LOUISIANA WETLANDS

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Plants and Animals / Food Chains & Webs
The source of all food is the activity of autotrophs, mainly photosynthesis by plants.
  • They are called producers because only they can manufacture food from inorganic raw materials.
  • This food feeds herbivores, called primary consumers.
  • Carnivores that feed on herbivores are called secondary consumers.
  • Carnivores that feed on other carnivores are tertiary (or higher) consumers.

Such a path of food consumption is called a food chain.

Each level of consumption in a food chain is called a trophic level.

The table gives one example of a food chain and the trophic levels represented in it.
Grass
→ 		Grasshopper
→		 Toad
→		 Snake
→		 Hawk
→		 Bacteria of decay
In general,
Autotrophs
(Producers)
→		 Herbivores
(Primary Consumers)
→		 Carnivores
(Secondary, tertiary, etc. consumers)
→		 Decomposers

Food Webs

Most food chains are interconnected. Animals typically consume a varied diet and, in turn, serve as food for a variety of other creatures that prey on them. These interconnections create food webs.

Energy Flow Through Food Chains

H. T. Odum analyzed the flow of energy through a river ecosystem in Silver Springs, Florida.

His findings are shown here. The figures are given in kilocalories per square meter per year (kcal/m2/yr).

At each trophic level,
  • Net production is only a fraction of gross production because the organisms must expend energy to stay alive. Note that the difference between gross and net production is greater for animals than for the producers - reflecting their greater activity.
  • Much of the energy stored in net production was lost to the system by
    • decay
    • being carried downstream
  • Note the substantial losses in net production as energy passes from one trophic level to the next.
  • The ratio of net production at one level to net production at the next higher level is called the conversion efficiency. Here it varied from
    • 17% from producers to primary consumers (1478/8833) to
    • 4.5% from primary to secondary consumers (67/1478).
  • From similar studies in other ecosystems, we can take 10% as the average conversion efficiency from producers to primary consumers.

    Animal husbandry often exceeds this 10% value. For example, broilers (young chickens) can gain half a pound of weight for every pound of food they eat. (Since the water content of the two is not the same, the conversion efficiency is somewhat less than the apparent 50%.) Nonetheless, the loss of energy as it passes from producers to primary consumers explains, for example, why it costs more to buy a pound of beefsteak than a pound of corn.

    Conversion efficiencies from primary consumers to secondary consumers (herbivores to carnivores) tend to be much lower, averaging about 1%.

  • In this ecosystem, all the gross production of the producers (20,810) ultimately disappeared in respiration (14,198) and downstream export and decay (6612). So there was no storage of energy from one year to the next. This is typical of mature ecosystems, such as a mature forest.
Some ecosystems do store energy, for example,
  • The slow rate of decay in bogs causes peat to accumulate (the source of the world's coal)
  • A young forest accumulates organic matter as the trees grow.
The pyramid is based on data acquired by Evans, Cain, and Walcott, and has been redrawn by E. P. Odum, Fundamentals of Ecology, 2nd. ed., © W. B. Saunders Co., Philadelphia, 1959. Copyright ©2004 Dr. John W. Kimball. All rights reserved.