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MagiMaster · 06-17-2010, 10:34 PM

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.

06-17-2010, 10:34 PM	#4
MagiMaster Join Date: Jan 2007 Location: Your imagination	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.