Ecosystem engineering by a gall‐forming wasp indirectly suppresses diversity and density of herbivores on oak trees

Abstract

Ecosystem engineers, organisms that modify the physical environment, are generally thought to increase diversity by facilitating species that benefit from engineered habitats. Recent theoretical work, however, suggests that ecosystem engineering could initiate cascades of trophic interactions that shape community structure in unexpected ways, potentially having negative indirect effects on abundance and diversity in components of the community that do not directly interact with the habitat modifications. We tested the indirect effects of a gall‐forming wasp on arthropod communities in surrounding unmodified foliage. We experimentally removed all senesced galls from entire trees during winter and sampled the arthropod community on foliage after budburst. Gall removal resulted in 59% greater herbivore density, 26% greater herbivore richness, and 27% greater arthropod density five weeks after budburst. Gall removal also reduced the differences in community composition among trees (i.e., reduced beta diversity), even when accounting for differences in richness. The community inside galls during winter and through the growing season was dominated by jumping spiders (Salticidae; 0.87 ± 0.12 spiders per gall). We suggest that senesced galls provided habitat for spiders, which suppressed herbivorous arthropods and increased beta diversity by facilitating assembly of unusual arthropod communities. Our results demonstrate that the effects of habitat modification by ecosystem engineers can extend beyond merely providing habitat for specialists; the effects can propagate far enough to influence the structure of communities that do not directly interact with habitat modifications.

Interaction engineering: Non‐trophic effects modify interactions in an insect galler community

“…Species can modify interactions via a trait change on one of the interacting species (predator causing fear on herbivore; plant‐mediated interactions, etc; Hammill et al, ; Prasad & Snyder, ) or via a change in the context of the physical environment (biotic and abiotic) where a pair of species interact. In the literature on interaction modification, empirical demonstrations of environment‐mediated interaction modification (Wootton, ) are very scarce and mostly restricted to herbivores modifying habitat structure with consequences for refuge availability, and as a result mediating predation risk for other herbivores (e.g., Pagè et al, ) Research on ecosystem engineering has mostly focused on effects of habitat creation on nodes (mainly species density; Prugh & Brashares, ; Wetzel et al, ) rather than effects on interactions (environment‐mediated interaction modification; Pagè et al, ). To the best of our knowledge, no previous work has looked at feedbacks of engineer effects via environment‐mediated interaction modifications.…”

“…All of these insect and spider species interacting directly or indirectly with the host plant were able to contribute to the effects demonstrated here. The focal galler, Baccharopelma dracunculifoliae (Sternorrhyncha: Psyllidae), is extremely abundant and gall induction can be viewed as allogenic ecosystem engineering (action of the engineer on other living or nonliving structure; Jones et al, ; Wetzel et al, ). The galls remain attached to the plant for a few months after emergence of the galler and gradually become dry and woody.…”

Modification of plant‐induced responses by an insect ecosystem engineer influences the colonization behaviour of subsequent shelter‐users

“…These ecosystem engineers are thought to alter herbivore diversity and abundance (Fukui ; Lill & Marquis ; Wetzel et al . ; Cornelissen, Cintra and Santos ) by providing novel habitats and shelters from natural enemies and abiotic stresses (Kudo ; Larsson, Häggström & Denno ; Fukui ; Danks ; Crawford, Crutsinger & Sanders ; Marquis & Lill ). Insect ecosystem engineers are also known to manipulate plant metabolism for their own benefit (Sandberg & Berenbaum ; Sagers ; Marini‐Filho & Fernandes ).…”

Blowing in the wind: a unique aerial descent on a leaf in a caterpillar

“…, Wetzel et al. ). In support of this hypothesis, I observed relatively low parasitism and no predation.…”