Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;


Ecology & Energy Flow

I.  Requirements of Life

II.  Energy and Life

A.  Sun.
    The ultimate source of energy for virtually all organisms on earth (with the exception of deep sea vent communities).  As an aside, the sun even powers our human activities via fossil fuels which were are the byproducts of long-dead plants.

B.  Plants are solar energy conversion factories

C.  Plants and animals are powered by the products of photosynthesis. 

D.  Life and Thermodynamics.
    According to the First Law of Thermodynamics - energy can be converted from one form to another;  Thus life is a process of thermodynamic energy conversions.  Plants convert solar energy into chemical energy - they are about 2% efficient.  They produce about 200 billion tons carbon fixed annually.  Consumers convert the chemical energy in plants to additional forms of chemical energy. 

E.  Producers and Consumers
    Autotrophs, like plants are producers; heterotrophs like animals are consumers.  Gross primary productivity refers to the total amount of photosynthesis by plants.  Net primary productivity = gross - energy used for maintainance.  Secondary productivity - amount of energy in consumers.


III.  Energy Flow.


A.  Energy flow is unidirectional - it never cycles!

B.  Passage of energy flows from: sun →  producers →  consumers →  decomposers

C.  Some general notes about energy flow:  

D. Detritivores (earthworms, insects) eat dead/decayed materials (larger), decomposers, like fungi and bacteria, eat smaller stuff; 

E.  Each biotic transfer is termed a trophic (feeding) level;

F.  Food chain - linear sequence of biotic feeding interactions.  i.e.,  plant →  mouse → hawk.  Uncommon in nature;

G.  Food web: complex, intertwined food chains.  More stable, because of more links (a chain is only as strong as its weakest link);  and 

H.  Energy is lost in the transfer of food from one trophic level to the next.  The energy is lost primarily as heat.


IV.  Energy Loss


A.  10% rule of thumb - as a very rough approximation, approximately 10% of the energy entering one level passes to the next.

B.  Predicted by the 2nd Law of Thermodynamics - no energy conversion is 100% efficient, or in other words, all systems tend to a state of greatest entropy - randomness


C.  Loss occurs as:  indigestible parts;  inability to harvest entire trophic level;  metabolism (heat)


D.  Pyramid model of energy flow  

V.  Lessons from the energy pyramid and 2nd Law

A.  Sets a limit on the number of trophic levels in a food chain.  
    The maximum number is usually five - there is not enough energy at higher levels to sustain a species.

B. Energy efficiency increases as you go up the food chain.  
    For example, Golley calculated the energy efficiency of a small food chain of plant mice weasel.  All of the plants, mice and weasels in an old field were collected and their total amount of energy was measured.  Biomass is one way of estimating energy levels.  Another more precise method is to "burn" the material in a calorimeter and note the heat output.  In any case, the total energy available to each level was then known and could be calculated based on the following equation:
  energy efficiency = energy assimilated/energy available x 100.  Check out the data in Table 1.

Table 1:  Energy relationships (study by Golley)


Energy efficiency (%)







.  Size of organisms.  
    Think about some big creatures.  What comes to mind?  Redwood trees, whales, elephants, brontosaurus (apatosaurus).  Where do they feed?  At or near the bottom of the energy pyramid where energy is plentiful.  Also note that many of the larger animals are aquatic or semi-aquatic.  They take advantage of the buoyancy of water to help them move and minimize energy requirements.0 

Principle of food size.
An individual must be large enough to capture and ingest its prey.  Thus, consumers in each level: (1) tend to get bigger, OR (2) have functional modifications to make them functionally bigger (i.e., teeth, claws, hunt in packs).  BUT, can't get too large because there is not enough energy to support and maintain large carnivores.

    But, what about T. rex, the largest carnivore to ever have roamed on earth?  Some speculate that T. rex was not the ferocious active hunter that s/he was portrayed to be in Jurassic Park; rather, T. rex may have spent lots of time sleeping and eating "relatively" easy prey to catch (diseased, crippled, young) and/or carrion.

E. Human cultural evolution.
    Humans have moved down the energy pyramid.  Initially humans were hunter/gatherers eating lots of meat (high on the pyramid).  Numbers were small.  Then, humans learned to herd animals (herbivores), thus feeding lower on the chain and insuring a more predictable food supply.  The development of agriculture permited large increases in human population size.

VI.  Energy and the future
   Will there be enough energy as our population increases?  Probably, but we will almost surely:

  1. Eat more plants (feed lower on the food chain).  Energetically, meat is very expensive (it takes about 10 kg grain to make 1 kg of beef.  As corn meal, the grain will supply the energy needs of 23 people, but feed to chickens, only 2) and is a sign of affluence

  2. Use energy-efficient animals (poultry and pork vs. beef and lamb)

  3. Use more energy-efficient plants (grains vs. tomato or broccoli or cauliflower etc)

  4.  Need to increase and protect (over development, salinization) arable lands.  About 1.4 x 109 ha. in cultivation.  This is about 10% of the 31 x 109 available - but only a small proportion of this total is suitable for cultivation

  5.  Improve the politics of food distribution

  6. Improve agricultural techniques (i.e., raise more crops per unit area)  Some ideas include mixed cropping, raised beds, no till, planting fertilizer species (Azolla).

  7. Genetically improve the quality and quantity (yields) of plants through conventional and molecular techniques.  However, there are currently many controversies about GMO's (genetically modified organisms)

  8.  Utilize non-traditional foods (fish farming, algae, quinoa, arid land crops, perennial crops)

VII.  Biological Magnification.  
    Minute quantities of pesticides released into the environment get concentrated in organisms at higher trophic levels.  This process occurs because as you go up the trophic pyramid, biomass decreases while fat-soluble pesticide levels remain about the same.  Thus, you have more pesticide per unit biomass at higher levels.  Fish consumption warnings in Minnesota are one sign of this phenomenon.  Table 2 provides a model for how biological magnification works. 

Table 2:  Model for Biological MagnificationAssume that DDT is sprayed on some plants at some low concentration.  The amount of biomass of successive trophic levels decreases exponentially (by about a factor of about 10 according to the 10% rule), but the [DDT] only decreases slightly.  Some is lost from the organisms at each level but the amount is relatively constant (this is an arithmetic decrease, not exponential).  Thus the ratio of DDT/biomass markedly increases at each level. Note - the units are arbitrary in all cases.

Trophic level




Secondary Carnivore




Primary Carnivore








Producers (plants)





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Last updated: April 22, 2004        � Copyright by SG Saupe