Chapter 5 - 6

06 Environmental Science

listen to a lecture in an environmental science class. fill in the diagram with the information that you hear.

W: Let’s go back in time a few years to the 1940s.

The world is at war… yeah, World War II is being fought.

Let’s say you’re the leader of a country that has men fighting in hot, tropical areas.

But here’s the problem−not only are your men fighting the enemy, they’re also fighting mosquitoes.

Yep, lots of your men are dying from malaria, a disease caused by the bite of a mosquito.

So you look around and find this stuff… DDT, which was actually made many years earlier.

But you’re smart, so you figure out that the substance, DDT, also does a great job of killing mosquitoes, it’s a pesticide.

You spray the staff in tropical areas, right where your troops are… and yeah, the mosquitoes die.

This is good, but little do you know that the pesticide DDT also has some very significant drawbacks.

DDT’s problems came to light pretty quickly.

Though scientists knew DDT killed mosquitoes, they hadn’t realized that the mosquitoes would be able to quickly adapt to the toxin through natural selection.

Over time, mosquitoes quite simply became resistant to the pesticide, and the parent mosquitoes passed on this resistance to their offspring.

You could spray the stuff on ‘em, and why, they’d drink it up and laugh at you!

So the number of cases of malaria rose again… um, really, really fast.

And mosquitoes reproduce fast, so the entire population could acquire resistance very quickly.

Pretty soon, almost all the mosquitoes were resistant to DDT.

Another downside is DDT’s effects on the environment: plants and animals.

In terms of animals, we’ll talk about birds.

Many birds were exposed to the pesticide in the process of spraying for mosquitoes, and it severely weakened their ability to reproduce.

Furthermore, many of the birds ate the mosquitoes with high DDT levels in their bodies.

The DDT caused the eggshell of the birds’ offspring to be thinner than normal, which failed to protect the baby bird until it could survive outside the egg.

This was a huge problem… you know the bald eagle of North America?

Well, it just about became extinct because of this.

There’s something else to consider.

Scientists initially figured that DDT couldn’t pose any real threat to the environment because it’s not water soluble, but they didn’t think about the fact that it can be carried in fats and oils.

So it is still possible for it to be transported to other areas.

So, yes, it did end up harming the environment.

There’s third negative effect of DDT that we need to discuss before moving on, and well, actually, I just mentioned it in passing.

Yeah, you see this pesticide causes something called biomagnification.

Basically, what this means is that the poison build up in the animals, and the higher up on the food chain in organism is, the higher the level of DDT it takes in.

Over time, the level of DDT builds up, OK. Hmm… let me explain: if an insect has been DDT in its body, and then a bird eats a lot of such insects, by the time it gets to the top of the food chain, there’s a whole bunch of DDT inside the animals.

It accumulates is what I’m saying.

This meant that the entire food chain was affected by the negative ramifications of this pesticide − all the way up to humans.

Now, despite all the many drawbacks of DDT, it did have a few strong points.

Like I said, it helped reduce the incidence of malaria.

Granted, in tropical areas, the number of cases rose again, but they still weren’t as high as they were initially.

And in countries that aren’t tropical, it drastically reduce the number of cases.

So, yes, DDT is absolutely ineffective pesticide.

There’s really no doubting that. It also had a positive result in that it limited to cases of yellow fever, which is another disease transmitted by mosquitoes.

If it weren’t for DDT, these diseases might still be causing major problems in countries around the world.

Now, the cases are much more contained than they were, although they are still a big problem in some parts of the world.

OK. Uh, in most of the developed nations, DDT isn’t used anymore, but a lot of countries are still using it for various purposes.

So the controversy over DDT is still a major issue.

However, if you look at the negative points in the positive points, my opinion is that the negative points easily outweigh the positive ones.

1) How does the professor organize the lecture?

2) Why does the professor mention WWII?

3) Why does the professor talk about biomagnification?