[R-sig-ME] Doubt about including random effects or not

[V. Coudrain]

Dear all,

I collected insects in 30 sites, 15 sites each with a level of isolation and all 30 sites are arranged along a gradient of habitat amount. Each insect species has a 
specialization value. I would like to test if specialization increases along the gradient of habitat and differs between isolation level. The number of insect species and 
individuals differ largely between sites. I have two doubts: should I specify site as a random variable? Should I specify insect abundance as an offset?

The model I thought about: lme(specialization~Isolation*habitat amount + offset(insect abundance) , random~1|Site)

However I am not sure about using site as a random effect and specifically if it makes sense to mix offset and random effect.

Thank you!

Valérie Coudrain
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