Negative indirect effects of an ecosystem engineer on seasonal assembly in non-engineered community

Background/Question/Methods

Ecosystem engineers, organisms that modify the physical environment, are generally thought to increase diversity and abundance of organisms at landscape scales by increasing habitat heterogeneity and facilitating specialists of engineered habitat. A remaining challenge is understanding the effects of ecosystem engineering on community assembly in adjacent unmodified habitats. It has been hypothesized that ecosystem engineers could have indirect negative effects on diversity and abundance in unmodified habitats if engineered habitats facilitated predators or competitors that interacted with species in both habitat types, but this has not been experimentally confirmed at the community level. We tested the indirect effects of a gall-forming wasp (Andricus quercuscalifornicus) on the seasonal assembly of the foliage-dwelling arthropod community on valley oak (Quercus lobata) by experimentally removing all galls from entire trees shortly before budburst and sampling the arthropod community on removal treatment and control trees one week before and two and five weeks after gall removal. We also sampled secondary-users of galls at the time of gall removal by collecting and dissecting galls.

Results/Conclusions

Gall removal before budburst led to 59% greater herbivore abundance, 26% greater herbivore richness and 27% greater total arthropod abundances. Gall removal also led to significantly lower beta-diversity among trees, and this was true even when accounting for differences in richness between treatments. The gall habitat was dominated by salticid spiders with an average of 0.87 (±0.12 SE) spiders per gall. This suggests galls on oak trees provided overwintering habitat for spiders, which, later in the growing season, suppressed herbivorous arthropods and increased beta-diversity by facilitating assembly of unusual arthropod communities. This work confirms that ecosystem engineers can indirectly suppress richness and abundance in the subset of a community that does not directly interact with an engineer’s physical modifications even while increasing beta-diversity at the landscape scale. This suggests that habitat modifications have the potential to influence seasonal community assembly trajectories.