The central idea is the same as the main idea. Most of the time, the main idea is mentioned in the Title. Without reading the essay, I do not know what the central/main idea is.
Salt Marsh: A marsh area that is usually found by estuaries and sounds; badlands
Prohibited: Something that's has been forbidden; banned
Hover: To remain in one place, or act of being in the air in one exact place; drift
Inhumane: When there is without compassion for misery or suffering; cruel
To introduce the idea or purpose of your essay and explain why and what you're writing about.
It would be qualitative because you are not researching for numbers or quantity.
Hope this helps :)