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The organisms in the ecosystem are linked through ecological interactions to form a food web, which has several trophic levels: producers (plants) at the lowest trophic level, primary consumers feeding on the producers, secondary producers feeding on the primary consumers and so on up to the apex predators that do not have any predators feeding on them, except for young stages in some species and some species where adults may be killed by humans. Because energy transfer from each trophic level to the next (e.g. the producers to the primary consumers) is not 100% efficient (as a general rule of thumb, the efficiency is only 10%) the biomass and production is highest in the lowest trophic levels and lowest at the highest trophic level. Organisms at different trophic levels influence each other through predation, whereas organisms at the same trophic level influences influence each other through competition – since they feed on the same food sources. Predation and competition are important biotic interactions that determine the dynamics of abundance and spatial distribution of the species in the ecosystem. Other kind of interspecies interactions, such as parasitism and mutualism, may also influence population dynamics, but these effects are less known and studied.