I have a population animals, and am looking at the effect of (yearly) environmental temperature on date of birth and litter size. Births in the population have been monitored for 15 years (one litter per year), consequently, many females have been repeatedly observed giving birth over this time. Environmental temperature is the same for all animals within each year.
I am currently modelling this as a linear mixed model, with yearly temperature as a fixed effect and maternal ID as a random effect, to account for repeated measures of mothers.
A reviewer has recommended that I fit the model with Year as a random effect as well. However, because only a single yearly temperature applies to all mothers within each year, this drastically changes the results. - Essentially, the random effect of year soaks up all the variation due to temperature, rendering the effect of temperature nonsignificant. For temperature to not have an effect would be VERY surprising since experiments manipulating temperature directly show a strong effect.
What is the best way to proceed?
summary of LMM without Year as a random effect:
AIC BIC logLik deviance df.resid 10378.6 10399.7 -5185.3 10370.6 1419 Scaled residuals: Min 1Q Median 3Q Max -2.35860 -0.60767 -0.02228 0.57020 3.08272 Random effects: Groups Name Variance Std.Dev. TrueID (Intercept) 39.22 6.262 Residual 58.43 7.644 Number of obs: 1423, groups: TrueID, 679 Fixed effects: Estimate Std. Error df t value Pr(>|t|) (Intercept) 36.3515 0.3320 582.1000 109.476 < 2e-16 CritTemp.c -3.0817 0.3757 1238.7000 -8.203 4.44e-16 Correlation of Fixed Effects: (Intr) CritTemp.c 0.010
summary of LMM with Year as a random effect:
AIC BIC logLik deviance df.resid 9819.7 9846.0 -4904.8 9809.7 1418 Scaled residuals: Min 1Q Median 3Q Max -3.2943 -0.4964 0.0186 0.4700 3.4843 Random effects: Groups Name Variance Std.Dev. TrueID (Intercept) 39.16 6.258 Year (Intercept) 27.80 5.273 Residual 31.73 5.633 Number of obs: 1423, groups: TrueID, 679; Year, 16 Fixed effects: Estimate Std. Error df t value Pr(>|t|) (Intercept) 36.550 1.358 16.900 26.917 2.66e-15 CritTemp.c -2.757 1.976 16.145 -1.395 0.182 --- Correlation of Fixed Effects: (Intr) CritTemp.c -0.095