4.2 Applications and skills

4.2.1 Practical 5: Sustainable mesocosms

A mesocosm is an experimental tool designed to mimic, as closely as possible, the conditions of a closed ecosystem. An ecosystem is closed when:

  • Energy in the form of heat and light can enter and exit the system. In most cases, this involves sunlight being added constantly, and heat energy being released through respiration.
  • Matter in the form of nutrients and biomass stay enclosed in the system. The amount of matter is constant in a closed system, but it is cycled through the system in various forms.

Natural ecosystems are open, in that they can exchange matter and energy with other systems in the environment.

Building a mesocosm is useful to demonstrate that ecosystems develop and change over time. This process is called ecological succession. Some of the key features of succession are:

  • Systems become more complex over time. The abundance of species tends to increase as a community develops.
  • Systems become more stable over time. Natural systems rely on feedback loops, which stabilise the structure of a community.

It is difficult to predict whether an experimental mesocosm will be self-sustaining over long periods of time. You may be creative in your design of a sustainable mesocosm, but the design must:

  • include photosynthetic organisms to provide oxygen for cellular respiration, as well as heterotrophic organisms (i.e. decomposers)
  • include relevant nutrients required for growth
  • not include any vertebrate or invertebrate animals (microbes are acceptable according to the IB’s animal experimentation policy).

Activity 1: A sealed aquatic mesocosm 

Pond water contains algae, microbes and plants, and makes a very simple, but possibly odorous, mesocosm in a bottle. For this activity each group needs:

  • a large pail or bucket
  • a wooden spoon
  • clear plastic or glass bottles with lids
  • pond water, weeds and mud from the same pond
  • sieves of varying-sized pores
  • paper towel.

Instructions

  1. Collect pond water and mud in the bucket. Mix it to create a muddy suspension.
  2. Pour some of the suspension through the sieve to remove large animal organisms. Return these to the pond.
  3. Use successively smaller sieves to remove smaller organisms.
  4. Fill the bottles about three-quarters full of the filtered muddy suspension. Wait for the mud to settle at the bottom. You may add pond weed or plant fertiliser at this point.
  5. Place a layer of paper towel under the lid, then close the lid on your aquatic mesocosm. The paper towel will absorb condensation at the top of the bottle, and regulate moisture in the air above the ‘pond’.
  6. Place your mesocosm in a well-lit and cool area. Observe changes over a period of weeks.

mixed mesocosmFigure 4.2.1a – Mixed mesocosm
The top half of the bottle at the bottom is used to join the two parts together.

Activity 2: A mixed mesocosm

This mesocosm has terrestrial and aquatic elements.

  • 2 transparent 2l plastic bottles
  • clear adhesive tape
  • cheesecloth or pantyhose
  • aquarium pebbles or gravel
  • elastic bands
  • bucket and shovel
  • pond mud and water
  • grass seeds or seedlings
  • soil from the school field or garden
  • dry leaves or litter

Instructions:

  1. Follow steps 1–3 from Activity 1. Keep the filtered suspension aside.
  2. Cut the top off one bottle. Invert the top, and cover the spout with the cheesecloth. Fix the cheesecloth in place with an elastic band. Save the bottom.
  3. Put about 3cm of pebbles in the inverted bottle. Layer the garden soil on top of the pebbles. (Make sure to remove any invertebrate animals like slugs and worms and return them to the garden.) Add the grass seeds, a bit of water and leaf litter to the top of the soil.
  4. Tape the bottom of the bottle to the inverted top.
  5. Cut the top off the second bottle. Fill the bottom about halfway with the muddy suspension from step 1.
  6. Cut the top of the second bottle again at a point before it begins to taper. Use this piece to couple the terrestrial part of your mesocosm to the aquatic part. Use tape to fix the part together as shown in Figure 4.2.1a.
  7. Leave your mesocosm in a well-lit place at room temperature for at least a few weeks. 

Open/closed systemFigure 4.2.1b – Open/closed system
In an open system, matter and energy are exchanged with the environment. In a closed system, energy is exchanged, but not matter. In real systems, the boundaries of systems are often difficult to identify.

Figure 4.2.1cFigure 4.2.1c – The Earth, as a whole, is a closed ecosystem 
Heat and light are reflected back to space, while matter is not exchanged past the boundary of the atmosphere. The concept of a closed system is conceptually useful. In reality, all ecosystems are open.

ecosphereFigure 4.2.1d – Ecosphere
This self-sufficient ecosphere is a closed system containing algae, microbes and shrimp. It was sealed in 1999 and is on display at the American Natural History Museum in New York. 

Did you know?

Biosphere 2 is now a museum and research centre owned by the University of Arizona, but it was originally designed as an experiment to determine if humans could live in a closed mesocosm, for the purpose of one-day colonising Mars?

biosphere 2 - internalFigure 4.2.1e – The internal facilities of Biosphere 2

biosphere 2 - externalFigure 4.2.1f – The external facilities of Biosphere 2

In the lab 1

For either mesocosm, try some or all of these variations and make hypotheses about which system is more sustainable:

  • Fill bottles with different combinations of organisms (based on the level of filtration) or change the ratio of mud:water.
  • Add different amounts of pond weed and fertiliser.
  • Place the bottles in different locations based on temperature and light exposure.

In the lab 2

Poke holes in the sides of your plastic bottles, so you can take sensor readings of oxygen and carbon dioxide levels in the air of your mesocosm. Be sure to seal the holes with transparent film and/or tape. Track the changes in gas concentrations over time.

In the lab 3

You can dechlorinate tap water by leaving it in a shallow pan for 24 hours. Chlorine will evaporate.

Course link

Learn more about succession and stability of ecosystems in 14.1.2 Communities and ecosystems.