Mad Kally

Me too elwood..

[ November 27, 2002: Message edited by: Mad Kally ]</p>

The Lone Ranger

One big reason is because of the phenomenon of bioaccumulation. DDT (and its breakdown product, DDE) is fat-soluble, which means that it tends to become incorporated into the tissues of organisms that ingest it.

Typically, about 10% of the food that an animal eats is actually converted into tissue -- actually, it's generally much less than that, especially for warm-blooded animals. The remaining 90-plus percent is "lost" as heat, bodily wastes, etc.

So, in order to produce, say 100 pounds of herbivore, you need something like 1,000 pounds of plant matter for them to eat. Furthermore, those 100 pounds of herbivores will support only 10 pounds or so of carnivore. Each step up the food chain, 90% or so of the available energy is lost.

***

Let's take a hypothetical chemical that can be stored in body tissues, as DDT/DDE are, and assume that it's lethal at a concentration of 1,000 ppm.

Okay, this stuff finds its way into a lake, where it is dissolved into the water with a concentration of 0.1 ppm. That means the phytoplankton in the water will probably also have a 0.1 ppm concentration in their cells. No big deal.

The zooplankton that eat the phytoplankton will have a concentration of about 1 ppm in their tissues, since it takes about 10 pounds of phytoplankton to make one pound of zooplankton. This means the chemical in question is much more concentrated in the body tissues of the zooplankton.

The small fish that eat the zooplankton will have a concentration of about 10 ppm in their tissues.

The big fish that eat the small fish will have a concentration of about 100 ppm in their tissues.

The ospreys that eat the big fish will have a concentration of about 1,000 ppm. Oops. Bad news for the ospreys.

***

Chemicals that can bioaccumulate like DDT are much more dangerous to animals high in the food chain than they are to animals that are lower in the food chain.

Cheers,

Michael

Ut

Are you sure of that? I would have thought the resistance of the population would have stayed constant while DDT wasn't in use.

If we find that 10% of pests are now resistant to DDT, and that we now stop using DDT and use agent X instead, we should find that the proportion of pests resistant to DDT stays constant, unless selecting for pests resistant to agent X implies selecting against pests resistant to DDT.

elwoodblues

As I understand things, that last phrase is generally correct, at least to some extent. Your math is correct; it just doesn't mirror the mechanism you're trying to describe.

Coragyps

At least some of the pesticide resistances that are heritable in insects give them some resistance to several classes of pesticides - sort of reminiscent of "superbug" bacteria that don't care which antibiotic you throw at them. Either Science or Nature had a review on this in a recent issue focussing on malaria - I'm painting my room right now and don't know which pile that issue is buried in, though.