In an ecosystem, food webs exist and all the components of a food web are connected to each other. Nutrients cycle through the various trophic levels, from producers to consumers (both primary and secondary) and detrivores (organisms who feed on dead or decaying matter, etc.). If one level is affected, so are all the other levels. For example, in an undisturbed food web, fishes consume smaller organisms and phytoplankton and obtain nutrients from them. These fishes are eaten by larger organisms and nutrients reach those larger carnivores. When these larger carnivores die, detrivores return the nutrients back to the environment. However, over fishing will remove the fishes from this food web and they will be used for food and oil (among other products) by human beings. The fishes will no longer be available to eat phytoplankton and zooplankton and the nutrient uptake would simply stop, larger carnivores being unable to feed on phytoplanktons and smaller zooplankton. This will prevent the flow of nutrients through the food web (and ecosystem), thus disturbing nutrient cycles.
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