Why does biomass decrease up the trophic levels?

1) some biomass is lost in faeces

herbivores cannot digest all of the plant material they eat, since they do not have the enzymes, e.g. to digest cellulose.
this means that large amounts of plant biomass cannot be broken down and absorbed into the body.
undigested material is passed out of the body as faeces.

carnivores produce less waste, since they mainly eat meat. however, parts such as bones or teeth are difficult, or impossible, to digest.
these parts are also lost as faeces.

2) some is lost in urine

if an animal eats more protein than it needs, the excess is broken down into amino acids. when excess amino acids are deaminated and converted into urea, they are passed out of the body, along with excess water taken in as part of biomass, as urine.

3) some is lost in respiration

glucose, part of the biomass taken in by animals, is used for respiration.
all cells require respiration for energy release, and in animals, both aerobic and anaerobic respiration require glucose.
respiration supplies energy needed for all the life processes in the body.

4) some is lost in movement

movement is one of the seven life processes, and requires a large amount of energy. active, contracting muscles require a lot of respiration.
the more an animal moves, the more biomass from food is used in respiration.

5) some is lost in keeping constant body temperature

breakdown of biomass in respiration transfer energy to the surroundings, making them warmer.
biomass in respiration can be used to keep bodies at a constant temperature regardless of surroundings.
in particular, mammals and birds use biomass for this purpose.