There is a fairly detailed discussion of fixed vs. random effects on CrossValidated here: http://stats.stackexchange.com/questions/4700/what-is-the-difference-between-fixed-effect-random-effect-and-mixed-effect-mode
Based on the discussion there, it seems like temperature, rainfall, and density could all be considered to be random effects for the following reasons: 1. You are unlikely to sample the entire populations for those variables. 2. They are not being controlled 3. They are likely continuous and distributed in some way (e.g., normal) rather than discrete values 4. You are unlikely to be interested in responses at a specific temperature, rainfall, and density; rather, it seems more interesting to understand effects relating to the underlying distributions of those variables. Those commenting in the CrossValidated forum cite a few sources, though they seem to be general/mathematical rather than ecology-specific. Hope that helps some. -Brian Church On Tue, May 17, 2016 at 11:12 AM, Gary Grossman <gdgross...@gmail.com> wrote: > I'm having a bit of difficulty getting a clear understanding of what > should be considered a fixed vs. a random effect in a linear mixed model > analysis of field data. Even the statisticians seem to say "it depends on > who's defining it" or "sometimes the same treatment/variable can be > either". Some examples may help, let's say I collected samples annually in > three sites and wanted to test for the effect of daily rainfall, daily > temperature, and density, on recruitment of individuals in the following > year. Using the lmer function in R which of these would be fixed effects > and which would be random? A reference or two would help. I really couldn't > find much in a google search on field studies, but I didn't go to anything > like zoological abstracts. TIA, g2 > > -- > Gary D. Grossman, PhD > Fellow, American Fisheries Soc. > > Professor of Animal Ecology > Warnell School of Forestry & Natural Resources > University of Georgia > Athens, GA, USA 30602 > > Website - Science, Art (G. Grossman Fine Art) and Music > www.garygrossman.net > > Board of Editors - Animal Biodiversity and Conservation > Editorial Board - Freshwater Biology > Editorial Board - Ecology Freshwater Fish > > Hutson Gallery Provincetown, MA - www.hutsongallery.net/artists.html > > >
