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17.1.2: Food Chains and Food 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

      Fig. 17.1.2.1 Silver Springs ecosystem

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 (227 g) of weight for every pound (454 g) 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 of Energy

          Fig. 17.1.2.2 Silver Springs

Conversions efficiencies are always much less than 100%. At each link in a food chain, a substantial portion of the sun's energy — originally trapped by a photosynthesizing autotroph - is dissipated back to the environment (ultimately as heat). Thus it follows that the total amount of energy stored in the bodies of a given population is dependent on its trophic level. For example, the total amount of energy in a population of toads must necessarily be far less than that in the insects on which they feed. The insects, in turn, have only a fraction of the energy stored in the plants on which they feed. This decrease in the total available energy at each higher trophic level is called the pyramid of energy.

Using Odum's data on net productivity at the various levels in Silver Springs, we get this pyramid. The figures represent net production at each trophic level expressed in kcal/m2/yr.

The Pyramid of Biomass

        Fig. 17.1.2.3 Silver Springs

How does one measure the amount of energy in a population?

Since all organisms are made of roughly the same organic molecules in similar proportions, a measure of their dry weight is a rough measure of the energy they contain. A census of the population, multiplied by the weight of an average individual in it, gives an estimate of the weight of the population. This is called the biomass (or standing crop). This, too, diminishes with the distance along the food chain from the autotrophs which make the organic molecules in the first place.

The graphic shows the pyramid of biomass for Silver Springs. (It, too, is based on the data obtained by Howard T. Odum.) The figures represent the dry weight of organic matter (per square meter) at the time of sampling. Analysis of various ecosystems indicates that those with squat biomass pyramids (with conversion efficiencies between one trophic level and the next averaging 10% or better) are less likely to be disrupted by physical or biotic changes than those with tall, skinny pyramids (having conversion efficiencies less than 10%).

The Pyramid of Numbers

   Fig. 17.1.2.4 Bluegrass pyramid

Small animals are more numerous than larger ones. This graph shows the pyramid of numbers resulting when a census of the populations of autotrophs, herbivores, and two levels of carnivores was taken on an acre (0.4 hectare) of grassland. The figures represent number of individuals counted at each trophic level. The pyramid is based on data acquired by Evans, Cain, and Walcott, and has been redrawn by permission from E. P. Odum, Fundamentals of Ecology, 2nd. ed., © W. B. Saunders Co., Philadelphia, 1959.

The pyramid arises because:

  • Each species is limited in its total biomass by its trophic level.
  • So, if the size of the individuals at a given trophic level is small, their numbers can be large and vice versa.
  • Predators are usually larger than their prey.
  • Occupying a higher trophic level, their biomass must be smaller.
  • Hence, the number of individuals in the predator population is much smaller than that in the prey population.

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