Data for: Plant and herbivorous insect communities respond in complex ways to rainfall manipulation in an oak savanna grassland

Changes in precipitation due to climate change will have consequences for
plant and herbivorous insect communities alike. Multiple hypotheses
explain how changes in plant diversity and productivity can lead to
changes in herbivore community composition. Yet as rainfall patterns
change, the bottom-up effects on the relationships between plant and
herbivore communities are less well understood. Using a long-term rainfall
manipulation experiment in a remnant patch of Garry oak (Quercus garryana)
savanna, we examined how plant diversity and productivity have responded
to variation in soil moisture over six years. This highly endangered
ecosystem is predicted to experience significantly wetter springs and
drier summers by 2080. We also investigated plant-mediated, indirect
effects of manipulated rainfall on herbivore diversity and abundance,
drawing on multiple hypotheses describing the relationships between plant
and herbivore communities. For example, the more individuals hypothesis
predicts that increased plant productivity results in increased herbivore
abundance which in turn results in increased herbivore diversity. We found
that plant productivity was influenced by soil moisture, but the direction
and magnitude of the response varied across years, and no support for
plant diversity influencing productivity. We also found that the cover and
productivity of grasses increased significantly with increasing
precipitation. In addition to a significant direct effect on herbivore
diversity, soil moisture had a significant indirect negative effect on
herbivore abundance, via the negative effect of plant productivity on
abundance, contradicting the more individuals hypothesis. Synthesis: Our
results highlight that not only can drought result in significant
reductions in plant productivity in this threatened ecosystem, but that
these changes will also result in increases in herbivore abundance. In
contrast, where soil moisture is higher, grasses will become more dominant
resulting in decreased abundance. Ultimately, predicting how this system
responds to changes in precipitation will depend on the ability to predict
whether growing season soil moisture will be consistently drier or wetter
in the future, a significant challenge. Going forward, investigating how
variation in precipitation due to climate change affects the links between
trophic levels, including how herbivores affect plant communities, remains
critical for understanding ecosystem processes and stability.