I'm going to begin this entry with a warning: I am not an ecologist. Some of the terms I use in this blog post may be inaccurate or incorrect, although I think I'm at least close to the technical meanings. I'm trying to generalize from what we know about ecosystems on Earth to what any world with life would be like, and so sometimes I may wind up using terms from Earthly ecology as analogies rather than their specific definitions.
What we call an "ecosystem" is a collection of organisms making use of available energy. Some of that energy comes from the environment, and some comes from other organisms. How organisms get that energy — and how much effort they have to put into getting it — defines their ecological "niche."
Autotrophs: We'll start at the bottom of what's called the "trophic pyramid" — the foundation layer, where ultimately all of the energy enters the system. These are organisms which exploit non-living energy sources, and are known as Autotrophs. On Earth, the best-known examples are plants, but there are also chemosynthetic organisms living in the oceans, making use of chemicals produced by geological processes. We discussed some of those potential chemical systems in the last blog post.
This kind of environmental energy source is likely to be fairly low-powered and diffuse. Sunlight provides 1.3 kilowatts per square meter — but that is hedged about with limitations and inefficiencies. Plants typically can only make use of certain wavelengths of light, which is why we have special colored "grow lamps" for indoor gardening. There are daily and seasonal variations in the intensity of light, so that maximum intensity is only available a fraction of the time.
And finally the process of converting sunlight to stored chemical energy has its own inefficiencies. When I burn a kilogram of oak or maple, I get about 14.8 megajoules of energy. That sounds like a lot! Except that it isn't, not really. It took years to make that wood. An oak typically takes 20 years to reach full size, which represents about 10 tons of mass, or 500 kilograms a year. During a single year an oak tree gets exposed to something like 2.4 gigajoules of energy, so the energy content of a full-grown tree represents less than a third of the solar radiation absorbed (and this ignores the not-inconsiderable energy involved in cutting, splitting, hauling, and drying that wood before it can be burned).
All of this is by way of explaining why plants don't move around much. The surface area needed to absorb solar energy is so large that the energy required to move would burn up everything that plant can absorb. This might be different on an energy-rich world with liquid-sulfur life, however.
Chemosynthesis gives a little more energy density, such that microorganisms may be able to swim up the concentration gradients of various chemicals in water, searching for the richest sources. Tubeworms hosting symbiotic bacteria can survive around vents, suggesting that on alien worlds chemosynthesis could support complex organism. But tubeworms don't move around much, either.
In my novel Arkad's World I did come up with an active life form based on plants. The Kchik are large plants resembling banyan trees, extending over a wide area to harvest lots of energy. Like many big plants, they have trouble getting their seeds to good locations far enough from the mother plant so they don't compete. The Kchik evolved motile seeds capable of self-propulsion, and that ultimately evolved into semi-intelligent beings capable of tool use. I assumed the seed-beings were short-lived, but shared memory with their parent plants, so they would "ripen" and drop to the ground as fully functional adults. Even so, the seed beings have a lifespan of just a year before they either take root and leaf out, or run out of stored energy and die.
Another kind of autotrophs are decay organisms, or detritovores, which break down dead matter — on Earth the most familiar types are fungi, plus countless varieties of microorganisms. They are technically making use of non-living matter for food, but the matter they use is once-living stuff. This is important because it's not bringing energy into the system. In fantasy roleplaying games you often see underground ecosystems based on "fungus farming" but there's no explanation of how that arrangement is viable for more than a few years.
Ex-living matter does hold considerably more energy than "inorganic" chemicals, so detritovores might be as active as animals. On Earth, slime molds move around when they need to. One could imagine a cross between my Kchik and a type of fungus — the fungus is a sessile mass of mycelia spreading through a forest floor to feed on decaying organic matter, but instead of mushrooms releasing spores, it produces mobile young which seek out likely spots to begin growing new mycelia. These mobile fungus creatures might also learn to make their own decaying organic matter, by killing plants and animals . . .
We distinguish between plants (autotrophs) and animals on Earth because they diverged very long ago. That may not be as true on other worlds. What distinguishes the role of an animal from that of a plant is that an animal has to move around to find food. A sessile animal is likely to starve unless it can bring prey to itself. So on an alien world there might be things moving around eating autotrophs which are nevertheless more closely related to those "plants" than they are to other "animals."
Herbivores: Animals which eat autotrophs can generally be lumped together as "herbivores." The main thing to notice about herbivores is that their food can't run away. It may be hard to find sometimes, but it's right there when you do. In many environments on Earth, plant food is so abundant that herbivores can specialize for eating different kinds of plants, or different parts of them. While there's a broad spectrum of herbivores, we can identify two ends of that spectrum: grazers and gatherers.
