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Let us consider a forest ecosystem

suppose a pesticide P is sprayed on grass to kill mosquitoes. When the grasshopper and other herbivorous animals eat the grass , the concentration of P increases by the no. of grasses eaten by the grasshopper. if the amount of P in 1 grass is x and the grasshopper eats 20 grasses then the amount of P in the grasshopper will be 20x. Now, if 1 rabbit eats up 15 grasshoppers in one day then the amount of pesticide in 1 rabbit will be 20*15x = 300x. Thus, the concentration of P in an animal belonging to a higher trophic level will be in a critical level. Therefore, P which did not kill an organism in a lower trophic level can kill organisms in higher trophic levels. This phenomenon is termed as biomagnification
The pesticides that get into the animal system does not get digested and degraded, rather it stays within the animal cells and gets passed on to the members of the next trophic level. for example, if a rabbit feeds on a plant sprayed with a pesticide, it can not degrade the pesticide by digesting. rather it remains within the system and when the member from the next level feeds on it, it feeds on many rabbits as a result of which the amount of pesticides increases with the increase in trophic levels.