Mathematically, the strength of this "regression" effect is dependent on whether or not all of the random variables are drawn from the same distribution, or if there are genuine differences in the underlying distributions for each random variable. Furthermore, when many random variables are sampled and the most extreme results are intentionally picked out, it refers to the fact that (in many cases) a second sampling of these picked-out variables will result in "less extreme" results, closer to the initial mean of all of the variables. This is known as “the 10 percent rule” and it limits the number of trophic levels an ecosystem can support.In statistics, regression toward the mean (also called reversion to the mean, and reversion to mediocrity) is a concept that refers to the fact that if one sample of a random variable is extreme, the next sampling of the same random variable is likely to be closer to its mean.
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On average, only about 10 percent of energy stored as biomass in a trophic level is passed from one level to the next. The waste and dead matter are broken down by decomposers and the nutrients are recycled into the soil to be taken up again by plants, but most of the energy is changed to heat during this process. At each level, some of the biomass consumed is excreted as waste, some energy is changed to heat (and therefore unavailable for consumption) during respiration, and some plants and animals die without being eaten (meaning their biomass is not passed on to the next consumer). Not all of the energy generated or consumed in one trophic level will be available to the organisms in the next higher trophic level. This happens at every step of the pyramid. The steps get smaller further up the pyramid because some of that energy is changed to a form that cannot be consumed by organism at the next higher step in the food chain. The width of each step represents the rate of energy flow through each trophic level. Each step of the pyramid represents a different trophic level, starting with primary producers at the bottom. To show the flow of energy through ecosystems, food chains are sometimes drawn as energy pyramids. In marine ecosystems, primary productivity is highest in shallow, nutrient rich waters, such as coral reefs and algal beds. In contrast, deserts have the lowest primary productivity. In terrestrial ecosystems, primary productivity is highest in warm, wet places with plenty of sunlight, like tropical forest regions. This is the portion that is available to be consumed by the primary consumers and passed up the food chain. Net primary productivity is what is left over after the primary producer has used the energy it needs for respiration. All the biomass generated by primary producers is called gross primary productivity. Both produce most the nutrients and energy needed to support the rest of the food chain in their respective ecosystems.
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In terrestrial ecosystems most primary producers are plants, and in marine ecosystems, most primary producers are phytoplankton.
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At the bottom of a food chain is always the primary producer. Each link in the chain represents a new trophic level, and the arrows show energy being passed along the chain. The easiest way to demonstrate this energy flow is with a food chain. Primary producers use energy from the sun to produce their own food in the form of glucose, and then primary producers are eaten by primary consumers who are in turn eaten by secondary consumers, and so on, so that energy flows from one trophic level, or level of the food chain, to the next. The primary source of energy for almost every ecosystem on Earth is the sun. Energy cannot be created from nothing, so it must be transferred through the ecosystem.
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Living things need energy to grow, breathe, reproduce, and move.