An excellent critique Andrew and I fully agree - see my earlier comments on
the Canberra Birds listderver. It is disturbing that such poor analysis is
promoted to justify concerted action on Mynahs - and it appears to have been
eagerly taken up uncritically by many others. Not only will this encouage
resources to be spent on actions that have not been appropriately justified
but a worse outcome from incorectly attributing danmmage due to a pest based
purely on correlation, is that the real causes are ignored and hhence not
addressed.
Another issue that disturbs me about this paper is that such poor science
got through the peer review process. I guess the reviewers were overawed by
the national scientific status of one of the authors rather than critically
reviewing the methods and the scientific analysis.
Regards
Mike Braysher
-----Original Message-----
From: Andrew Taylor
Sent: Sunday, 19 August 2012 2:33 PM
To:
Cc:
Subject: modelling of Common Myna impact in Canberra
A recently published paper [1] claimed to have established that Common Mynas
had harmful effects on Canberra populations of 11 bird species - the first
time this has been demonstrated.
I posted some quick comments on birding-aus but as these claims have been
widely publicized [2] and they'll be used to justify myna control programs,
I've written a more detailed explanation of why I think the paper's claims
are unsupported and the paper provides no useful evidence of myna impacts.
The paper [1] describes a model built on COG Garden Survey Data.
The authors divide Canberra into 4 regions and convert the survey results
into biannual population estimates for 20 species of birds over the 29 years
the survey has run.
The external model variables are essentially population, dwellings, extent
of 5 vegetation types, mynas and year.
The model attempts to explain the changes in 20 bird populations over
1981-2009 in the 4 regions using these model variables.
The demographics of Canberra myna populations, as estimated in the paper
vary between regions but overall there is a peak 5-15 years ago followed by
a decline. In two regions the decline is recent and in 2009 myna
populations are still near 50% of their peak. In the other two regions the
peak is earlier and by 2009 myna populations are below 20% of their peak.
The effects mynas are suspected to cause are largely density-dependent
(proportional to the number of mynas) so the obvious model input to estimate
myna impact is myna density. The model instead uses a proxy variable, years
since myna establishment. This yields a variable which linearly increases
from 1991, 1993, 1981 & 1989 in the respective regions.
The paper doesn't compare this variable to myna density but its clear there
will be a large disparity. For example from eyeballing the graph, the
region 3 1986 myna density looks to be about double the 2009 myna density,
but the proxy variable will be 5 in 1986 and 28 in 2009.
In other words 2009 myna density is about half 1986 but the modelled myna
impact is a factor of over 5 times larger.
The justification in the paper for this choice of variable makes no
numerical or ecological sense to me. This variable choice is crucial
because all the claims in the paper are based solely on attributing changes
in the COG data to this myna variable.
This reduction of changes in myna density to a simple linear increase raises
concern about correlations with variables not included in the model. Its not
uncommon for variables to show roughly linear increase or decrease for a
period of time. For example there conceivably might be a bias in the COG
data related to observer recruitment, and if it increased or decreased
roughly linearly and roughly over the same time period as myna
establishment, part of its effects might erroneously be attributed to mynas.
Its important to consider what variables are included in the model.
One obvious omission is rainfall. Its easy to obtain. It has been linked
to bird abundance and it shows large variation through the time period.
For example, on a quick look at BOM data for the the Botanic Gardens,
Sep-Nov rainfall appears to vary by a factor of 6 between 1981-2009.
Rainfall may be less important in a suburban environment its still seems
some of the variation in some species will be explained by rainfall.
Another omission is the effects of birds other than mynas. For example, Pied
Currawong, Red Wattlebirds and Noisy Miners have all been linked to negative
effects on other bird species in suburban environments, but the model
excludes this possibility. Particularly problematic is the omission of
Noisy Miners because their Canberra population is roughly constant until
2000 then shows a large roughly linear increase [3].
As Noisy Miners aren't included the model its impossible to exclude their
effects being erroneously attributed to mynas. There is good reason to
suspect there might be such effects because a number of researchers have
noted such effects elsewhere and there has been such a large increase in
Canberra's Noisy Miner populations. There is also good reason to suspect
such effects could be erroneously attributed to mynas in the model because
the growth in Noisy Miner population follows a pattern that correlates with
the mynas proxy variable.
In other words the concern is if that if the authors had taken the same data
and same approach to modelling Noisy Miner impact they would have found
similar impacts but attributed them to Noisy Miners.
There are other species whose omission as model variables is problematic.
For example, cockatoo numbers triple through the time period.
Their density and the variation in their density is similar in magnitude to
mynas. While the authors suggest mynas affect other cavity nesters by
competing for nest sites, their model assumes cockatoos do not affect other
cavity nesters by competing for nest sites. This asymmetry is hard to
justify.
So, for example, it is impossible to exclude the negative impact on Galahs
the model attributes to mynas being actually caused by cockatoos.
Its suspected that mynas have a negative impact on cavity nesters.
However the model's outputs for cavity nesters are equivocal: 4 species
supposedly negatively affected and 3 positively affected.
3 of the 4 species supposedly negatively affected species populations grew
through the time period, for example cockatoo numbers tripled, but the model
output indicate mynas slowed this growth.
The only cavity nester which showed a large decline, starling, supposedly
benefited from mynas - an unexpected result. The authors suggest this large
decrease in starling numbers is hiding the negative effect from mynas. This
is plausible. The model attributes the majority of this decline to the
linear pseudo-variable year. If this large unexplained effect is non-linear
then it could mask a negative effect from starlings.
Eyeballing the starling & myna density graphs raises the suspicion that if
the model had used myna density as a variable they might instead have found
the expected negative effect.
This problematic presence of large unexplained effects also occurs for other
species. For example, a large part of the increase in cockatoos is
unexplained by the model, and is also attributed to the linear
pseudo-variable year. If this large effectively unexplained effect is
non-linear then part of it could be being erroneously attributed to the myna
variable.
Note when the model produced an unexpected positive impact from mynas
(starlings) the model output was rejected, but when the model produced an
unexpected negative impact (cockatoos) the model output was accepted, and
conveyed via press release to Australia's major media outlets.
Another concern is the extent to which the model is opaque. The PLOS paper
presents only a table of coefficients. We aren't given the values of the
model variables, e.g. as graphs, to which these coefficients apply.
The authors discuss only the signs of the coefficients and only for the myna
variable. Some of the coefficients look to have implausible values but the
information needed to interpret them is absent.
It easy to criticise models - all models are wrong, some are useful - and
even given the lovely 29 years of COG data its no small task to tease apart
competing effects - but this model fails to do so and fails to support the
claims made.
Personally I think is likely given the high density they reached mynas did
affect at least a few Canberra bird species, but this modelling leaves me
none the wiser as to which species or the magnitude of these effects.
On the other hand 2 recent observational papers [4,5] (both paywalled) on
Sydney & Newcastle mynas I found while writing this did suggest to me I (and
others) might be over-estimating myna impacts.
Andrew
[1]
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0040622
[2]
http://www.smh.com.au/environment/conservation/pushy-myna-birds-a-major-nest
-pest-20120812-242v2.html
[3] Canberra Bird Notes 37 (2) June 2012 Noisy Miners in the COG Garden Bird
Survey Martin Butterfield [4] http://dx.doi.org/10.1071/MU11046 [5]
http://dx.doi.org/10.1007/s10336-011-0674
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