Why are primary producers the first trophic level?

Answer 1

It is not necessarily. Imagine a large tree, insects eat its fruit and leaves, birds eat the insects on it, foxes eat the birds. In a week the tree might feed 10000 insects, who in turn feed 100 birds who in turn feed 5 foxes. There is only one tree and every level has more organisms on it than the producer level.

It is not a question of number of organism, it is of Biomass, Biomass is the mass of any living matierial in a food chain. There is more biomass at the bottom level because lots of it is wasted as the chain progress - some leaves drop of and are not eaten, some insects are not eaten, the feathers and bones of the birds are not eaten. All of this means that some biomass is not passed on from one level to the next - some is wasted. Therefore, in orderto have enough to support the 5 foxes there needs to be a lot of biomass at the bottom. This is the tree. 1 organism, but very big, so los of biomass. If this whole food chain lived in an open field, the first level might be grass, in which case there is less biomass per organism and so the numbers of organisms would begreatest at the bottom. As it is, the numbers do not increase "like a pyramid". Biomass pyramids always do.