Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert & Mary Jean Holland

Organisms and Their Environments

Ecology is the division of science that examines the interactions between living organisms and their environments. It is an interdisciplinary field involving biology, chemistry, and physics and several subdivisions of those fields. The interaction between the many different phenotypes of individuals of a species with each other, with other organisms, and with the chemical and physical environment, determines who will have the most progeny in the next generation. This is the grass roots level of evolutionary studies.

A biological species includes all organisms of a group that reproduce and produce fertile offspring in nature. In simplest terms, organisms that do not achieve this in nature are different species or kinds, because gene flow among them is not possible without reproduction. Most species are divided up into local populations that are partially separated from one another by a physical (e.g., mountain) or chemical (e.g., saline) barrier. Gene flow is reduced but not cut off. The several populations of different species that live together and interact form a community. The community of organisms and their physical/chemical environment constitute an ecosystem. Organisms play different important roles in an ecosystem. The description of the role of a particular species in the community, its interaction with all the biotic and abiotic factors in the ecosystem, is termed the ecological niche of that species.

Food Chains and Food Webs

The hierarchy of feeding, who eats whom, is referred to as trophic levels.

Producers (first trophic level) are mostly photosynthetic autotrophs (self feeders), such as plants and cyanobacteria, that extract energy from sunlight and chemical building blocks from the inorganic world, e.g., water, carbon dioxide gas, etc.

All other organisms are consumers or heterotrophs (other feeders) that eat other organisms as the source for their own organic molecules. Heterotrophs are divided among the following subcategories or trophic levels:

Primary consumers (second trophic level) are herbivores, e.g., cows and caterpillars; they eat plants, the producers. Some parasites are primary consumers of plants.

Secondary consumers (third trophic level) eat primary consumers. A cat is a carnivore (meat eater) that eats a mouse that ate seeds; the cat is a secondary consumer. A shrew is an insectivore that eats insects that ate plants. Parasites, too, may live off the organic molecules made by the consumers at any level. An organism that feeds on a secondary consumer is called a tertiary consumer (fourth trophic level).

Decomposers, which include fungi, most bacteria, and some Protista, break down the dead tissue of organisms of all trophic levels. Without them, the world would quickly fill up with corpses, and certain elements of organic molecules would become extremely scarce. These organisms are the recyclers of nature.

Omnivores eat both producers and consumers. People, pigs, and rats are omnivores (great company, huh?).

Plants absorb about 1.2 % of the sunlight that strikes them, and about a third of that energy becomes incorporated in organic molecules that another organism might eat. These organic molecules constitute the biomass of the plant. Organisms at higher trophic levels are able to incorporate about 10 % of ingested biomass as biomass of their own. Thus a herbivore that eats 100 pounds of plant matter may gain 10 pounds in weight.

Consider a food chain with beans as primary producers, mice as primary consumers, weasels as secondary consumers, and eagles as the top carnivores that eat the weasels; how many pounds of beans are required to support a 10 pound eagle? Can you figure out why there are usually no third level carnivores?


Why is it possible for the world to support more people if people eat plants than if people eat cows?


A single line from primary producer through primary consumer to a higher trophic level is called a food chain. Interlocking chains make up the a food web.


Pond Ecosystem

In the following example you are given information about the feeding preferences of a variety of pond organisms. Indicate the environmental role of each one in the right-hand column:

  • Producer
  • Consumer: indicate primary, secondary, tertiary or higher level consumers
    also indicate carnivore, herbivore, or omnivore
  • Decomposer: indicate whether they are organisms of decay—true decomposers
    or scavengers on small organic debris—detritivores


Role in Ecosystem

Grasses (at edge of pond) carry out photosynthesis  
Algae carry out photosynthesis  
Protista eat algae  
Snails eat grasses  
Worms eat dead grasses  
Rotifers eat protista  
Small Fish eat worms, snails, and rotifers  
Medium-sized Fish eat worms, snails, and small fish  
Large Fish eat small fish and medium sized fish  
Turtles eat medium-sized fish and large fish  
Eubacteria decompose any dead material  
Fungi decompose any dead material  
Small Crustacea (on the bottom of the pond) eat pieces of dead animals and plants  

Assignment: On a separate piece of paper, construct a food web for the pond ecosystem using the feeding relationships given in the pond ecosystem chart (above). Arrows should be drawn in the direction of energy flow from producers (autotrophs) toward final consumers and decomposers.

Homework: Review the many fresh water pond organisms that you have studied and those still to be considered in future labs. Fill in their foods and trophic levels:



Trophic Level

Kingdom Eubacteria    
Kingdom Protista    
Kingdom Fungi    
Kingdom Plantae: Green Algae    
Spirogyra, Elodea, etc.    
Kingdom Animalia: Cnidaria    
Kingdom Animalia: Platyhelminthes    
Kingdom Animalia: Mollusca    
Kingdom Animalia: Annelida    
Kingdom Animalia: Arthropoda    
Crustacea: crayfish, etc.    
Insecta—butterfly adult    


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Last updated 30 August 1999 (JHW)