Food Chains and Food Webs

There are two food chain systems on every planet: the grazing food chain and the detrital (decomposition) food chain. Autotrophs and magiotrophs form the foundation of every food chain because they're the creatures that don't require organic material to create life. Contrary to appearances, the detrital chain has the most energy going through it at any one time. Only deep-water aquatic systems (with their characteristic low biomass, rapid turnover of organisms, and high rate of harvest) have more energy flowing through the grazing chain. But if you're a gnome or a halfling, you may not be so surprised being intimate with the vast amount of life contained in the soil and its detritus, or so I've been informed.

The grazing food chain is most easily observed. Deer in the forest, rabbits nibbling on lettuce, insects eating everything green, cattle ruminating on grass, and gorgons in their rocky lairs all represent basic consumer groups of the grazing food chain. Only a small percentage of an environment's net primary production is used by herbivores: only 2.6% in poplar forests, while 30-50% on heavily grazed grasslands. Below ground herbivores like nematodes, scarab beetles, and ground beetles account for the vast majority of herbivorous assimilation.

The detrital food chain is the major pathway of energy flow because grazers utilize so little of the net production. Millipedes, snails, mushrooms, cave crickets, maggots, slugs and most of the oozes (although some can be quite predatory) are all examples of detrital chain organisms. They play an important, if somewhat disgusting, role in the maintenance of a healthy ecosystem. Everything that's not eaten by herbivorous grazers eventually ends up as fodder in the detrital chain. We're all food for the worms.

The detrital chain is based upon decomposition, the reduction of energy-rich organic material by consumers (generally detritivores and decomposers). Whereas photosynthesis and magiosynthesis involve the incorporation of solar energy or magical energy into organic matter, decomposition involves the loss of heat energy and the conversion of organic nutrients into inorganic ones. To test this theory, just go to a farmer's compost heap and stick your hand in. You'll find that the inside of the heap is quiet warm because of all the heat lost through decomposition. Decomposition includes many processes: the leaching of soluble compounds from dead organic material, fragmentation, bacterial and fugal breakdown, consumption of bacterial and fungal organisms by animals, excretion of organic and inorganic compounds by organisms, and the clustering of colloidal organic matter into larger particles. Every non-magical animal is involved with decomposition, as its waste products are primary source material.

These two chains are easily represented thusly. The grazing chain is light/magic > autrotrophs/magiotrophs > herbivores > carnivores > top carnivores. The detrital chain is detritus/magic > microsaprophages/magiotrophs > microbial grazers > microbial predators> top detrital predators. There are many other relationships occurring at the same time that cannot be explained simply, but better seen pictorially.

But most relationships in nature are not simple, straight-line food chains. Numerous food chains interlink into complex food webs, with all links leading from producers through an array of primary and secondary consumers. Interestingly enough, food webs (once unraveled) rarely exceed four links because every new layer adds another level of energy transfer inefficiency. Highly productive ecosystems rarely support more links, termed trophic levels. They usually support more species and have more complex webs instead of longer ones. The few land-based ecosystems that exceed four links usually stress magic as a primary energy source. Food webs are usually shorter in fluctuating environments (temperature, moisture, salinity) and longer in environments that have more stable conditions. Highly stratified environments, like forests or pelagic water columns, have longer food webs than poorly stratified environments, like grassland, tundra, and stream bottoms. The widest food webs (those with the greatest number of herbivores) tend to be the shortest while narrow food webs have the greatest fraction of top carnivores. More complex food webs are actually less stable than shorter ones and are easiest to disrupt. Generalist species most easily invade simple food webs while specialists, capable of exploiting a restricted source of energy, are best able to invade a complex food web.

One would think that omnivores (those who can eat both meat and plant) would dominate food chains, but the reality is quite different. Although an excellent survival mechanism, omnivores tend to be generalists and hence, cannot digest either meat or plants to the efficiency of true carnivores or herbivores. This means omnivores are not highly prevalent in food chains dominated by larger creatures. Most omnivores can only feed on adjacent trophic levels, but detritivores, insects, and their predators and parasitoids can often feed on non-adjacent trophic levels.

Predators may overlap in their exploitation of prey species. Many predators feed upon mice, for example. Top predators may feed on a number of species, or they may concentrate on a few particular species found on trophic levels right below them. In general, the more species of prey an animal exploits, the fewer predators it faces. This isn't a truism, but it helps indicate a creature's general trophic level in its environment.


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  • Gherardo
    Why does a food web tend to be more stable than a food chain?
    1 year ago

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