Stress can directly undermine health and physical well-being by

Stress could be triggered because of doing hard things, decisions, problems and many more that will make a person think and feel problematic. The stress can directly undermine health and physical well being by having people to start or to engaged in behaviors that are unhealthy. It is because stress makes people feel vulnerable and do things in order to relieve the stress that they have even if it isn't appropriate or good. That's why it is causing a person to engage in unhealthy behaviors.

Unavailability of food decrease their population.

Explanation:

The population of Galapagos Finches decreases because of unavailability of food. Galapagos Finches feeds on a specific insects whose population decreases on that Island that leads to the death of the Galapagos Finches and they decrease in number. Positive population  growth occurs only if more food is available to the Finches or they adapt another food item for themselves which fulfill their need of food.