Environmental Correlates
I began by investigating changes in the percentage of stems and galls
knocked over among years. Note that while the extreme weather event
occurred in 2018, the affected gall collection occurred in 2019 and is
referred to as such (i.e. the 2018 extreme event affected the 2018/19
generation). I began by using a linear mixed model (LMM) to test for
differences in the ln-transformed percentage of stems knocked down
across years, including year as a factor and population as a random
effect. This model is designed to test for changes in the mean of a key
environmental condition. However, extreme events can also change
patterns of environmental variation among populations and communities.
As such, I next used a Levene’s test to test for difference in variance
among pairwise combinations of years. As above, I controlled for
consistent population-level differences using a random effect. Together
these two tests quantify the mean change and among-population variance
change in stem knock down.