The Ecological Pyramid

The Ecological Pyramid

An Introduction;          
        We described in the previous sections how energy and organic compounds are passed from one trophic level to the next. What was not mentioned is the efficiency of the transfer. In a highly efficient transfer almost all of the energy would be transferred -- 80% or more. In a low efficiency transfer very little energy would be transferred -- less than 20%.
          In a typical food chain, not all animals or plants are eaten by the next trophic level. In addition, there are portions or materials (such as beaks, shells, bones, etc.) that are also not eaten. That is why the transfer of matter and energy from one trophic level to the next is not an efficient one.
        One way to calculate the energy transfer is by measuring or sizing the energy at one trophic level and then at the next. Calorie is a unit of measure used for energy. The energy transfer from one trophic level to the next is about 10%. For example, if there are 10,000 calories at one level, only 1,000 are transferred to the next. This 10% energy and material transfer rule can be depicted with an ecological pyramid that looks like this:

    

        This pyramid helps one visualize the fact that in an ecological system there need to be many producing organisms at the bottom of the pyramid to be able to sustain just a couple of organisms at the top.
       In looking at the pyramid, can you guess how much larger the volume of each layer is as compared to the one just above it? Take a guess. It might not look like it but they are close to 10 times larger.

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 of Energy

Pyramid of Energy

         Indicates the total amount of energy present in each trophic level

     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.

Some ingested materials, e.g., fat-soluble pesticides like DDT, tend not to be dissipated back to the environment. As they pass from one trophic level to the next, their concentration in living tissue actually increases.

The Pyramid of Biomass

Pyramid of Biomass

Indicates the total dry mass of the organisms in each trophic level

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

Pyramid of Numbers

Shows the number of organisms in each trophic level and does not take into consideration the size of the organisms and over-emphasizes the importance of small organisms

      Small animals are more numerous than larger ones. This graph shows the pyramid of numbers resulting when a census of the populations of producers, herbivores, and two levels of carnivores was taken on an acre of grassland.

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.

     The graphic represents a pyramid of numbers in an acre of bluegrass. 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.