Welcome to Awful Symmetry, my primer on speculative biology for beginners. If you were a member of my mailing list, you could have read this last month! Sign up to get more stuff ahead of the disgusting peasants.
Living things capture and store energy in order to make copies of themselves. Theoretically, they could get energy from all sorts of places: the wind, tides, radioactive decay, chemical breakdown, light from a star. It’s that last that accounts for the vast majority of energy on Earth, and I suspect the same will be true for alien planets, too. (See Biochemistry for more on how life collects and stores sunlight chemical and radioactive energy).
We call the organisms that collect energy from nonliving sources autotropes (“self feeding” organisms) and the most familiar examples are plants. That’s the base of your food ecosystem. Plants gather sunlight (or whatever) and concentrate it in their bodies, making a tempting target for other organisms (heterotropes or “other feeding” organisms ). At this point, you’re probably thinking of an herbivore like a cow, but parasites like Rafflesia or aphids are just as guilty of stealing energy.
Other parasites work differently. Viruses, crown gall bacteria, and gall wasps trick a plant into using its energy to build structures useful to the parasite (more viruses, crown galls, and wasp galls respectively). But the terminology only matters insofar as you can google it to learn more.
What’s important is you have energy accumulators like plants and energy thieves like animals. The plants will probably be immobile, since moving around costs energy and you don’t need to do it to collect sunlight. Some animals might also be immobile (barnacles for example), but others will have to be able to move around if they want to gain access to new stores of energy.
You can get creative here. In what circumstances would a producer need to move (alien trees wrestling for access to light from a stationary sun on a tidally locked world?) and consumers might be able to stay still. Barnacles allow seawater to carry plankton to them, spiders wait for bugs to blunder into their webs, crocodiles ambush prey that wanders too close, cats stalk and pounce on mice. That’s a nice gradient from a food-rich environment where you don’t need to invest much in hunting, to a food-poor environment where you have to work harder.
In general, the further you move up the trophic levels (away from plants to herbivores to carnivores to hypercarnivores), the faster on its feet the organism has to be. More richness – more energy – means more trophic levels. Tropical jungles and coral reefs have more species than polar tundras and ice floes1.
Energy isn’t all that matters, though. The mechanisms life forms use to gather and store energy have to be made of something. Without basic materials like water and nitrogen, you get a barren desert or a tropical lagoon (that water is crystal clear because it doesn’t have any plankton living in it2). Polar islands manage some lichen, bugs to eat it, and maybe birds to eat the bugs before some local resource runs out and the food chain stops.
Evolution also plays a role as species become better at extracting what they need from the environment. Beans and their relatives, for example, use symbiotic Rhizobium bacteria to convert atmospheric nitrogen into nitrates, enormously increasing their range of habitats. Evolution takes time, though, so you can only expect organisms to “crack” a hostile environment if that environment is stable over evolutionary time scales. Expect lots of species diversity in old, stable, resource-dense, high energy environments.
Stability is important everywhere. The big hyper-predators are always the first to go when the environment gets unpredictable. Look at the extinction of every kind of dinosaur bigger than a quail. Look at which animals went extinct after the evolution of humans. A planet that just went through a global catastrophe or is home to a geo-engineering sapient species is not likely to have anything like tigers.
1Although the real reasons species diversity decreases with latitude are more complex. See Currie et al. (2004). “Predictions and tests of climate-based hypotheses of broad-scale variation in taxonomic richness”. Ecology Letters. 7 (11): 1121–1134. doi:10.1111/j.1461-0248.2004.00671.x.
2Behrenfeld1 et al. (2006) “Controls on tropical Pacific Ocean productivity revealed through nutrient stress diagnostics” Nature 442, 1025-1028 ) doi:10.1038/nature05083