What I've come up with so far isn't really cutting it so I need a little help on this for a college assignment. Please cite sources if you have an answer for me :) If if comes from personal knowledge add your name and I will use that as a reference if I use part of your answer.
formerly_bob
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Here's a minimalist way to look at it, and I'm not sure this is what you prof is looking for - this is just my way of thinking:
A hypothetical autotrophic system would require:
* organisms that convert energy to biomass (plants)
* water
* nutrients
* suitable atmosphere
* suitable temperatures
A minimal heterotropic system would need the above plus decomposers
A complete ecosystem adds more trophic levels to the previous two components:
*herbivores
*carnivores - primary & secondary
*omnivores