease as you go up the food chain, but that’s about as specific as you can be, given the range of ecosystems out there.