Mesocosms and keystone species

Mesocosms and keystone speciesSpec D4.2.4, D4.2.5

Using a model: mesocosms

A mesocosm is a small, enclosed experimental ecosystem that models a natural one, so that one variable can be changed while others are controlled. Mesocosms can be set up in open tanks, but sealed glass vessels are preferable: entry and exit of matter is prevented, but energy transfer is still possible, as light enters and heat leaves. Aquatic or microbial ecosystems are likely to be more successful than terrestrial ones.

1. Set up several identical sealed glass jars with pond water, sediment, aquatic plants and algae.
2. Change one variable between jars, for example light intensity, temperature or added nitrate, with at least three replicates of each.
3. Keep other variables the same and leave the jars for several weeks.
4. Monitor stability, for example dissolved oxygen, pH, water clarity and the number of organisms of each type.

Keystone species

A keystone species has a disproportionate impact on community structure compared with its abundance. If it is removed, the community changes greatly and the ecosystem may collapse.

• Sea otters in Pacific kelp forests eat sea urchins. Where otters were hunted out, urchins multiplied and grazed the kelp away, leaving 'urchin barrens' with far fewer species.
• Ochre starfish (Pisaster) on rocky shores eat mussels. When they were removed in experiments, mussels covered the rocks and the number of species fell sharply.