r/K selection theory
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Basic Information

In ecology, r/K selection theory suggests that life forms have two paths to success, and that each animal species will be on one of those paths. Natural Selection and various environmental pressures cause them to evolve along one of those two paths, or go extinct.

Animals on the r- path survive off unexploited ecological niches. There's not as much competition for the resources they live off of, so they produce large numbers of offspring, and tend towards minimal parenting. Only a small percentage of the offspring survive to full maturity, and average life span tends to be short. The tend to be small, reproduce rapidly, and excel at surviving in unpredictable or unstable environments.

Animals on the K- path are strong competitors for resources in crowded niches. As the environment cannot support large numbers, they tend to have fewer offspring, and invest more heavily in them. Expect longer parenting, slow maturation, and larger body size. They are well adapted to stable environments, but suffer heavily when some outside force (such as man) alters or damages the ecosystem.

Many species are predominantly r- or K- strategists, but there is a full spectrum, with some organisms showing traits of both strategies.

There's some dispute and controversy about this theory, but that's a topic for some other site, …say, Wikipedia. Here at Arcanawiki, we just care about how this concept can enhance our games.


1. GURPS Space - the alien life form generation system in this book use this to help generate alien psychology and behavior.

Game and Story Use

  • Understanding of ecological and evolutionary principles can help you design more logical Species, Aliens, or Monsters for your gameworld.
    • Fantasy games sometimes have monsters that eat gold, steel, magic items, or similar resources that few others eat, but which can be hard to a steady supply of. Sounds like it's probably going to be an r-strategist, producing many offspring and abandoning them to their own fate as it roams a wide territory.
  • r/K selection theory may suggest which creatures will survive (or even prosper) in an After The End campaign setting, or if some ecological disaster happens mid-campaign.
    • r-strategists produce many more offspring than K-strategists, and tend to have shorter lives and mature more quickly, as well. This means more opportunities for random adaptive mutation.
      • A game could be set just far enough into the future that the r-strategist species have evolved into all sorts of fanciful new organisms, but several K-strategist species exist in the same (or nearly the same) forms they have now.
  • An r-selected intelligent species might be exceptionally violent due to population pressure. A species that creates a civilization will likely reduce its infant mortality dramatically — for a species in which each parent produces thousands of offspring, if even a small percentage survive, the population will grow explosively. Such a species will either be radically expansionist or reduce its numbers through constant warfare and infighting — perhaps both. A species like this could form the major antagonist in a fantasy or SF campaign.
  • A version of r/K strategy can even be seen within different cultures of the same species - compare our 19th century ancestors with their large families to our modern peers with a birthrate in many cases below replacement …
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