so less ddt leads to a more acceptable level of damage in the egg shells
its not a sum zero game
interesting
i didnt know that
I just winced so hard I almost broke the chair-back -- that use of "acceptable" in relation to DDT hits me as just... wrong!
As NickCole said, the stuff's persistent; it endures a long time (I hunted again and can't find how long; Wiki, inspite of a lengthy article, doesn't seem to have it). It doesn't metabolize; it's stored in fats in the body and IIRC nerve sheath tissue. So, like the wheels on the bus, it goes 'round and 'round, from one generation to the next in an ecosystem, continuing to deal death.
"Acceptable levels of damage" sounds so, well, military. I'd say, "reduced mortality rate", or maybe "species-survivable damage".
With a persistent toxin, there are two ways to have an "acceptable" level of use, i.e. one that lets a species survive. One is to know the half-life or persistence term of the substance, calculate how much the species can tolerate, and compute how much can be put into the environment without raising the presence above the threshold; the other is to have a removal vector.
Fortunately for the condors and eagles and their cousins, there turned out, in coastal areas especially, a very nice (for them) vector at work: when an individual dies, a substantial amount of the toxin gets eaten from it by thing which do not belong to the bird species' food chain. Ultimately, DDT ended up in aquatic critters that over time spread the stuff all over the planet -- solution by dilution (pun intended). Eventually, I learned in my second term of oceanography at OSU, the planet itself will recycle the stuff (if it doesn't break down first), because it's showing up in the sea floor as fish carrying it die -- as as we all know, the sea floors are ever-so-slowly diving into oblivion under other crustal plates.
Of course that's a bit tongue-in-cheek; DDT doesn't last tens of millions of years (well, lying on the sea bottom, it just might; that's a pretty bland environment), but it makes a point: be afraid, be very afraid.
From a course in Human Ecology I also picked up that every person in the U.S., as of about 1980, had DDT in the fatty tissues. (Be more afraid.)
Something I didn't mention earlier about its dealiness: the stupid molecule is one that, for some reason, insects can grow tolerant of
fast. Althoug not banned in the tropics, DDT is reserved mostly for severe outbraks, because it was discovered in the 70s that mosquitoes were becoming tolerant of it within just a half dozen generations -- but when its use is tabled for a few years, they begin losing that tolerance to an extent. The result is that the longer and more DDT is used, the less useful it becomes -- but remains just as deadly to vertebrates, especially egg-laying ones.
I have reason to appreciate the ban often. I put many hours into a conservation/safety project overlooking the beach here on the northern Oregon Coast. The first year of the project, a nearby resident pointed out that an eagle had been hanging around. The next year, the eagle built itself a nest... and showed up with a friend. The third year we learned that the "friend" was a mate, because shortly after the nest was completed, momma eagle began staying there 24/7 (or at least one eagle did; I could never tell them apart). The little eagles arrived, and by now, the seventh year of the project, the entire mountain/cape area has three eagle couples -- and from time to time they all come soaring down over the project area.
DDT may be useful, but like the man in the film
The Candidate says, "There's got to be a better way!"
