[TN-Bird] Re: Pulliam BBS contribution
- From: "Van Harris" <shelbyforester1223@xxxxxxxxxxxx>
- To: <littlezz@xxxxxxxxx>
- Date: Tue, 18 Oct 2011 12:44:32 -0700
The major problem with the Breeding Bird Survey is getting people to run them.
There are plenty of people (like Bill & Kevin) who are smarter than I am to
crunch the numbers and discuss them.
Van Harris
Millington, TN
--- littlezz@xxxxxxxxx wrote:
From: Bill Pulliam <littlezz@xxxxxxxxx>
To: kbreault <kbreault@xxxxxxxxxxxxx>
Cc: tn-bird@xxxxxxxxxxxxx, kbreault@xxxxxxxx
Subject: [TN-Bird] Re: Pulliam BBS contribution
Date: Tue, 18 Oct 2011 14:13:38 -0500
Thanks for the thoughts, Kevin. A few comments about statistics and
the purpose of the BBS. First, statistics. Statistics do not
determine what is real, they determine the degree to which the data
do or do not conform to the model of "significant difference" on
which a particular test is designed. All tests have their own
particular assumptions such as linearity, normality, constant
variances, etc. to which real world data rarely conform precisely. A
non-significant trend is not in fact "spurious by definition." A
"spurious" trend is an apparent trend that is in fact not real; as
statistics do not determine what is real, they do not determine what
is spurious either. Apparent trends or differences that fail to be
statistically significant may be real, they may not. They may in
fact be highly important. The first Starling that showed up in
Tennessee was not a statistically significant change from the long
string of zeros that came before it; it was however extremely
significant ecologically. And, of course, as Kevin points out, a
"statistically significant" change is not necessarily a meaningful
one. The mass media love to play up studies that show significant
differences, ignoring that these differences are often tiny. This is
why I presented the raw trend graphs for the species, regardless of
their statistical outcomes. A trend is a trend, there are many
factors that determine if it is a trend worth thinking about, the
results of statistical tests being only one of them.
About confidence levels -- when you have 84% of the species showing a
statistically significant change at the p = 0.05 level, that does
mean something. In contrast, when I applied a similar analysis to my
own data from Lewis County, using data from just three routes and
only a 5 year time interval, I only had 6% of the species show a
"significant" difference at the p=.05 level -- in other words about
what one would expect from spurious differences. So I think it is
pretty safe to conclude that there has been real, meaningful,
extensive, large-scale change, affecting the majority of the 104
species considered, not just among the species that show dramatic 90%
drops or 800% increases. Now any individual trend might be spurious,
but the big picture of generally shifting populations is worth
talking about. The REASONS why these shifts have occurred are
another matter entirely, and the BBS was not designed to determine that.
Now about the purpose of the BBS. It was always intended to be a
broad-scale monitoring program -- the big picture, the coarse view.
It's number one job is to detect signs of population changes. It was
never designed to actually measure populations themselves. Its
severe limitations caused by observer effects, roadside biases,
drastic changes in detectability between species and between habitats
within a species, etc. were recognized from the beginning and had to
be baked right in. All sampling protocols require tradeoffs and
compromises, all have severe limitations, all data are flawed in some
ways. The bigger your scope in space, time, and number of species,
the more limitations you have to accept. The BBS serves to give an
overview and sometimes to sound an alarm bell. The more extensive
"disaggregated" studies provide much more info; but I can absolutely
guarantee you that they will NEVER be conducted for every species
across every state in every year! Thorough research on individual
species only trumps more superficial research on large numbers of
species if you only want to know about one species. You need the
overview to select which species are of the most interest; otherwise
you might be playing your trump cards up the wrong tree (how's that
for a mixed metaphor?). Within the context of their limitations and
intentions, the BBS data are actually rather exceptional; there is
nothing comparable for the big picture, even if that picture is
viewed through a tinted, blurry, smudged, constantly wiggling glass.
As the saying goes, it is better to have flawed data we can argue
about than no data at all!
Bill Pulliam
Hohenwald TN
=================NOTES TO SUBSCRIBER=====================
The TN-Bird Net requires you to SIGN YOUR MESSAGE with
first and last name, CITY (TOWN) and state abbreviation.
You are also required to list the COUNTY in which the birds
you report were seen. The actual DATE OF OBSERVATION should
appear in the first paragraph.
_____________________________________________________________
To post to this mailing list, simply send email to:
tn-bird@xxxxxxxxxxxxxx
_____________________________________________________________
To unsubscribe, send email to:
tn-bird-request@xxxxxxxxxxxxx
with 'unsubscribe' in the Subject field.
______________________________________________________________
TN-Bird Net is owned by the Tennessee Ornithological Society
Neither the society(TOS) nor its moderator(s)
endorse the views or opinions expressed
by the members of this discussion group.
Moderator: Wallace Coffey, Bristol, TN
wallace@xxxxxxxxxxxxxxxxxxx
------------------------------
Assistant Moderator Andy Jones
Cleveland, OH
-------------------------------
Assistant Moderator Dave Worley
Rosedale, VA
--------------------------------
Assistant Moderator Chris O'Bryan
Clemson, SC
__________________________________________________________
Visit the Tennessee Ornithological Society
web site at http://www.tnbirds.org
* * * * * * * * * * * * * * * * * * * * * * * * * * * * *
ARCHIVES
TN-Bird Net Archives at //www.freelists.org/archives/tn-bird/
MAP RESOURCES
Tenn.Counties Map at http://www.lib.utexas.edu/maps/states/tennessee3.gif
Aerial photos to complement google maps http://local.live.com
_____________________________________________________________
=================NOTES TO SUBSCRIBER=====================
The TN-Bird Net requires you to SIGN YOUR MESSAGE with
first and last name, CITY (TOWN) and state abbreviation.
You are also required to list the COUNTY in which the birds
you report were seen. The actual DATE OF OBSERVATION should
appear in the first paragraph.
_____________________________________________________________
To post to this mailing list, simply send email to:
tn-bird@xxxxxxxxxxxxxx
_____________________________________________________________
To unsubscribe, send email to:
tn-bird-request@xxxxxxxxxxxxx
with 'unsubscribe' in the Subject field.
______________________________________________________________
TN-Bird Net is owned by the Tennessee Ornithological Society
Neither the society(TOS) nor its moderator(s)
endorse the views or opinions expressed
by the members of this discussion group.
Moderator: Wallace Coffey, Bristol, TN
wallace@xxxxxxxxxxxxxxxxxxx
------------------------------
Assistant Moderator Andy Jones
Cleveland, OH
-------------------------------
Assistant Moderator Dave Worley
Rosedale, VA
--------------------------------
Assistant Moderator Chris O'Bryan
Clemson, SC
__________________________________________________________
Visit the Tennessee Ornithological Society
web site at http://www.tnbirds.org
* * * * * * * * * * * * * * * * * * * * * * * * * * * * *
ARCHIVES
TN-Bird Net Archives at //www.freelists.org/archives/tn-bird/
MAP RESOURCES
Tenn.Counties Map at http://www.lib.utexas.edu/maps/states/tennessee3.gif
Aerial photos to complement google maps http://local.live.com
_____________________________________________________________
Other related posts:
