The biological/ecological approach focuses on the environment and how that influences illnesses.
http://www.cimlondon.co.uk/newsletters/2007_3.htm
The ecological approach views more on how the surroundings on a person can lead to an illness. In this case, depression is greatly influenced by stress. Looking at the graph we see that the more stressful someone's job is the more likely they are diagnosed with depression. This concept is also seen in the documentary "Not Just a Paycheck". The people of Greenville Michigan, experience stress and depression from the Electrolux plant shutting down. After losing their jobs and unable to find another the people of Greenville began to experience many health issues. They started to self-medicate with alcohol and pills. This shows that things outside of medical factors can greatly influence depression.
On the biological side, depression is something that can be genetic. If it runs in your family you are more sustainable to getting depressed. In the video below it talks about what exactly happens in the brain when someone become depressed. There is a chemical imbalance that influences moods and behavior.