There is abundant good general advice for writing an abstract for your thesis, research talk or published paper from the web but it is perhaps useful to add a few points specific to ecological studies. I suggest five points for a good abstract as a condensed version of the traditional writing advice: Who, What, When, Where, How and Why.  

1. What is the problem, question, or controversy? You must grab the reader in the first sentence or two.
2. What is your contribution to answering, testing, or changing the question? In a few sentences you should explain what you did, where and when you did it if a field study. If you are testing a hypothesis, you should state the alternative hypotheses as well.
3. How did you reach your conclusions, what methods did you use? The design of your study should include what species or group of species you included, some general points about sample sizes.
4. Rotate back to match your conclusions to your prior hypotheses, or the new hypothesis you present.
5. Finally, state what needs to be done next to further these ecological issues.

The trick is to do all of this in concise sentences, to state clearly your advances in understanding, and equally to state clearly what failed to work the way you had originally postulated.

So, if you can do all of this in 200-300 words, you win the prize. A good abstract is like gold and worth the work.

There is much literature on writing well. Sayer (2019) gives a concise statement of writing for ecological journals. Pollock (2020) emphasizes the responsibility scientists bear for their writing, and Mammola (2020) makes a plea for reducing superlatives in over-selling our conclusions,

If you would like an exercise in a seminar or lab meeting, go through your favourite journal and rank the abstracts in an issue on a scale of 1-10 for both clarity and for enticing you the reader to read the complete details of the rest of the paper.

And go through this same writing routine if you are giving a seminar or lecture and must present a short abstract. We may all be attracted to hear an address on whatever from the Prime Minister or the President, but alas that is not always the case for we mere mortals who must attract an audience to our talks on the basis of our abstract. 

Mammola, S. (2020). On deepest caves, extreme habitats, and ecological superlatives. Trends in Ecology & Evolution 35, 469-472. doi: 10.1016/j.tree.2020.02.011.

Pollock, N.W. (2020). The responsibility of scientific writing. Wilderness & Environmental Medicine 31, 129-130. doi: 10.1016/j.wem.2020.04.005.

Sayer, E.J. (2019). The essentials of effective scientific writing – A revised alternative guide for authors. Functional Ecology 33, 1576-1579. doi: 10.1111/1365-2435.13391.