If you get a general question on ecosystems at risk for vulnerability you would first talk about
what are ecosystems and the dynamic equilibrium;
how they are subjected to change ie, natural and human induced;
define vulnerability;
explain how this is linked with the resilience of the ecosystem and how this correlates to classifying an ecosystem at risk.... etc.
To illustrate you answer use the knowledge you have from your case studies to make examples. You will definately need your case studies in an HSC answer.
Finish your essay with a summation about how ecosystems are vulnerable and what it is that makes them so.
it might be an idea to raise the need for protection and management strategies so that the rates of change dont exceed the ecosystems ability to respond to these changes (resilience)
hope that helps