re (1) In terrestrial ecosystems, the sun's energy is captured by primary producers (plants). That energy is used for plant metabolism, reproduction, and growt

re (1) In terrestrial ecosystems, the sun's energy is captured by primary producers (plants). That energy is used for plant metabolism, reproduction, and growth (building tissues, storing energy, etc.). Net primary productivity (NPP) of an ecosystem is the the difference between what energy primary producers capture from the sun and onvert into sugars during photosynthesis, and the energy loss to maintenance and metabolism. It is principally measured as the accumulated biomass. Importantly, NPP is an upper bound, since the primary, if not only way, for all other organisms to obtain their energy is to consume plants (herbivores), or consume those who consume plants (carnivores). NPP is different between terrestrial ecosystems. Deserts have the lowest. Tropical rainforests the highest (excluding marshes, etc.). Temperate forests are quite high, and much higher than grasslands or farmlands. e.g. 9000 kilocalories/square meter/year for rain forests, 6000 for temperate forests, about 2000 for farmland. Now what most modern farming does do, is bring in energy from elsehwere. For example, cattle might be fed corn brought it from elsewhere. Not to mention the energy inputs from fossil fuels (fertilizers, tractors, etc.) (2) You are right that comparisons in terms of biomass imply fungibility and it is not always appropriate. But it is also true that individual species often simply fill niches that arise as a property of the system (the system of energy inputs and limiting factors). How else would you compare, say, a forest in Washington with a forest in northern Europe? different species, and all that. But the niches are largely the same.