I am attempting to determine if a type of pesticide is influencing the abundance of a particular species of bird. I have 35 years of data, which was collected along roadside survey routes that are run annually (Breeding Bird Survey). I am also considering environmental variables, such as land use and climate variables.
Should I include "year" as a fixed or random effect? I need to account for the fact that bird abundance is decreasing over time whereas pesticide use is increasing.
My current code looks like this:
BBS.1 <- glmmadmb(abun ~ pmdi + avail + rte.pest + rte.dev + rte.ag + rte.precip + rte.temp + (1|year) + (1|eco) + (1|rte.num), data = mybbs, family = "nbinom")
- abun = bird abundance
- pmdi = drought index
- avail = 1/(distance to ag + 1)
- rte.pest = kg pesticide used within route (which was buffered by .5 km using GIS)
- rte.dev = developed area within route
- rte.ag = agricultural area within route
- rte.precip = annual precipitation within route
- rte.temp = mean max monthly temp within route
- year = year which survey was conducted (factor with 35 possible years)
- eco = ecoregion (factor with 6 possible ecoregions)
- rte.num = route id number
I found one similar question that had been asked before, but was never answered. I am very appreciative of any help you're able to give!