Magnification of Toxicants: Persistence, Bioaccumulation & Biological Video

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  • 0:06 A Warning Sign
  • 2:28 Toxicant Build-Up
  • 4:34 Silent Spring
  • 5:59 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this lesson, you will learn about the persistence and magnification of toxicants in the environment. You will also learn about how these toxicants affect organisms as they move up the food chain.

A Warning Sign

From the 1950s to the 1970s, populations of birds of prey in North America fell at an astounding rate. The peregrine falcon was almost completely wiped out from the eastern U.S., the brown pelican disappeared from the Atlantic coast, and the bald eagle (our national bird) was mostly eliminated from the entire lower 48 states. Other large predatory birds, such as osprey and hawks, also declined substantially during this time.

The cause of this alarming situation was a highly toxic pesticide called DDT. At the time, DDT had not been tested for safety and was being sprayed indiscriminately throughout residential neighborhoods and other public areas as mosquito control.

When DDT accumulates in the tissues of birds of prey, it affects their ability to lay normal eggs with thick shells. When the female birds went to sit on and incubate their eggs, the shells were so soft that they broke underneath the mother's weight. Because they were not able to produce viable offspring, the populations declined sharply during the time of heavy DDT use.

But, the birds of prey were not in direct contact with DDT, so how did it get into their tissues and cause the thin egg shells? This was due to the persistence and magnification of toxicants in the environment. These are toxicants that linger in the environment for long periods of time and can even build up in animal tissue.

Persistence of Toxicants

Toxicants break down at different rates, and the ones that are persistent in the environment are the most dangerous. Persistent toxicants are those that resist breaking down in the presence of environmental factors, like sunlight, temperature, and moisture.

Persistent toxicants can be natural or human-made and are found in all sorts of things, from cosmetics to plastics to household cleaners to pesticides and more.

Think about it this way. You wouldn't want your patio furniture to degrade just from sitting out in the sun because there would be nothing left after a few days! The materials in your lawn chair are specifically designed to withstand environmental conditions. The problem is that when you no longer want that chair, it doesn't magically start breaking down.

The substances in the plastic and fabric materials that made it stand up to the sun and weather will keep standing up to those factors for a long time to come, even after you throw it away.

Toxicants Build Up in Tissues

Some persistent toxicants are especially dangerous because they not only persist in the environment, but also build up in animal tissue as they are inhaled or ingested. This process is called bioaccumulation. DDT is one such toxicant as it is absorbed and stored by fatty tissue. Other toxicants may build up in muscle or bone.

Toxicants that bioaccumulate are easily transferred to other organisms as they move up the food chain. When one organisms eats another, it eats all of the toxicants that are stored in the muscle, fat, and bone of its prey. The predator then takes these toxicants into its own tissues, as well as those of all the other prey items it eats. So, with each step up the food chain, we get biomagnification, the magnification of toxicant concentrations.

How Bioaccumulation Works

Let's look at how this works using the birds of prey in North America. DDT is sprayed liberally and gets into the water supply either through runoff or through seeping into groundwater. The DDT in the water is then taken up by algae, which are eaten by tiny organisms in the water called zooplankton. Small fish eat millions of these tiny zooplankton and are therefore eating a higher concentration of stored DDT. Larger fish eat many smaller fish, also increasing the amount of DDT that gets stored in their bodies. Finally, a large predatory bird (such as our bald eagle) comes along and eats the large fish, which ate the smaller fish, which ate the zooplankton, which ate the algae, which initially absorbed the DDT.

So, you can see that even though the birds of prey were not in direct contact with DDT, they ingested a large amount of it because they didn't eat just that large fish - they ate everything that came before that large fish through the food chain! The soft eggs of the falcons, eagles, osprey, and other birds of prey were therefore the result of biomagnification because the concentration of DDT increased as it moved up the food chain.

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