Brian Dunning’s Skeptoid does an excellent job of debunking pseudoscience, so his podcast on DDT is profoundly disappointing. Dunning claims that DDT use did not have a large impact on bird populations, that elitist environmental groups were killing brown children by blocking DDT use and that DDT is effective even if mosquitoes are resistant. None of these claims are true, as I will detail in this post. But first, why did a sensible fellow like Dunning get it all so badly wrong? Well, his primary source for information about DDT was Steve Milloy‘s junkscience.com. One commenter remonstrated:
I trust JunkScience on health and environment like I trust the Discovery Institute on evolutionary biology, or Prison Planet on history.
The JunkScience guy has a blatant libertarian agenda. The SourceWatch guy has a blatant anticorporate agenda. Big whoop! They’re both still researchers. I have no problem citing either if they’ve done the research I’m looking for.
If you must insist that this makes everything coming from either guy always right or always wrong, then you should demand to see the voting history of every scientist or researcher in order to determine the quality of their work.
This misses the point. Biased sources aren’t necessarily wrong, but they are unlikely to give you the whole picture and it would be wise to be skeptical about what they say and fact check their claims. This post is the fact check that Dunning failed to do.
Silent Spring’s principal thesis was that DDT harms bird populations through eggshell thinning.
Silent Spring doesn’t say anything at all about eggshell thinning since it wasn’t discovered until after the book was published.
It’s been about five decades since Silent Spring was published, and we’ve learned a lot in those years. One thing we’ve learned is that DDT is only one of many causes of eggshell thinning. Other culprits include lead and mercury toxicity, oil, phosphorus and calcium deficiency, and dehydration. Perhaps most significantly, birds in captivity in order to undergo testing are under stress, and this stress alone is enough to produce eggshell thinning. Although DDT’s mechanism for eggshell thinning is plausible, many studies throughout the 1970’s and 1980’s failed to correlate such thinning with high levels of DDT, even extremely high levels. Other studies have confirmed Rachel Carson’s findings. My own conclusion based on a review is that there probably is a correlation, but it’s not a strong one; and at best it’s only one of many causes. Whether DDT is used or not would probably not have a large impact on bird populations.
This is based on points 39-64 in Milloy’s 100 things you should know about DDT, but Dunning goes beyond Milloy’s claims with the unsupported assertion that the stress of captivity causes thinning. Dunning’s “failed to correlate such thinning with high levels of DDT” corresponds to Milloy’s points 43 and 44. These all cite obscure reports and journals so we can’t check them. Except for this one:
44 Among brown pelican egg shells examined there was no correlation between DDT residue and shell thickness.
[Switzer, B. 1972. Consolidated EPA hearings, Transcript pp. 8212-8336; and Hazeltine, WE. 1972. Why pelican eggshells are thin. Nature 239: 410-412]
Ah, Nature. That we can check. Here’s what Hazeltine’s letter says about that correlation:
“The CFDG data (Table 1) show a nearly perfect correlation of lipid DDE residues to shell thicknesses, and the relationship is positive.”
OK, that’s not exactly what Milloy had, but the point is that the correlation goes the wrong way. Trouble is, Milloy fails to mention that there were four responses to Hazeltine published in Nature. Let me summarize some of the problems with Hazeltine that these letters pointed out. First, Hazeltine’s CFDG data comprised just nine eggs. Those eggs mere a mixture of incubated and non-incubated, and the positive correlation is caused because incubated eggs have thicker shells and higher DDE concentration in the lipid. How? Well, incubation consumes most of the lipid and concentrates DDE in the remaining lipid. And thin eggshells are less likely to survive incubation (that’s the reason why eggshell thinning is a problem in the first place.) If you look at the relationship between whole egg DDE and eggshell thickness there is no statistically significant relationship in Hazeltine’s set of just nine eggs. But other studies with larger samples have found a significant negative relationship between DDE and brown pelican eggshell thickness. Unlike Milloy, Hazeltine cites them in his paper and states:
That DDE is the cause is of thin brown pelican or peregrine eggs is well established in the … scientific literature.”
