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MagiMaster 06-17-2010 10:33 PM

GURPS: Ecosystems and Evolution
Originally, I had intended to write this up for Pyramid, but I didn't get everything together in time for the issue it was most suited to. It might still fit in the Monsters in Space, but I don't know how posting it here would effect that. Either way, since I still haven't worked out all the details, and I posted a good chunk in the Sewer Ecology thread already, I decided to go ahead and post what I have.

Whenever a GM is preparing an environment for an adventure to take place in, the question of "what are these things eating" always comes up. Depending on the setting, answering that question may or may not be important enough to bother with. This is for those times that it is. This information is meant to supplement the information given in chapter 6 of Space.

Table of Contents
Trophic Levels
Food webs
Worked Example

The second part was/will be Evolution which gives some guidelines for creating evolutionary trees using the alien creation rules, but I've got significantly more to do on them, so I'll put it off until later.

MagiMaster 06-17-2010 10:33 PM

Re: GURPS: Ecosystems and Evolution
For world building purposes, a good definition of an ecosystem would be an environment and the things that live there. The first step in designing most ecosystems is to determine how big it is (in square miles for this system). Few creatures move between two different ecosystems, but those can be dealt with later.

For any ecosystem, the most important question is, where is its energy coming from. For most places, the answer would be the sun, but other answers exist (deep ocean vents and seeps, chemosynthetic life inside rocks, etc.). Where the energy comes from decides what the life will look like, but those details will be left to the GMs imagination. How much energy the ecosystem is receiving will determine how many creatures will live there. For all ecosystems, this can be boiled down to how much biomass (in pounds here) is generated at the first trophic level, the producers.

Earth ecosystems that depend on sunlight vary greatly in how much energy is converted into biomass, from about 50 lbs per sq. mile per day to about 40,000 depending on the water and other nutrients available. For comparison (from Wikipedia) -
  • swamps and marshes: 37,500 lbs per sq. mile per day
  • tropical rain forests: 30,000
  • algal beds and reefs: 30,000
  • river estuaries: 27,000
  • temperate forests: 19,000
  • cultivated lands: 10,000
  • tundras: 2,100
  • open ocean: 2,000
  • deserts: 50

For planets other than Earth, pick a number based on similarities to the examples above, multiply it by the star's luminosity (in solar luminosities) and divide by the square of the orbital radius (in AU).

Other Energy Sources
On Earth, about 900,000,000 watts per sq. mile is absorbed by ground and water on the surface. This is averaged over the entire planet though, so it will be greater near the equator (about 25% more) and less near the poles (about 25% less). Of this, roughly 1% to 5% is converted by plants into chemical energy.

Combined with the numbers above, any energy source that can be rated in watts can optimally be converted to biomass at the rate of about 30,000 watts per (lbs per day). Unfortunately, not many energy sources can easily be expressed in watts.

Radiation, for example, is not directly measurable in watts, but some natural neclear reactors give us an idea of what to expect. These natural reactors only lasted for a few hundred thousand years and only produced an average of 100,000 watts, which would only support a few lbs per day. Fictional planets might have some geological processes that could increase this, but such places would be extremely dangerous to adventurers.

Thermal energy is probably a better place to look. While hot objects carry a lot of energy (in fact any object at about 75 F radiates about 40 watts per sq. foot), it's generally useless if everything's the same temperature. To get energy from heat, it has to be moved from somewhere hot to somewhere cold. Transforming a temperature differential into work is not straightforward. Currently, I have no idea how to transform a hot temperature, a cold temperature and an area into a maximum wattage.

Some forms of energy can be converted directly into biomass without worrying about watts. Things like chemical plumes and waste runoff from another ecosystem are best measured in weight per day. Simply assume that some portion of the input, depending on the type, is transformed into biomass at the first trophic level. I don't have numbers for hydrothermal vents or rainforest canopy refuse, but both would be appropriate examples.

MagiMaster 06-17-2010 10:34 PM

Re: GURPS: Ecosystems and Evolution
Trophic Levels
Trophic levels are a simplified view of how food webs work. If you assume that things only eat from the trophic level immediately below themselves, it basically works, but many real animals eat from multiple levels.

