Why does a pollutant end up multiplied as it travels up the food chain from producers to consumers?

When there is a gradual increase in concentration toxins(pesticides) in the tissues of organisms progressively in the food chain,the term for describing this is called -bio magnification.

The progressive multiplication in the concentration of these toxins may be due to non-biodegradable nature of the toxins,which are absorbed into the tissues of mostly the producers from the environment faster than they could metabolized by the body.

.Therefore these could not be removed by the producers, during excretion and respiration,and hence build up progressively upwards in the consumers.

A typical example is the phytoplankton which absorb toxins from sea water and accumulate this in the body.The build could not be ,metabolize by the body system,and is transferred to the zooplankton along the food chains when they feed on the phytoplanktons- this is Bio magnification.