Hazeltine tried and failed to overturn this. Milloy misrepresented the science by deliberately concealing the existence of the studies that found that there was a correlation between eggshell thinning and DDE.
Now let us examine Dunning’s next paragraph:
But despite the likelihood that it would have some impact, it’s now known that the species Rachel Carson focused on (most notably bald eagles) were already in massive decline from unrelated pressures even before DDT’s introduction. Habitat loss and hunting had been, by far, the greater causes of bald eagle deaths. Hunting had reduced the populations to just a few hundred nesting pairs in the mountains, and lowland eagles were already gone from habitat loss. Rachel Carson did not ignore these issues in her book, but the popular perception that banning DDT was all that was needed to magically restore bald eagle populations was naÃ¯ve. In the end, it was the Bald Eagle Protection Act and the bird’s 1967 placement on the endangered species list, combined with increased penalties for poaching, that ultimately led to the bald eagle’s successful return to remaining habitats.
One of Dunning’s five references is to a 2007 news story in Science Can the Bald Eagle Still Soar After It Is Delisted?, which states
Just 40 years ago, the bald eagle seemed headed for extinction in the conterminous United States. Nesting females were accidentally crushing their eggs, which were weakened by the ubiquitous insecticide DDT. … Banning DDT has helped the national population of breeding bald eagles to grow.
Looking just in Science there is also a 1994 story:
[Wildlife ecologist Stanley Temple of the University of Wisconsin] says that the return of the eagles and other raptors, including peregrine falcons and and ospreys, “is due almost wholly to the ban on chlorinated hydrocarbon pesticides.” The ban of the number one offender, DDT, took effect in 1972, eliminating a group of pesticides that thinned eggshells, causing the birds’ reproductive rates to plummet.
And a 1982 study:
Reproduction of bald eagles in northwestern Ontario declined from 1.26 young per breeding area in 1966 to a low of 0.46 in 1974 and then increased to 1.12 in 1981. Residues of DDE in addled eggs showed a significant inverse relation, confirming the effects of this toxicant on bald eagle reproduction at the population level and the effectiveness of the ban on DDT. The recovery from DDE contamination in bald eagles appears to be occurring much more rapidly than predicted.
Now it is certainly true that shooting and habitat loss also contributed to the decline of bald eagles as the US FWS states:
By 1963, with only 487 nesting pairs of bald eagles remaining, the species was in danger of extinction. Loss of habitat, shooting, and DDT poisoning contributed to the near demise of our national symbol.
As the dangers of DDT became known, in large part due to the 1962 publication of Rachel Carson’s book Silent Spring, the Environmental Protection Agency took the historic and, at the time, controversial step of banning the use of DDT in the United States. That was in 1972, and it was the first step on the road to recovery for the bald eagle.
But Dunning is wrong to dismiss DDT as a cause — preservation of habitat and bans on hunting and DDT were all necessary to save the bald eagle.
On to Dunning’s next paragraph:
Brown pelicans are another species often cited as having been decimated by DDT use in the United States, along the Gulf coast and in California. Massive declines were indeed correlated with DDT use, but it may have been a coincidence in each case. Along the Gulf coast, hunting by angry fishermen had reduced the pelican population in Texas from 5,000 annual births to just 200 in 1941. The California populations suffered a double whammy in the years following Silent Spring’s publication; first with an oil spill off Santa Barbara in 1969, and then with an outbreak of Newcastle Disease in 1971 that unfortunately required the culling of millions of brown and white pelicans. DDT certainly didn’t help; but it was another case where the bird populations would have dropped sharply whether DDT was in the picture or not.
