[leps-talk] Credible sources

Stanley A. Gorodenski stan_gorodenski at asualumni.org
Thu Oct 9 14:23:14 EDT 2003

Although more research is needed, possibly along the lines Patrick 
suggested, I think until then Paul's time series could provide some 
valuable general information concerning the alleged demise of the 
Monarch migrations, but only if, as Patrick suggested, we can know more 
about the data making up the time series.

For example, random measurement error in the dependent variable will not 
result in a biased estimate. The dependent variable in this case could 
be a measurement of the roosting area by a technician. Differences 
between technicians will increase variability and affect significance 
test but it will still give unbiased and consistent estimates. If this 
was the only factor affecting the reliability of the data, conceptually 
one could develop a model that might explain whether the current drop in 
migration numbers, although comparable in magnitude to the numbers at 
the start of the series, is human caused. Or could the migratory numbers 
at the beginning and end of the series be the result of a common cause, 
such as climate, solar variation (a wild one), geomagnetic activity 
(causing some adults to lose their way during migration, another wild 
one), etc.

On the other hand, measurement error in the independent variable will 
lead to inconsistent and biased parameter estimates. The independent 
variable in this case would be the technique, such as a photograph of a 
roosting area, used as a measure of the  number of adults. However, 
there will be measurement area if the numbers of adults represented by 
this area is a complex function of other variables, such as thinning, 
new growth, etc., that is not taken into account. Also, changes in 
measurement techniques (such as outright sample counting instead of a 
photograph) could result in measurement area. For example, if the large 
migratory population in the middle of Paul's time series is the result 
of researchers having learned how to best measure migration size during 
the beginning of the time period, then the more recent drop in numbers 
(although comparable to the magnitude level at the beginning of the 
series) would be something to be even more concerned about.

Patrick Foley wrote:

> I especially appreciate the population time series. But for Monarch's 
> sake and all our sakes, can't we get a source and methodology for this 
> data and some idea of the uncertainty in this data?
> If we are to get a clearer and more honest picture of the Monarch's 
> status and prospects, we need a _quantitative_ description of the 
> changes in critical winter habitat. Your pictures are suggestive, but 
> we all know how easy it is to pick out samples for our own convenience.
> We need to ask 1) how does Monarch winter survivorship depend on the 
> amounts of land in old growth, secondary growth, clear cut and milpas? 
> And 2)how much of the winter sites fall into these (or other land use 
> categories) and how will the land use distribution change in the 
> future? Question 2 is fairly easy to answer (given a little money to 
> pay scientists to measure it, and given a little stability in land use 
> planning).
> Question 1 is much harder. Your population time series would help. Is 
> there any historical data about land use distribution during this 
> time? And is it possible to predict survivorship from such data? These 
> are not rhetorical questions nor am I putting you on the spot. The 
> entire Monarch community (including the black sheep), should be 
> addressing these questions. Are they? Chip or others, please tell?


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