Just like a food chain, energy enters the food web at the trophic level of the producers, which can also be called autotrophs. In the desert, autotrophs are often some type of cactus, grass, or shrub.
The main purpose of the INTRODUCTION is to give a description of the problem that will be addressed. In this section the researcher might discuss the nature of the research, the purpose of the research, the significance of the research problem, and the research question(s) to be addressed. Three essential parts of a good introduction are: RATIONALE.
1 The main idea is the central concept or topic for the piece of writing. Reviewing the first and final paragraph will usually clarify the main idea.  The purpose of persuasive writing is the reason the author is writing, which is the belief or the action the author wants to convince the reader to believe or do.