How short? you can answer the question in 3 sentences or in 2 pages. it depends. but it usually takes some skillz to answer it in 3 sentences. So if I were you, I'd elaborate. AP people like it that you put down everything that you know that is related to the question. And, explain everything that you write in biological terms. If it asks like, what type of seed dispersal is common, don't just say wind dispersal. Explain how wind helps move seeds around, why it's important, what structures are on the seed that facillitate this, and other details such as the seed not needing animal/bug pollination and so it's not as pretty or sweet etc. And then go on to explain why it's beneficial to be wind-pollinated. This is just an example, and is what I would do. Basically I'm the type of person who writes 2 pages. Most of which probably is not irrelevant but I could probably do without. I say, since they don't take off points for it being wrong, I go for it
And essay form just means complete sentences. YOu can even draw pictures if you want.