Measurement of the weight of biological material in an ecosystem can be hard to do. Dry weight is used to measure the biomass in an ecosystem. Let’s take a tree for example. To measure the biomass of the tree you would have to chop the tree down, deplete its water content (because the water content of organisms [even of the same species] varies), and then measure the weight of what is left. The size of the organism in question is also taken into account. This is the most unethical way to measure biomass, but it is one way to get the job done.