Center your biomass variable on this value: either create a
mydata$invert_biomass_c <- mydata$invert_biomass-ref_value
or include it directly in your formula:
bird_dens ~ I(invert_biomass-ref_value), ...
On 17-01-16 05:44 PM, Joaqu?n Aldabe wrote:
Thankyou very much Ben. Can you please suggest a way of fixing some
sensible reference quantity for Invertebrate biomass?
All the best,
Joaqu?n
2017-01-16 18:59 GMT-03:00 Ben Bolker <bbolker at gmail.com
<mailto: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
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, Joaqu?n Aldabe
<joaquin.aldabe at gmail.com <mailto:joaquin.aldabe at gmail.com>> wrote:
> Dear all, I'm interested in modeling the effect of invertebrate
> the density of a grassland shorebird (they eat invertebrates). For
> picked 8 plots and sampled invertebrates and birds 6 times in each
> about 30 days. This is, I went to each plot and did repeated
> invertebrates biomass and shorebird density separated in time by
> five days, as invertebrates biomass may change over time and it is
> that birds density change accordingly.
>
> So, I'm trying to see a general pattern of the effect of changes
> on the density of this shorebird species at a plot scale. Plot
> not important; I consider them as particular events of a random
>
> Is this model correct:
>
> lme(Bird.density~Invertebrate biomass, random=~1|Plot_identity,
>
> Thank you very much,
>
> Joaquin.
>
>
> --
> *Joaqu?n Aldabe*
>
> *Grupo Biodiversidad, Ambiente y Sociedad*
> Centro Universitario de la Regi?n Este, Universidad de la Rep?blica
> Ruta 15 (y Ruta 9), Km 28.500, Departamento de Rocha
>
> *Departamento de Conservaci?n*
> Aves Uruguay
> BirdLife International
> Canelones 1164, Montevideo
>
> https://sites.google.com/site/joaquin.aldabe
>
> [[alternative HTML version deleted]]
>
> _______________________________________________
> R-sig-mixed-models at r-project.org
<mailto:R-sig-mixed-models at r-project.org> mailing list