TROPHIC LEVELS

PRODUCERS - autotrophs

• on land usually plants, in aquatic ecosystems usually algae

• use light to make carbohydrates as sources of energy for all organisms in food chains

• 99% of all organic matter is made up of plants and algae

• all heterotrophs combined make up only 1% of organic matter

• key factors influencing productivity are

• duration and intensity of sunlight

• temperature

• precipitation

• availability of essential mineral elements (esp. in aquatic ecosystems)  

• gross productivity - measure of the rate at which energy is assimilated by the organisms in a particular trophic level  

• net productivity = gross productivity - cost of all the metabolic processes/activities of those organisms

  • the increase in biomass (grams or tonnes in a particular time)

  • biomass = total dry weight of all the organisms measured at any one time

  • measure of the rate at which organisms store energy

  • energy available to organisms of the next trophic level

  • increase in mass of individuals and the mass of offspring  

• net primary production is the increase in the amount of plant or algal material between the beginning and end of a specified time period e.g. agricultural ecosystem - the standing crop for a season

• primary production is very variable

CONSUMERS - heterotrophs

Primary Consumers - herbivores

• eat plants and/or algae

• each ecosystem has its characteristic herbivores

• much of what is consumed is eliminated undigested

• only a fraction of the chemical energy consumed by the herbivore is converted into animal biomass (a very rough guide is 10%)

• most of what is consumed is used to maintain metabolic processes and to provide energy for activities e.g.:

  • searching for food

  • eating and digesting food

  • mating

  • caring for young

  • fleeing predators

Secondary Consumers - carnivores

• animals that eat other animals

• only a small amount of the organic material of the herbivore is incorporated into the body of the carnivore

• some food chains have 4^th or 5^th levels (i.e. 3^rd and 4^th consumer levels)

• there are usually only 3 links (4 levels) in a food chain

   plant  a herbivore a carnivore a carnivore

• the limit is usually 5 links regardless of the ecosystem

• at each level there is a decrease in the total amount of energy stored as biomass

Detritovores

• scavengers and decomposers (fungi and bacteria)

• live on the refuse or detritus of a community

• scavengers - feed on dead organisms (carrion)

• decomposers - break down cellulose and nitrogenous waste products as well as dead organisms

• more than 90% of net primary production is consumed by detritovores rather than herbivores

• some of this energy is fed through the food web by consumers of the detritovores

Food Chains 

• usually short because the transfer of energy from one level to the next is rather inefficient  

• approx 10% at each level (1% - 20% depending on the species involved)

• 1% - 3% of sunlight falling on vegetation is used in photosynthesis

Food Pyramids

• depict relationships at each level of a food chain in quantitative form  

  • energy

  • biomass

  • numbers of organisms

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Copyright © Jennie Walters: 1998 ~ 2004
Last changed: May 10, 2003

since 10/02/2002