IB ESS SL 2.1.23 Keystone Species Question Bank
Explain how keystone species can have disproportionate effects on community structure and function.
- Syllabus
- First assessment 2026
- Course
- ESS SL
- Level
- SL
Explain how keystone species can have disproportionate effects on community structure and function.

Figure A simplified food web in a kelp forest ecosystem

Figure Kelp forest ecosystems with and without the presence of sea otters
The arrows indicate the flow of carbon between the atmosphere and the carbon stored in kelp.
Outline why sea otters are considered a keystone species.
a. They protect the integrity of the food web / regulate herbivore populations;
b. Without them the biodiversity is reduced/food web may collapse;
c. Without them the kelp forest/ecosystem structure is severely weakened;
Explain how sea otters contribute to the resilience of the kelp forest ecosystem.
a. The presence of sea otters increases the resiliency of the kelp forest ecosystem;
b. The otters control the sea urchin/snail/crab/herbivore population which allows the kelp forest to remain intact;
Resiliency increases with:
c. ...greater overall biodiversity with sea otters present;
d. ...more biomass/greater storages with sea otters present;
e. ...greater productivity leads to greater cycling of nutrients;
f. ...more complex food webs/species interactions;
Note to examiners: 2 max if no explicit connection to resilience