[R-sig-ME] modeling question

[Joaquín Aldabe]

Thankyou very much Ben. Can you please suggest a way of fixing some
sensible reference quantity for Invertebrate biomass?
All the best,
Joaquín

2017-01-16 18:59 GMT-03:00 Ben Bolker <bbolker at gmail.com>:

> 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>
> >
> >         [[alternative HTML version deleted]]
> >
> > _______________________________________________
> > R-sig-mixed-models at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>



-- 
*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>

	[[alternative HTML version deleted]]