Hi All,
It's interesting where I live, hearing the reaction to the SMH article, and the
PLoS One paper. I live on Magnetic Island off Townsville, where there are
usually no mynas. Right now, there are at least 6 that have made it over from
the mainland, presumably accompanying the ferries, since the ferry terminal is
where the birds are stationed. The overwhelming reaction among my good friends,
who are typically conservation minded, has been that the SMH article
(misconstrued as it is compared to the less clear conclusions of the original
publication) is justification for what they've always "known" about mynas, i.e.
they are a serious pest and culling should occur. I agree, we should try to
cull them on the island, provided it's not at the expense of some other
conservation outcome. However, my experience of mynas in northern Australia is
very different to "rats of the sky", "cane toads of the air" type sentiments
sometimes applied. I (and my immediate colleagues) undertake systematic surveys
of fauna (and flora) across north Queensland and have been involved in over
1600 permanent fauna (sometimes just birds) monitoring sites (1 ha each), many
visited repeatedly. We conduct 8 bird counts per hectare site. Our surveys
typically avoid anthropogenic influence as much as possible. We obtain
somewhere between 10,000 and 50,000 bird records annually (not necessarily that
many individual birds of course), and we have counted exactly zero mynas in our
travels. And they've been in north Qld a loooong time.
I seriously doubt that they represent a significant threat to any Australian
bird species, and where a threat may occur, it is in an urban, periurban or
agricultural setting, where the mynas are riding the habitat modification. Try
finding a myna away from serious anthropogenic habitat alteration in
mid-eastern to northern Australia. From my (significantly less) experience
around Canberra and further south, it's the same there - except the challenge
is turned on its head - i.e. try finding an area away from serious
anthropogenic habitat alteration.
I think mynas are a lesson in human psychology - many of us just don't like
them. I think that's ok as long as it's acknowledged for what it is and not
taken to be part of an important conservation outcome.
I think noisy miners (and yt miners) may be a problem to other native birds in
some circumstances. They are not especially common in our surveys, but we do
record them from time to time in seemingly intact woodlands and forest. Those
sites are invariably depauperate in a select set of woodland birds. Correlation
or causation? I think causation, but I'm not sure.
Eric Vanderduys
-----Original Message-----
From:
On Behalf Of 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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