Ecology And Environment

Why the use of Ddt Harms Ospreys

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Back in the 1940s, DDT was considered a miracle chemical. For the first time, mankind seemed to be winning the war against malaria, and it seemed that even insect-caused crop failures would soon be a thing of the past. Paul Muller, the chemist who had discovered that DDT could be used as a cheap, effective insecticide that could eradicate large swarms of mosquitoes and other insects with no apparent toxicity to humans, was awarded the 1948 Nobel Prize in Medicine and Physiology.

Unfortunately, previous toxicity testing had been based only on the sprayed concentrations of DDT. Testers had not taken into account how DDT concentration would biomagnify as it traveled through the food chain. Even today, DDT apologists argue that if DDT had indeed been responsible for raptor population declines, the effects should have been noticeable immediately and not with a delay of several years.

However, the full effects of DDT biomagnification cannot show up immediately. It has to wait until a few years have passed and DDT fully saturates the food chain.

Biomagnification is what happens when a chemical that does not break down enters the food chain. Because it does not break down, concentrations at each level of the food chain keep increasing until it reaches a lethal level in one of the organisms involved. At that point, that tier of the food chain will break down.

Plants and plankton receive a basic dose of DDT when it first enters the environment. Plankton eaters receive a higher dose, accumulated from the amount of plankton they have to eat. Creatures which eat the plankton eaters receive a still higher accumulated dose, and so on. As a result, the highest concentrations of DDT will be found in apex predators.

The problem was first identified in the 1960s. By 1966, the rate of decline in the osprey population was estimated at 30% annually. After abnormal behaviors and fledging difficulties were ruled out, the problem was traced directly to a failure to hatch eggs. Osprey usually lay between 1 and 3 eggs per nesting. At times, fewer than a single nest in every 10 would hatch a healthy chick.

Ospreys are especially vulnerable to the effects of DDT biomagnification because the chemical structure of DDT affects calcium absorption. This did not greatly affect the adult bird, because raptor bones are naturally thin, hollow, and light. However, even raptor bones need some calcium if they are not to break.

In 1968, Joseph Hickey and Daniel Anderson found that the limited supply of calcium went to the bones at the expense of making strong eggshells. Without calcium, the eggshells thinned to the point where many of them broke in the nest before they could hatch.

DDT does break down slightly, into DDE. However, DDE is completely stable. It may even have a stronger effect upon eggshells than DDT.

Back in the 1960s and 1970s, the bioaccumulated DDT and DDE decimated the osprey population to the point where some doubted the species could survive. There is no question that urgent bans on the prolific use of DDT in the environment saved the osprey from what could have become complete extinction.

Today, the osprey is no longer a threatened species. The estimated world numbers are back to a healthy half million or so. Yet the problem has not completely gone away. Even today, raptor eggshells in some marine areas are still thinner than they had been before DDT was introduced.

More about this author: Michael Totten

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