[R-sig-ME] GLM question

[dorothy borowy]

Hello,

I have a question about GLMMs and it was recommended to me that I send my question to this group.  My question is below.  I appreciate your input. 

> I designed an experimental greenhouse study whereby I grew 67 plant
> species from seeds collected from urban vacant land habitats in 2 soil
> types (urban vacant lot and topsoil).  Each species was replicated 10
> times (10 pots each w/ an individual plant) in each soil type. I
> measured a suite of continuous traits (i.e. height at maturity,
> % germination success, aboveground biomass, belowground biomass and
> specific leaf area) on each individual for each species over the duration
> of the study.  /
> /
> /
> /My study question is based on understanding functional trait responses
> of a pool of urban plant species to different soil environments. Because
> trait responses can include both differences in mean values and/or
> differences in the variances, and because these represent different
> potential strategies for surviving in urban habitats, I would like to
> test two types of models: the first is a model that tests the mean
> differences between soil groups for each species (species is the unit of
> replication) and the other testing differences in variance as response
> variables.  I am unsure, however, how to structure the model and I
> cannot find any studies that have done anything comparable--not to say
> they do not exist. /
> /
> /
> /I am considering designing 2 generalized linear models that have soil
> and functional group (a separate factor that I have not described in
> this email) as explanatory variables and mean difference and difference
> in variance/SD for each species as the response variables.  My question is
> as follows: /
> /
> /
> /1) Is this possible and how would I structure each model? Specifically,
> how do I represent mean differences and differences in variance,
> separately?  As I mentioned, I am interested in capturing general trends
> in mean and variance across all species.  Therefore, do I simply calculate and
> compare effect size to capture mean differences (model 1) and F values
> for a Levene’s test (or something comparable) (model 2)?  Or, because sp.
> is the unit of rep., would I have the mean and variance/SD for each sp. as
> the response for each model and include sp. as a nested random effect? 

> For example-
> lmer ( trait_mean ~ functional_group + soil + (1|species))
> lmer ( trait_var ~ …)

Kind Regards,



Dorothy Borowy 
PhD Candidate
Department of Geography and Environmental Systems
University of Maryland Baltimore County 
http://ges.umbc.edu <http://ges.umbc.edu/>
www.researchgate.net/Dorothy_Borowy <http://www.researchgate.net/profile/Dorothy_Borowy>






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