Final answer:
At the first trophic level, there would be approximately 10,000 joules of energy if the third trophic level has 100 joules, assuming a 10% energy transfer efficiency between levels.
Step-by-step explanation:
Considering the trophic levels and energy transfer within an ecosystem, we know that only about 10% of the energy from one level is available to the next level due to metabolic processes, heat loss, and other factors. If at the third trophic level there is 100 joules of energy, this means at the second trophic level there would have been about 1000 joules available (assuming a 10% transfer efficiency), and thus at the first trophic level, there would have been about 10,000 joules available.
It’s important to understand this diminishing energy because it explains why ecosystems rarely have more than four trophic levels, as there isn't enough energy left to support additional levels above this.