modeling question

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, Joaqu?n 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.


--
*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
<https://sites.google.com/site/perfilprofesionaljoaquinaldabe>

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