according to the law of conservation of matter, matter is never created or destroyed. Yet in a food pyramid the amount of biomass decreases as we move up through the pyramid, from producers to top level consumers. why?

According to the law of conservation of matter, matter is never created or destroyed but it can change from one form to another. Most of the biomass in a food web gets converted at each trophic level and it no longer remains apart of the food web. Most of the biomass is released into the atmosphere in the form of heat and hence cannot be transferred to the other trophic level. Also, matter is used up by the organism itself and converted into different forms. Most of the matter might get accumulated in the soil when the organisms of a trophic level die.