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Old 06-17-2010, 10:34 PM   #3
MagiMaster
 
Join Date: Jan 2007
Location: Your imagination
Default 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.
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