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Hi Chris & All, Apr 24, 2012
I had to sit this one until now. busy with income tax.
We could get bogged down in small points but, quite apart from muddy =
presentation, the big picture bothers me more. The statistics in the =
paper deal with measurement error and associations between variates but =
can not define experimental error because they had only one roost.=20
From this case study of one roost it is therefore entirely unsound =
to apply the conclusions to the entire Swift population. Measuring one =
hen egg a million times, to obtain absolute precision, does not provide =
any insight into the dimensions of eggs in general or even of hen eggs =
in general.
A quote from DISCUSSION says it all; "Their (Swift) population =
declines are probably a product of the general decrease in relative =
abundance of Coleoptera from the early 1970 to 1992 (Fig. 2c) [sic"]: 2b =
intended.
To swallow the above (no pun intended) you have to accept that--
1) Beetles and true bugs constitute the Swift's diet almost entirely,
2) A questionably significant decrease in relative abundance of Beetle =
debris from 0.6 to 0.4 contributed to a 90% decline in Swift population =
and
3) Essentially the same decrease in Beetle relative abundance would be =
found in roosts throughout the range of Chimney Swifts.
Yt, Dave Webster, Kentville
=20
=20
----- Original Message -----=20
From: Christopher Majka=20
To: naturens@chebucto.ns.ca=20
Sent: Monday, April 23, 2012 8:43 PM
Subject: Re: Swift again: was Re: [NatureNS] 50th Anniversary of =
Silent Spring
Hi Steve,=20
On 23-Apr-12, at 7:33 PM, Stephen R. Shaw wrote:
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' =3D 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 =3D =
2.08).
Why not ask Joe Nocera directly? I've corresponded with him and he's =
very open to providing information with respect to the paper. Whether =
it's 2.2 cm/year or 3.1 cm/year (200 cm/64 years =3D 3.1 cm/year), what =
salient difference does it make?
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?)
There's no such problem in the version I have which I downloaded from =
the site. Maybe there is a software glitch at your end
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?
But Steve, without inquiring into this, you are assuming this is an =
error. Maybe you should find out the basis of the calculation. Do you =
honestly think that a paper published in the Proceeding so the Royal =
Society by ten recognized academics which employed really sophisticated =
science and mathematics (read their description of their statistical =
analyses) suffers from simple mistakes in arithmetic? Wouldn't the first =
assumption be that one needs clarification on this point rather than the =
authors are sloppy and erroneous with simple math? =20
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.=20
I don't know that that's the case. Does guano really compact?
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.
Reworking is mentioned in the paper. Note, for example: "... changes =
in nitrogen provenance of post depositional processing such as =
ammonification or nitrate utilization ..." (paragraph 2, page 5) which =
refers to bacterial or other reworking of the deposits in such a way as =
to change the ^14N/^15N ratios leading to the comparatively large =
fluctuations in ^15N as shown in Figure 2c.
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?
My understanding is that all analyses were done at levels of the =
1928-1992 deposit. The paper reports data from ~ 1945-1992 since the =
authors are focusing on the impact of DDT on the Chimney Swifts. If you =
look at Figure 2d you will see that the first data point shown (from =
circa 1945) indicates that levels of DDE were at almost 0