2.5 Estimating Biomass and Energy Flow

Estimating Biomass and Energy Flow

It's quite a challenge for students to gain any experience of this aspect of the ESS guide's knowledge and understanding. Techniques which might be used to obtain data are not ethical and not at all in-line with the IB's Animal Experimentation Policy. Students can gain direct experience using primary producers but for higher trophic levels students will have to understand that this must be done by extrapolation, estimation and using standard conversion factors from wet weight. It is useful to discuss with students how scientists originally conducted seminal studies such as Silver Springs and make a good link to TOK and ethics. 

"Methods for estimating the biomass and energy of trophic levels in a community include measurement of dry mass, controlled combustion and extrapolation from samples. Data from these methods can be used to construct ecological pyramids." IB ESS Guide 2015.

See also  2.5 Measuring Dry Biomass 

Significant ideas: 

  • Ecosystems can be better understood through the investigation and quantification of their components.

From the Guide (IB ESS Guide 2015):

Methods for estimating the biomass and energy of trophic levels in a community include measurement of dry mass, controlled combustion and extrapolation from samples. Data from these methods can be used to construct ecological pyramids.

A Famous Case Study: Howard Odum (1957) Trophic Structure and Productivity of Silver Springs, Florida

This study was the first comprehensive attempt to look at a system, using interdisciplinary approaches, counting and measuring as many components of the system as possibly. Odum investigated the flow of energy and materials in the system.

Here's the original paper in pdf format.

And here is a 50 Year Retrospective Study of Silver Springs in pdf format.

Student Task:

Using the two studies of Silver Springs, identify the following:

  • how was plant biomass estimated?
  • how did the scientists identify which trophic levels organisms belonged to?
  • how did Odum's study and the more recent study differ in their techniques for biomass estimation of 2nd and higher trophic levels?
  • how were respiration rates for organisms calculated?
  • how were biomass pyramids constructed

 Teacher Tip: These are long papers. Divide the class up to find the different information then review everyone's findings. If possible, try to draw similarities to your own class fieldwork.

 Why do you think the methodologies changed in some areas during the 50 years between the two studies? Is this an improvement to scientific understanding?

Some interesting notes from the study:

  • Odum comments about the importance of repeatability in ecological studies - he give clear descriptions of methodologies used
  • plant biomass was estimated by collecting the above ground material and making wet and dry biomass measurements
  • algae succession studies were done using glass slides submerged in the water
  • total counts and samples were used for the 2nd and higher trophic levels
  • once rough counts were made, rough biomass was obtained by obtaining the dry mass of each organism, this included all aquatic organisms up to predatory fish
  • decomposer bacteria and fish parasites were even included in the studies
  • respiration rates for some organisms were calculated by placing the organisms in jars and measuring Oxygen levels before and after a short period of time.
  • biomass pyramids were constructed by multiplying counts for organisms with estimated biomass
  • organisms were allocated to trophic levels depending on the percentage of feeding they did at that level
  • Odum made calculations of ecological growth efficiencies, trophic level production ratios and trophic level efficiency along with an impressive variety of other measures and estimates
  • He used a combination of observational sampling and experimental measures and calculations

In the more recent study (2006), standard wet weight/dry weight conversion factors were used for estimations of biomass. It is also noted in Henderson & Southwood (2016)[1] that

"The study of energy budgets generated great interest between the 1960s and 1980s, but has subsequently declined in popularity, probably because the great effort required to obtain the data did not generate sufficienty novel insight to justify the additional costs incurred over the use of number or biomass as a currency."

Measuring Biomass

To obtain biomass estimates, there are no standard procedures and methods vary from organism to organism. Dry mass is preferred as is comparable across species and systems but even the temperature used for drying varies across species. Again for IB coursework, only plants should even be dried and a temperature of about 60 degrees can be used. Some suggest using a desicator but with larger quantities of biomass this can be too slow and encourage the material to go mouldy! Higher temperatures are not advised as they evaporate oils as well as water.

To determine the amount of energy in a substance, the most accurate and widely used method in ecology, is bomb calorimetry. This requires the substance to be burned in oxygen the amount of heat emitted is recorded. A more basic version of this method can be seen here Energy Content in Biomass .

For a method to obtain dry biomass for plants see Measuring Dry Biomass .

For a method for estimating the biomass of macroinvertebrates in aquatic systems from wet mass see Estimating Dry Biomass from Wet Biomass.

Constructing Ecological Pyramids

There is a really promising exercise produced by the HHMI Biointeractive website which shows students how to produce ecological pyramids from the Gorongosa Region in Mozambique. The exercise is quite detailed and would require several periods of investigation but would be a great data analysis exercise.

When collecting data from kick sampling in streams, the data collected can be used to build ecological pyramids based on the feeding type of the organism, the number collected (per unit area) and the wet mass of the organisms collected. See Investigating Streams.

Student Task 1:

In his study, Odum (1957) identified the following organisms as belonging to different trophic levels. He was also able to estimate their biomass per square metre.

Primary producers: Sagittaria / Aufwachs 809 g/m2

Herbivores: turtles, snails and mullet 36.8 g/m2

Primary consumers: fish and midges 10.7g/m2

Top consumers: Bass, birds and alligators 1.53 g/m2

Detritus Feeders: bacteria and crayfish 5 g/m2

  1. Draw a food chain for the Silent Springs system.
  2. Draw a Pyramid of Biomass for the Silent Springs system.
  1. sagittaria → mullet → midge → birds
  2. The pyramid should decrease at each level going up and be labelled with the trophic level and biomass at each level.

Student Task 2:

Footnotes

  1. ^ Henderson & Southwood (2016) 4th Edition Revised by Henderson. First published in 1966. Ecological Methods. Wiley Blackwell, UK.
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