Just can not grasp it
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Organisms at the top of food chains eat more of the organisms along the food chain. Using the following food chain as an example, it can be easily explained:
Plankton -> Small fish -> Bigger fish -> Seagull
A small fish eats a lot of plankton to survive, only to be eaten by the bigger fish. In turn, the bigger fish eat a lot of small fish to survive. Then seagulls eat the bigger fish to survive.
If you introduce a bioaccumulating toxin, such as the pesticide DDT, the toxin will enter the plankton first. The small fish eat a lot of plankton, which transfers the toxin to the small fish. The small fish get eaten by the bigger fish, which increases the amount of toxin transferred to the bigger fish because the bigger fish eat a lot of small fish. Finally, when you get to the top of the food chain with the seagulls, they eat a lot of big fish, which transfers a lot more toxin to them.
Plankton -> Small fish -> Bigger fish -> Seagull
A small fish eats a lot of plankton to survive, only to be eaten by the bigger fish. In turn, the bigger fish eat a lot of small fish to survive. Then seagulls eat the bigger fish to survive.
If you introduce a bioaccumulating toxin, such as the pesticide DDT, the toxin will enter the plankton first. The small fish eat a lot of plankton, which transfers the toxin to the small fish. The small fish get eaten by the bigger fish, which increases the amount of toxin transferred to the bigger fish because the bigger fish eat a lot of small fish. Finally, when you get to the top of the food chain with the seagulls, they eat a lot of big fish, which transfers a lot more toxin to them.