Ecosystems are the basic functional units in ecology, which include the organisms that form communities, and their environment; freshwater lakes and deciduous forests are two types of ecosystem (E. P. Odum, 1959).
Comparisons of plant productivity
389( 2 ) Criteria and units Measurements used as evidence of the productivity of different communities may be published as standing crop, crop, yield, biomass, production or productivity, and are expressed according to a great variety of criteria and units. Such different entities cannot be compared directly and the problems of selecting conversion factors require detailed consideration.Results expressed in different units, such as lb./acre and kg./m.2, can be readily inter-converted by constants, but when different criteria are involved, such as dry weight and ash-free dry weight, conversion factors must be selected to suit the particular community.Conversions from volume have been used in preparing this article only for some forest and phytoplankton crops. They are particularly unreliable and are of limited importance. Volume is usually measured by displacement, which may be inaccurate when the plant contains collapsible air spaces or has adherent water (Westlake, 1961), or is calculated by mensuration, which is not easy for microscopic plants or for complicated shapes. Conversion factors (i.e. densities) for volume to fresh weight are very variable from species to species, usually depending on the proportion of air 25-2