The first trophic level consists of the primary producers. In most Earth ecosystems, this consists of plants. The second trophic level consists of things that eat the primary producers. If plants are the first level, these would be herbivores. The third and higher levels are carnivores. How many levels can be supported depends on how rich the ecosystem is. Besides the main trophic levels, there are two secondary energy sources: excrement and dead bodies. Depending on how rich the ecosystem is, both of these sources can produce their own series of trophic levels.

At each trophic level, half of the biomass is lost to non-predatory deaths. This portion goes to the scavengers and decomposers. The consumers from the next trophic level eat the other half. Half of what is eaten is lost as excrement. Half of what's left is lost to respiration. This portion becomes various gasses and vapors and is generally unrecoverable on the small scale. (On a planetary scale, it's recycled as part of the conversion from energy to primary producers, in general.) Finally, the remaining portion is used for the growth and reproduction of the consumers. That means the each trophic level contains approximately 1/8 the total biomass of the trophic level below it.

The biomass at each level needs to be divided among the various species at that level. While there are no rules for this, a reasonable guess would be to give somewhere between 1/2 and 1/5 the biomass to the most abundant species, between 1/2 and 1/5 of what's left to the next, etc. until the biomass remaining is too small to support another population (see below for transforming biomass into populations).

The amount of biomass in the first trophic level has already been dealt with in the previous section. Use this to determine the biomass for each new level until there isn't enough left to support any populations within the new level. In some ecosystems, the levels might stop one short of this, but few ecosystems are likely to be short more than one level short in the long term. From the final trophic level, all of the biomass goes to scavengers and decomposers as there's nothing that hunts the creatures there.

Finally, determine the totals are for waste and corpses. As a shortcut, about the dead plant matter totals about 50% of the biomass of the 1st level, the dead animals total about 7%, and the animal waste totals about 28%. If these numbers are high enough, there might be creatures that prey on these creatures, so fill in any further levels there. I don't have any numbers, but it's reasonable to assume that both of these numbers should have some conversion efficiency factor applied to them. I'd guess that it'd be in the range of 1/10 to 1/2 for the waste and 1/4 to 3/4 for the corpses. After all, things are willing to expend the energy to avoid having to eat these things, so there must be a reason.

MagiMaster 06-17-2010 10:34 PM

Re: GURPS: Ecosystems and Evolution
Food webs
Trophic levels are a useful simplification, but a real ecosystem is rarely that simple. A better model is the food web. Most of the same ideas are useful in building a food web, but the process is a little more involved.

Start by making a list of all the primary producers in the area, and distribute the biomass among them. At this point, the scavengers and decomposers can be added using the percentages listed above to determine their biomass. (Ideally, it'd be possible to list all of the species before continuing, but it's very likely that add species as you go will be easier.) Add further species one at a time, drawing arrows from what they eat to them. (This is what makes it a food web.) Each of the arrows needs to be labeled with how much biomass is taken due to that particular predator-prey relationship. It's easiest to ignore the non-predatory losses for a second and say that all the arrows out of one species should add up to the biomass available to that species.

When deciding how much goes which way, a good rule of thumb is that a predatory gets most of its food from one or maybe two target species, but occasionally eats some other things. However things gets layed out, each species, besides the primary producers, gets a biomass of 1/8 the total of all the arrows coming in to it. It's possible that some of the species may end up with too little biomass to support a stable population and must be trimmed or find something else to eat.

MagiMaster 06-17-2010 10:35 PM

Re: GURPS: Ecosystems and Evolution
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 06-17-2010 10:37 PM

Re: GURPS: Ecosystems and Evolution
Reserved for expansion

MagiMaster 06-17-2010 10:38 PM

Re: GURPS: Ecosystems and Evolution
Worked example

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

Refplace 06-18-2010 12:47 AM

Re: GURPS: Ecosystems and Evolution
Logged as a reference thread :)

TheNinjaD 06-18-2010 06:41 AM

Re: GURPS: Ecosystems and Evolution
I'm looking forward to seeing how this turns out.

MagiMaster 06-18-2010 04:20 PM

Re: GURPS: Ecosystems and Evolution
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.

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