In an ecosystem, the trophic level with the least biomass is typically the tertiary consumers. Conversely, the organisms at the trophic level that consume the most energy are the primary consumers.

To understand why this is the case, let's first define the trophic levels mentioned:

Now, let's look at why tertiary consumers have the least biomass and why primary consumers consume the most energy:

In summary, the tertiary consumers have the least biomass due to biomagnification of pollutants and toxins, while the primary consumers consume the most energy because of the relatively low energy transfer efficiency from plants to herbivores and the need to consume larger quantities of plant material to meet their energy needs.

Dr Gaurav H Tandon thank you for your contribution to the discussion

The trophic level that has the least biomass is usually the tertiary consumers. Tertiary consumers are at the top of the food web and subsequently have the least access to energy, according to the 10% rule.The level with the least biomass is the highest predators in the food chain, such as foxes and eagles. In temperate grassland, grasses and other plants are the primary producers at the bottom of the pyramid. The top level of an energy pyramid has the fewest organisms because it has the least amount of energy. Eventually there is not enough energy left to support another trophic level; thus most ecosystems only have four trophic levels. The biomass is inversely proportional to the energy density at a particular trophic level. The energy density is lowest for producers as it is spread out and hence they have the highest biomass. Whereas consumers at the highest levels have high energy density and lower biomass. With less energy at higher trophic levels, there are usually fewer organisms as well. Organisms tend to be larger in size at higher trophic levels, but their smaller numbers result in less biomass. Biomass is the total mass of organisms at a trophic level.Biomass decreases with each trophic level. There is always more biomass in lower trophic levels than in higher ones. Because biomass decreases with each trophic level, there are always more autotrophs than herbivores in a healthy food web. Only the green stored energy is available to the consumer. Thus, a primary consumer is going to be more efficient than a secondary consumer. A secondary consumer is going to be more efficient than a tertiary consumer. In the image below, the ground squirrel consumes the plant but then the fox consumes the squirrel. The top level of an energy pyramid is made up of tertiary consumers which are usually apex predators. They are carnivorous animals who obtain their energy by eating other animals. The secondary consumers tend to be larger and fewer in number. This continues on, all the way up to the top of the food chain. About 50% of the energy in food is lost at each trophic level when an organism is eaten, so it is less efficient to be a higher order consumer than a primary consumer. The bottom and largest level of the pyramid is the producers and contains the largest amount of energy. As you move up the pyramid, through the trophic levels to primary, secondary and tertiary consumers, the amount of energy decreases and the levels become smaller.