The proportion of insect taxa carrying pollen and the taxonomic
richness of pollen carried
To examine the effect of land-use intensity on the average abundance and
proportion of pollen carrying insects in different land-use types and
pollen richness present in each land-use and carried by different insect
taxa, we constructed generalized linear mixed effects models using thelme4 package (Bates et al. 2014). In these models, either
average abundance of each of the four common insect orders, proportion
of insect individuals carrying pollen or pollen richness on those insect
individuals was the response variable and land-use (categorical) was the
fixed effect. For the pollen carrying and pollen richness models, insect
order was also included as a fixed effect. We included month as a random
effect to account for repeated sampling across the three time periods.
All models were validated by examining the distribution of residuals
plotted against fitted values, and we set a negative binomial
distribution to account for overdispersion where necessary (Zuuret al. 2009). We ran separate models for each of the four most
abundant insect orders (Coleoptera, Diptera, Hymenoptera and
Lepidoptera), which represented 93.5% of pollen carriers and 94.5% of
all sampled individuals. The sample sizes of the remaining taxa were too
small for further analyses. Post-hoc multiple pairwise
comparisons between land-uses and insect orders were tested using
estimated marginal means (EMMs, using the emmeans package, Lenth
2018). EMMs are generated using the fitted model to make predictions
over a grid of predictor combinations in order to assess the effects of
individual factors. These predicted differences in abundance of
individuals and proportion of pollen carrying insects from each of the
four dominant insect orders between land-uses, and differences in
average pollen richness by land-use type and insect order. We estimated
the significance of differences between terms, using Tukey’s HSD
(honestly significant difference) test with false discovery rate (FDR)
corrected P values (at α = 0.05) (Verhoeven et al.2005).