BioMagnification
Chemicals that humans release
into the environment quickly make their way into the bodies of various
animals. Many of these chemicals lodge
in the fat tissue of the animals and are not excreted from the body. The chemicals have a relatively low
concentration in zooplankton and other low-level consumers, but the chemicals
quickly accumulate in top predators in the ecosystem. For instance, various raptors suffered the
effects of DDT during the 1960's. DDT
caused various problems for the birds especially by causing thin egg shells,
which lead to a reproductive decline of the species.
Chemicals accumulate in top
predators as a result of biomagnification. Put simply, if I eat a chicken, I also eat
all of the chemicals that the chicken ate.
So if my diet is entirely chickens, I eat seven times as many chemicals
in a week than a chicken would have eaten up to that point in their life. Similarly, if an osprey relies on fish as a
major part of its diet, than it consumes all of the chemicals stored in the
bodies of those fish after they have spent a lifetime building up chemical
residue.
Demonstrate biomagnification by spreading a variety of milk jug lids
around a grassy area. Pick six
participants to serve as predators. The
rest of the group represents zooplankton and must gather the milk jug lids as
their food. After all
of the campers have gathered some lids, send three campers into the field as
small fish. Instruct small fish
to capture two or three zooplankton, and take their milk caps. Then, send two campers, representing large
fish, into the field to catch the small fish.
Finally, the last camper represents the osprey and eats both of the
large fish, inheriting their store of milk caps.
After the demonstration, enter
the situation and look at the hawk's store of lids. Inform the campers that all of the green lids
represent DDT, a harmful chemical that causes thin egg shells and thus poor
reproduction rates because of diminished
chances that newly-laid eggs will survive the mother's weight. Walk through each stage of the food pyramid,
asking the campers how many green caps they had at each level. Help the campers to see that gradually, lots
of DDT becomes embedded in the hawk from throughout the entire ecosystem. Discuss the problems of DDT and what was done
to solve that problem. Also mention the
current problem with PCBs and the problem they lead to in killing fish and
causing reproductive problems in birds.
The chemical's effect is so potent that even a small amount is
detectable, such as through nest building irregularities among Tree Swallows.