You are Matt Damon stranded on Mars. Alone.
You recall an ecology course you took in training to be an astronaut (but do not remember the prof). Perhaps ecology can you keep you alive. Interactions: biotic-biotic-abiotic. Your air filtration system can keep you alive for years. There is ample water too. Food is the real issue. You realize that the formula for NPP is the most important principle you need right now to stay alive. What is it?