K2-18 b is a sub-Neptune exoplanet that may have a hydrogen-rich atmosphere over a global ocean — a “hycean world.” If microbial life exists there, what would its ecology look like? Not the organisms themselves, which are unknowable, but the population dynamics: how many species can coexist, what controls their abundance, what makes the system stable or unstable.
Cooke, Madhusudhan, and Mitchell (arXiv:2603.22491) applied Lotka-Volterra competition and predation equations to vertical water columns under hycean conditions — anoxic, hydrogen-rich atmospheres, no solid surface. One to five bacterial species, including phototrophs, chemotrophs, and bacteriophages.
The results map a possibility space. Phototrophic bacteria dominate the surface layer and suppress deeper-dwelling species through competitive exclusion — light is the limiting resource, and it's only available at the top. Bacteriophage inclusion either collapses the ecosystem (if viral pressure is too high) or enhances diversity (if it selectively controls dominant species, opening niches for competitors). The standard ecological paradox of the plankton appears: coexistence requires mechanisms that prevent competitive exclusion.
Tidally locked planets — with one hemisphere permanently facing the star — produce stabler but less dense populations. Seasonal variation (from eccentric orbits or obliquity) drives boom-bust cycles with higher peak abundances.
The through-claim: ecological diversity on hycean worlds is constrained by the same competition dynamics that operate on Earth. The physics is alien; the ecology is familiar. Lotka-Volterra doesn't care whether the ocean sits under a hydrogen atmosphere or a nitrogen one.