Bottom‐up and top‐down forces in plant‐gall relationships: testing the hypotheses of resource concentration, associational resistance, and host fitness reduction

Abstract

1. The abundance of insect galls may be influenced by the local host abundance as well as by the structure of the surrounding plant community, while insect galls may reduce host fitness. So far, few studies have been done in order to understand the relationship between galling insect abundance and the surrounding non‐host plant community.2. In the present study, we explored the relationship between galling insect abundance and plant community structure and diversity. We tested the bottom‐up forces of host and non‐host plant density and non‐host diversity on gall abundance. We also tested the top‐down force of gall abundance on host fitness.3. We sampled all trees, measured their basal area, and estimated gall abundance in 25 plots (1 ha in total) located in a Brazilian savanna. In addition, we recorded the presence of flowers and fruits in all trees studied.4. We found a positive relationship between host basal area per plot and gall abundance and a negative relationship between non‐host basal area and species diversity with gall abundance. We also observed a negative association between gall abundance and the probability of the host plant to have flowers or fruits.5. Our data provide evidence that plant communities in which one host species dominates can be more susceptible to herbivore pressures than communities with less host dominance and more non‐host neighbours, and that the non‐host species create disruptive cues limiting galling insect infestation.