Dear list members,
I have species composition and abundance data of 17 plots sampled in two
different years. From the first year to the second a general decrease in
Ca-content and pH occurred in the area.
For each plot and year I also have two explanatory variables (Ca-content
and pH).
My response variable is species richness (count data). I want to test if
species richness is related to Ca-content and pH and if there is
a difference between the two years.
As I understand it should be a generalized linear mixed effect model
with Poisson error distribution.
What I am not sure about is how to build the random term. Each plot is
sampled twice which means that plots are pseudo-replicates.
Should the random term consist of plot within year or year within plot?
glmer(species richness ~ Ca +(year|plot), family=poisson, data=data)
or
glmer(species richness ~ Ca +(plot|year), family=poisson, data=data)
Also, if I include "year" in the random term,
how can I see if there is a difference between years and test the
strength of that difference?
Thank you very much for your help!
--
*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*^*
Thomas Möckel, PhD student
Department of Earth and Ecosystem Science
Lund University
Sölvegatan 12
SE-223 62 Lund
Sweden
Tel:+46 (0)46 - 222 4887
E-mail: [email protected]
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