The link to both articles are below:

1. What evidence is there that sea otters are a keystone species?
2. Explain how the killer whale-sea otter-urchin food web is an example of top-down control.
3. Explain the difference between Figure 2a and 2b from Estes and Palmisano (1974).
4. What are the benefits of large-scale/long-term approaches in ecological research?

https://pdfs.semanticscholar.org/3758/ae458e57c16b…

https://werc.ucsc.edu/Estes/Estes%20Publications/1…