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Hi Chris, Dave: [buyer beware -- this contains some numbers but no locations]
Putting off marking some student reports .... Chris' comments seem
reasonable, but I share some of Dave's general misgivings in his
earlier post about the quality of the some parts of the swift guano
article.
Where does the '2.2 cm per year' guano accumulation that they
estimate, come from? The deposit is said to be 2 m deep ('2' = one
significant figure -- a guess is that it is not uniformly flat on top
so that this may be an estimate of only the edge of the part
excavated). There are three guano horizons available in the main
paper, [1] the top (0 cm, sealed 1992), [2] the cesium spike (70 cm
down, supposedly 1963) and [3] the bottom (200 cm down, opened 1928).
As Dave said earlier, none of these give their '2.2 cm/year' answer:
[1] & [2] give 2.41, [2] & [3] give 3.82, while [1] & [3] give 3.17
cm/year. Another estimate is possible, buried in their supplementary
Figure S1, where an estimated 100 cm were sampled from ~1944-1992,
which gives 2.08 (different analyses start at different years and I
couldn't find it stated when Fig S1 started, and its X axis is too
sparsely labelled to make this out accurately, but it says '48 years'
in the Abstract, and 100cm/48yr = 2.08).
In the only data Figure (Fig. 2a-d) of the main paper, the last three
lines of the legend are rendered unintelligible by some sort of text
transposition. Didn't any of the 10 authors notice this to correct
it? (Did most even read it?)
Do these peculiarities affect the paper's conclusions? It probably
doesn't really matter if the depth per year was at the extremes of 3.8
or 2.1 cm/yr, but it does shake confidence in the reader: if they
can't even get such simple arithmetic across clearly, how
careful/sloppy were they about the other measurements?
Two other issues could use airing. Two meters of solid guano must
press down heavily, so you might expect that the layers near the
bottom would become compacted relative to those at the top, over the
years. The numbers above (2.41 for the top part versus 3.82 cm/year at
the bottom) suggest the opposite. How could that be,
mis-identification of the Cs spike, bad sampling, or what? An obvious
need is for a tree-ring type analysis to really identify the years,
perhaps looking for annual pollen spikes. Their sampling method is
too crude to reveal this, but driving a vertical geology-type core
down an undisturbed part of the deposit should work, and one of the
authors lives in a geology department. This issue of possible
compaction or reworking is not even mentioned in the paper.
Second and perhaps more ominous, no metabolite DDE measurements are
reported in the paper from before the mid-1940s. On the face of it,
this may seem sensible conservation of effort because DDT was first
detected in the wild in 1939 (they say somewhere). On the other hand,
the article's reviewers should have absolutely required 2-3
measurements from the 1929-1938 interval. If any substantial DDE had
been discovered at these depths before 1939, it would mean that DDE
had been eluted and washed down from the levels above. If such eluted
amounts were substantial, this could compromise the basis for the
entire depth analysis. Why didn't they look at this? Reviewers
asleep at the switch?
On Chris' point 2, '...indeed there was a slight (and statistically
significant) rise until 1992 (when the chimney in this study was
capped)...' The supplementary material discusses with some
sophistication the curve fitting, which seems to involve fitting
linear functions to chunks of data then combining these in the stats
program 'R'. This produced a best fitting overall line which does
indeed rise a bit from 1970-1992, but that's not the same as saying
that it's a statistically-significantly better fit than (say) a
horizontal flat line, 1970-1992. There are only 4 points in that time
zone in Fig. 2d, and these 4 don't exactly hug the fitted line.
Perhaps a longer less hasty paper will tell it all more clearly.
Steve (Halifax)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Quoting Christopher Majka <c.majka@ns.sympatico.ca>:
> Hi DAve,
> On 20-Apr-12, at 9:09 PM, David & Alison Webster wrote:
>
>> Hi Again, Apr 20, 2012
>> In my earlier post I forgot the mention that the commentary in
>> the top URL has some significant misinformation i.e. "Pesticides
>> get more concentrated as they move up the food chain..."; a half
>> truth and therefore a half falsehood.
>
> I don't think erecting this straw man is warranted. If you read the
> study itself (in the Proceedings of the Royal Society B) or the
> popular exposition of it in Science Now (URLs of both below) you
> will see that the study is confined to the effects of DDT and its
> metabolites, which as an organochlorine, does bioaccumulate in
> lipids. Thus its clear in both contexts that the use of the word
> "pesticide" is strictly in relation to DDT and its metabolites. So,
> the context makes clear: no half-truths or half-falsehoods.
>
>> Lipid-soluble pesticides are prone to bioaccumulation because
>> they partition into lipids and are therefore less exposed to
>> metabolic breakdown but even lipid-soluble pesticides require a
>> multistage food chain to have much effect.
>>
>> Shortly after this consequence of lipid solubility was
>> recognized, a great deal of effort was expended to develop
>> pesticides that were not prone to bioaccumulation and the earlier
>> lipid-soluble ones were largely retired
>
> DDT and other organochlorines were banned in 1973 in North America. However:
>
> 1. Chimney Swifts migrate through central America and the Caribbean
> to wintering grounds in northwestern South America and some of
> those jurisdictions did not ban the use of DDT until substantially
> later.
>
> 2. Although the amounts of DDE (a stable metabolite of DDT) in this
> study peaked in the late 1950's and declined until 1973 (when DDT
> was banned), it did not decline subsequently, indeed there was a
> slight (and statistically significant) rise until 1992 (when the
> chimney in this study was capped). Indeed this continuing
> persistence of DDE in the environment is still apparent in the
> proportions of Hemiptera/ Coleoptera in the Chimney Swift diet.
>
> Moreover, since Coleoptera are hit harder by organichlorines than
> are Hemiptera, and (overall) Hemiptera are at a higher tropic
> position in the foodchain, and they consequently bioaccumulate more
> DDE then do Coleoptera, so the fact that Chimney Swifts were
> consuming a greater proportion of Hemiptera in their diets, means
> they were being receiving a greater d