Trees are few in numbers in a tree ecosystem but have large biomass that supports organisms which are smaller but much in number up the food chain.
Organisms in an ecosystem can be arranged in three different types of ecological pyramid: pyramid of numbers, pyramid of biomass, and pyramid of energy. See image of the different types of ecological pyramids in the attachment below.
Pyramid of numbers takes account of the number of organisms occupying each trophic level.
Thus, in a tree ecosystem, we have fewer large trees at the bottom of the pyramid that serve as primary producers serving other organisms above in the pyramid, which are usually microscopic or smaller but large in number. This gives us an inverted pyramid of numbers for a tree ecosystem.
Therefore, trees are the smallest trophic level in a pyramid of numbers because they are large in size but fewer in number with large biomass that can support and provide enough nutrition for the consumers in other trophic level.
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