Ecological Interactions
After quantifying the environmental effects of the storm, I next sought to characterize the consequences of those environmental differences for survival and species interactions. Because attack byMordellistena and Eurytoma occurs in summer, they should be unaffected by galls having been knocked to the ground. Conversely, bird attack is likely to decline if galls are knocked to the ground and are therefore less high off the ground (Start 2018a), and also tends to decline further away from forest edges where woodpeckers live (Start et al. 2018). I tested for these effects in two ways. First, I used an LMM to estimate total mortality and attack rates of each enemy independently, including year as a factor and population as a random effect. I then performed pairwise comparisons of attack rates among years using Tukey’s tests. Second, I used another LMM to estimate the same measures of mortality and attack rates, but while including distance to trees and percentage of stems knocked down as main effects. In combination, these two tests (i) estimate among year differences in demography and species interactions, and (ii) tie any differences to specific environmental conditions.
While the above tests quantify mean differences among years and environmental conditions, they do not estimate among-population variation (i.e. whether the extreme event homogenizes the environment or creates environmental variation). I again tackled this issue using Levene’s tests. I independently tested for differences in among-population but within-year variation in mortality and each attack rate. I again compared years using Tukey’s tests to gain pairwise comparisons. I also tied differences in variable species interactions to environmental variation. Specifically, I calculated residuals for each environmental variable and each ecological interaction. Residuals were measured from the grand mean across all populations, but within a given year. I then used linear models to estimate the residuals of each species interaction using residual variance in the distance from trees and the residual variance in the percentage of stems knocked down. As above, this combination of tests quantifies among-population variance in species interactions while tying that variation to corresponding environmental variation.