Ecosystem structure and functioning is the focus of much ecological research because many ecosystem properties such as production, energy flow, nutrient cycles, and stability lie at the core of understanding ecological processes. Net primary production (NPP) is primarily influenced by climate and nutrients. On a global scale, NPP in terrestrial biomes tends to be greatest near the tropics, where the combination of constant and moderately high temperature and adequate rainfall promote plant growth. NPP in marine biomes peaks at about 40° S latitude, which is associated with large areas of upwelling and high nutrient availability. On a regional and local scale, the availability of nutrients such as nitrogen and phosphorus influence terrestrial, marine, and freshwater production. Ecosystem structure is based on the interactions between producers, consumers, detritivores, and decomposers. A substantial but variable amount of energy is lost with each transfer from one trophic level to the level above, which has the effect of limiting food chain length (FCL). In some aquatic systems, longer food chains are associated with CO2 export from the water into the atmosphere, and with the biomagnification of toxic substances.