[R-sig-eco] Help with nonlinear population trends & binomialregression

[Aitor Gastón]

Matthew,

A more flexible model may be a better option than fitting two separate lines 
(e.g. restricted cubic splines, rcs function of the rms package or a 
generalized additive model, mgcv package).  A restricted cubic spline with 4 
knots may be enough to model the response curve that  you describe.

Hope that helps,

Aitor


--------------------------------------------------
From: "Matthew Forister" <forister at gmail.com>
Sent: Friday, December 17, 2010 7:19 AM
To: <r-sig-ecology at r-project.org>
Subject: [R-sig-eco] Help with nonlinear population trends & 
binomialregression

> Hello, I'm hoping someone can point me in a new direction with this
> particular issue...
>
> I have count data for many species across 20-30 years.  I know that many
> species are declining, probably in association with habitat destruction, 
> and
> I have been using binomial regression to model the declines
> (e.g. glm(cbind(presence,visits-presence)~years,binomial).  I have also 
> used
> the glm.binomial.disp function for overdispersion.  So far so good, but
> here's the issue: not all species decline in the same way...
>
> Some go down steadily over the years, while others will be holding steady
> and then suddenly start on a decline.  There are other patterns, but those
> two are dominant and I would like to be able to say that different species
> have these different dynamics.  But how do I quantify those different
> curves?  I have played with fitting quadratic and cubic terms within the
> binomial
> regression (e.g.
> glm(cbind(presence,visits-presence)~years+I(years^2),binomial)),
> and then comparing models with AIC to think about the better fit of the
> model with the quadratic.  That kinda makes sense for some species, but 
> it's
> far from satisfying... In the case I described (a species holding steady 
> for
> 2 decades and then going into a steady decrease for another 10), what it
> really looks like is two different lines, one flat and one precipitous.
>
> Is there a way to ask if a given relationship is better fit by two lines
> than one?  any other hints on how to describe these kinds of dynamics?
>
> thanks!
> Matt
>
>
> -- 
> Matthew L Forister
> Assistant Professor
> Department of Biology / MS 314
> 1664 N. Virginia St.
> University of Nevada, Reno
> Reno, Nevada 89557
> --
> E-mail: forister at gmail.com
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