[R-sig-ME] modeling question

Ben Bolker bbolker at gmail.com
Mon Jan 16 22:59:52 CET 2017

That seems perfectly reasonable.  There are a couple of things to
consider, although you may or may not find that your data supports
that much complexity.

(1) The relationship between bird density and invert biomass, as well
as the intercept (i.e., expected bird density at invert_biomass=0, or
better invert_biomass=<some sensible reference quantity>)

lme(Bird.density~Invertebrate biomass,
random=~invert_biomass|Plot_identity, data=)

(2) The relationship might be changing over time?

lme(Bird.density~Invertebrate biomass+sample_time,
random=~invert_biomass|Plot_identity, data=)

(3) In principle you could consider random effects of both time and
invert biomass, but that will almost certainly overwhelm your data.

  Don't forget to do the standard post-fitting checks: are your
residuals *approximately* equal-variance and (even more approximately)
Normally distributed?  Is the relationship between bird density and
invert biomass *approximately* linear?  (See ?plot.lme)

On Mon, Jan 16, 2017 at 2:06 PM, Joaquín Aldabe
<joaquin.aldabe at gmail.com> wrote:
> Dear all, I'm interested in modeling the effect of invertebrate biomass on
> the density of a grassland shorebird (they eat invertebrates). For this, I
> picked 8 plots and sampled invertebrates and birds 6 times in each plot for
> about 30 days. This is, I went to each plot and did repeated measures of
> invertebrates biomass and shorebird density separated in time by four or
> five days, as invertebrates biomass may change over time and it is expected
> that birds density change accordingly.
> So, I'm trying to see a general pattern of the effect of changes in biomass
> on the density of this shorebird species at a plot scale. Plot identity is
> not important; I consider them as particular events of a random process.
> Is this model correct:
> lme(Bird.density~Invertebrate biomass, random=~1|Plot_identity, data=)
> Thank you very much,
> Joaquin.
> --
> *Joaquín Aldabe*
> *Grupo Biodiversidad, Ambiente y Sociedad*
> Centro Universitario de la Región Este, Universidad de la República
> Ruta 15 (y Ruta 9), Km 28.500, Departamento de Rocha
> *Departamento de Conservación*
> Aves Uruguay
> BirdLife International
> Canelones 1164, Montevideo
> https://sites.google.com/site/joaquin.aldabe
> <https://sites.google.com/site/perfilprofesionaljoaquinaldabe>
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