Compare with Milloy:
96 An epidemic of Newcastle disease resulted in millions of birds put to death to eradicate the disease.
[United Press International. "Newcastle disease epidemic in California (April 1972)] The epidemic among U.S. birds was caused by the migration of sick pelicans along the Mexican coast.
Note that Milloy doesn’t actually say that the birds put to death were pelicans — he just implies that they were and Dunning didn’t stop to think whether it even made sense that an endangered species would have a population in the millions.
Now look at what the USDA says
For example, parrots from South America are believed to have caused an outbreak of exotic Newcastle disease (END) in southern California in the early 1970s. Eradicating that disease outbreak cost $56 million over a 3-year period. During that outbreak, more than 12 million birds were destroyed, the vast majority of which were commercial poultry.
In 1992, 26,000 turkeys in North Dakota were destroyed after APHIS diagnosed virulent Newcastle disease in the flock. Cormorants were the suspected source. Hundreds had died from the disease at a lake near where the turkeys were being reared on range. Seal confirmed this suspicion when he compared the genomes of turkey and cormorant isolates.
The episode was part of the United States’ first known Newcastle-related die-off of free-ranging wild birds.
So the number of pelicans dead as a result of the 1971 outbreak of Newcastle Disease was not “millions” as Dunning claimed, but zero.
As for the Dunning’s claim that the pelicans’ troubles were coincidental, this interview Professor Daniel W. Anderson of the University of California at Davis, summarizes the evidence:
DA: Dr. Ray is a little slippery when she says there is no supportive evidence of population declines and that bird populations actually increased. There are many cases, but let me mention one that I personally worked on. She does not mention one of the best case histories in North America of DDE-induced population effect which involves the decline and resurgence of the California brown pelican.
ER: California brown pelican populations were hurt by DDT?
DA: By DDE. But in this case, they were adversely affected not by DDT used as a spray but as DDT and metabolites were put into the sewage system in Los Angeles. It entered the Southern California Bight in large quantities. [A bight is a curve in the coast that forms a bay and also often entrains an oceanographic gyre. ed.] Some people in Professor Risebrough’s group looked at the sediments in Southern California and saw where DDT and its metabolites first appeared.
ER: When did DDE start showing up in the sediments?
DA: About the 1950s. It was like a detective case and when the source was discovered, input stopped pretty quickly.
ER: What was the source?
DA: The major source was found to be a manufacturing plant in Torrance.
ER: When did they make the changeover to landfill disposal?
DA: It started in about 1970. And within a year the DDE residues began a long decline in the pelicans and other wildlife of the bight and the pelicans (and other species too) started to show signs of recovery.
ER: Was there experimental work with the pelicans that showed eggshell thinning after exposure to DDE?
DA: No experimental work like controlled feeding experiments was done in brown pelicans because that species was on the endangered species list. But there was a positive relationship between the amount of DDE residue female pelicans had in their bodies at the time they were laying eggs and the amount of eggshell thinning. Similar relationships have been shown over and over again, all over the world, for many bird species and by many, many investigators.
For more details and references to scientific papers on brown pelicans
see the US FWS’s California Brown Pelican Recovery Plan.
As Daniel Anderson notes in the interview above, there were no
controlled feeding experiments on brown pelicans so you could, I
suppose, argue that the correlation between thin eggshells and DDE,
and the collapse and recovery of pelican breeding associated with the
increase and decrease in DDT use was all a big coincidence. But a
1975 paper by Jeffrey Lincer
would seem to rule that out.
Here’s Figure 3 from Lincer’s paper. The x’s show that in controlled experiments the more DDE fed to kestrels the thinner the eggshells. And it shows that the same relationship occurred in wild kestrels, so it is not true that dosages used in the experiments were unrealistic.
For more on DDT and eggshell thinning see this WHO report written by expert toxicologists.
This post is more than long enough already and we are only half way through the Skeptoid podcast, so the rest of the fact check will be in another post. Part 2 will fact check what Dunning says about DDT and malaria.