a.) Using organisms from four different trophic levels from these four networks as examples, explain how energy is obtained at each trophic level. Since energy is a necessary part of how an organism survives, how it obtains its energy is crucial. When you look at an ecosystem and all the organisms living in that ecosystem, you can connect many species together through their dependence on each other. Scientists will examine these connections to see how they depend on each other, or in other words, where their main source of energy and nutrition is. Once this step is determined, scientists will assign this group of species to a trophic level; primary producers, primary consumers, secondary consumers or tertiary consumers. The bottom of the chain and the trophic level on which all others depend are the primary producers. These primary producers consist of autotrophs, capable of obtaining their source of food and energy without consuming organisms or substances from other organisms. In Alaska's arctic lake, one of its main producers is aquatic plants and algae. These algae and algae contain chlorophyll, which means they can use light energy from the sun to synthesize glucose and other organic compounds, which they can use for cellular respiration and as a building material for growth. In other words, we are talking about photosynthesis. Photosynthesis requires light energy, but some autotrophs use chemosynthesis, meaning they can convert nutrients into organic compounds without the presence of light. While the primary producers' trophic level is that of autotrophs, the next remaining levels represent all heterotrophs. Heterotrophs can only obtain their energy by consuming other organisms. At the tropical primary consumer level, these herbivores depend on these primary producers and other plants for their food. Chironomid larvae, or a type of aquatic insect, are an example of a primary consumer. The next trophic level consists of secondary consumers. These secondary consumers are also heterotrophs and these organisms are carnivores that obtain energy from consuming other herbivores. One organism that falls into this category is the sculpin, a small fish, which uses organisms such as chironomid larvae to obtain their energy for survival. The fourth and final trophic level remaining is that of tertiary consumers..