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.

[MagiMaster]

Reserved for expansion

[MagiMaster]

Worked example

The worked examples are being saved for the final publication, at least for the time being.

[Refplace]

Logged as a reference thread :)

[TheNinjaD]

I'm looking forward to seeing how this turns out.

[MagiMaster]

Thanks for the comments. There are still some holes in things that I might need some help filling in. Anyway, I'm going to be away from the internet for a week or two so it might be a bit before I get the worked example up.

[Gizensha]

...And I think this is exactly what I've been saying I wanted in every Beastiary thread...

While something official would be nice, this will do nicely for now. Thankyou.

[Vaevictis Asmadi]

This would make an awesome Pyramid article or supplement and you should definitely submit it once it's finished.

Originally Posted by MagiMaster

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.

I'm finding this paragraph difficult to understand and interpret. It would help if you could clarify how to calculate things. For example, is "3/4 power of its mass" = mass^(3/4)? And what are the proportions based on? For one thing, "cold-blooded" and "warm-blooded" are actually short-hand for a number of different metabolic traits that can each have a variety of values. Even the GURPS advantages and disadvantages related to them should really be divided up.

[MagiMaster]

(I'm still away, so I haven't really gotten any actual work done on this yet.)

Thanks for all the support. If I can make this fit into either format, I'll submit it and see how it goes.

Originally Posted by Vaevictis Asmadi

This would make an awesome Pyramid article or supplement and you should definitely submit it once it's finished.


I'm finding this paragraph difficult to understand and interpret. It would help if you could clarify how to calculate things. For example, is "3/4 power of its mass" = mass^(3/4)? And what are the proportions based on? For one thing, "cold-blooded" and "warm-blooded" are actually short-hand for a number of different metabolic traits that can each have a variety of values. Even the GURPS advantages and disadvantages related to them should really be divided up.

Yeah, 3/4 power of mass means mass^(3/4). I'm not sure of a better way to put that, but maybe I should just do what Space does and write out the equations.

I'm not sure what you mean by "what are the proportions based on". For most things like "warm-blooded" and "cold-blooded" I intend to use the alien rules, which covers things with a little more granularity than the disadvantage does. (I suppose for a standalone publication, I'd need to make it so that it doesn't really too much on Space, but for a pyramid article, you'd probably have to have that book to use this.)

[Vaevictis Asmadi]

This may belong in a separate thread, but I've never learned if the 4th ed. Space rules for aliens are the updated version of the rules for aliens in Uplift, or an entirely new set of rules.