Grazers eat very abundant, typically low-energy food. Cows munching grass, elephants eating the leaves of shrubs and trees, and so forth. Their whole lifestyle is based on eating nearly all the time. Just keep shoveling it in, and let your symbiotic gut bacteria break down all that cellulose into something you can absorb.
Baleen whales can be seen as a kind of grazer, too: while the food they're eating is tiny animals rather than plants, they can't get away from a whale so the main issue is just how much a given whale can eat, rather than hunting down the food.
Grazers benefit from economies of scale in things like temperature regulation, food absorption, and such. They can be big — I mentioned whales, and there were also the sauropod dinosaurs, the biggest land animals that ever lived. I'm going to go out on a limb and say that in just about any evolved ecosystem (as opposed to artificial ones), the largest animals will be grazing herbivores.
At the opposite end of the herbivore spectrum are gatherers. On Earth these are creatures which take advantage of the bribery-based system plants have evolved to spread their seeds: encase the seeds in attractive, energy-rich fruits and let animals eat the fruit and spread the seeds. Other gatherers may look for energy-rich seeds even if there is no bribe, or dig up energy-storing roots and tubers. The important point is that gatherers eat stuff that is rich in concentrated energy, but is harder to locate. They have to search, or dig, or defeat plant defenses to get at the seeds.
This keeps gatherers smaller than grazers — mostly. You might define elephants as gatherers, and they're really big. It also (and here I'm definitely generalizing very broadly) makes gatherers tend to be smarter and more mobile than grazers. They have to be able to distinguish between what is food and what isn't (because plants don't care how long the animals which eat their fruit live, and sometimes lace those fruits with insect-killing chemicals that can kill a vertebrate). They may have to do some puzzle-solving to locate edible roots, or get through a thick rind or hard shell to get at the tasty fruit and seeds. All of which requires a bit more brainpower than just "Take a bite of grass. Take another bite of grass." Our own distant ancestors were probably gathering herbivores living in trees.
To me it seems likely that any intelligent tool-using pure herbivores would evolve from gatherer ancestors.
Carnivores: The next big division in how you get your food is eating other animals instead of plants. Animals can run away, they can hide, they can even fight back. Carnivores have to be able to move faster than what they eat (though possibly just for a short distance), they need some kind of "weapon" to quickly kill the prey, and they probably need good senses and the ability to sneak up on what they're planning to kill and eat. Often they team up with other individuals, even a whole pack sometimes, to bring down a big prey animal. This means they need good communication and social abilities.
They aren't sporting about any of this. Predators prefer to kill animals smaller and weaker than themselves. Infants are especially prized — or better yet, unhatched eggs! At this end predators function in a very similar way to gathering herbivores, and it's not surprising that many species combine both roles. Another type of carnivore behavior that overlaps with gathering herbivores is scavenging. Meat is meat, whether you killed it yourself or not. And we see lots of omnivores combine scavenging, opportunistic predation, and gathering tubers and fruits.
Again, venturing into massive generalization territory, it seems that predators are also on the cleverer side. They have to outwit other animals to survive, and often evolve fairly complex social systems and methods of communication to organize hunting groups, both of which require brainpower.
On land, most carnivores prey on herbivores. There are very few carnivores I can think of that directly hunt other carnivores. A coyote or a mountain lion will snap up a house cat or a small domestic dog — but I suspect that may simply be due to the domesticated animals not recognizing a threat until it's too late. I'd be willing to bet that feral cats and dogs have a better track record against bigger predators.
In the ocean, it's different. Most of the pure herbivores there are pretty small, if only because a lot of the plants are tiny algae. This means that oceans have a trophic pyramid with a lot more levels than most environments on land: plants, small herbivores, small carnivores, bigger carnivores, even bigger carnivores, and apex predators. Shrimp eat algae, herring eat shrimp, jacks eat herring, tuna eat jacks, sharks and orcas eat tuna.
One would expect this would make ocean creatures even more hyper-intelligent than their dry-land counterparts. After all, they not only have to outwit their prey, they probably have to outwit predators as well. But fish, at least, seem rather dull-witted, driven more by pure instinct than cognition. Cetaceans — descended from land animals — are very clever indeed, and we've learned that cephalopods may well be in the same league. I don't know why there aren't smarter fish.
Naturally, I've stuck to very general terms in this discussion. There are loads of weird and amazing adaptations by animals which defy these broad categories. Trappers, like spiders or antlions, which use non-living structures to catch prey. The whole vast world of parasites, and symbiotes. I certainly can't describe the entire variety of life on Earth in a single blog post. But these general categories should serve in the next few entries as we discuss intelligent alien beings.
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