GURPS: Ecosystems and Evolution

[MagiMaster]

Creatures
Knowing that a species has 100,000 lbs of biomass available for growth and reproduction doesn't tell you much about the individuals making up that species.

First make sure that the creatures can eat what the diagrams say they should be able to eat. The evolutionary arms race necessarily leans in favor of the predators, though it's usually very close. Also, predators are rarely smaller than their prey by more than one or two SM.

The first thing we want to find out about the species is how individuals there are. The population of a species should be (how much food the species is eating)/(how much food an individual eats). The first will be 4 times the biomass for growth available to the species. The second is more difficult.

Bigger animals tend to require less food in proportion to their body weight, as do cold-blooded animals and animals with particularly efficient hunting methods. The alien creation rules from Space will be helpful in narrowing down the possibilites.

As a baseline, we should consider which Earth animals eat the most and the least compared to their body weight. On the high end, Hummingbirds can eat 5 times their bodyweight per day in nectar, and shrews can eat almost their bodyweight per day in insects. Both of these are small and warm-blooded. The hummingbird flies and the shrew digs. On the other end, large constricting snakes can eat around 1/200 of their body weight per day (on average).

Kleiber's law says that an animal's metabolic rate is proportional to the 3/4 power of its mass, meaning what the eat realted to their mass is proportional to the -1/4 power of body mass. Using this to correct for size, the flying, warm-blooded, gathering herbivore has a metabolism of about 1.25 while the slithering, cold-blooded, pouncing carnivore has a metabolism of about 0.0125. This suggests that the general formula would be 1.25 times (weight to the -1/4 power) times some correction between 1 for the most active animals and 0.01 for the least. The table below gives some guesses for this correction factor based on the alien creation categories from Space. Multiply the numbers from each subtable.

Tables to be added as I can figure out the numbers.

After working out how much an individual eats, get the population from 4*(biomass of species)/(consumption of individual). If the population is less than around 100 or so, it's almost certainly too low to survive in the long run. Either find something else for them to eat, or remove them from the area. If it's between 100 and 1000, they'll probably survive, but it'll be a struggle. Adventurers or settlers could wipe them out fairly quickly. Over 1000 and they'll probably be fine. No small group of adventurers passing through will significantly dent their numbers, though settled people can systematically eliminate them over time.