The impact of herbivore regulation on the competitive outcome between Ailanthus altissima and Robinia pseudoacacia
The Enemy Release Hypothesis (ERH) states that the success of nonnative plants is due to reduced regulation from specialized herbivores or pathogens. Nonnative plants therefore have more resources available for growth and reproduction, potentially providing them with a competitive advantage over natives. Most evidence for the ERH does not directly test whether differential herbivory alters the interactions between competing nonnative and native species. We tested whether the outcome of competition between a native and invasive tree (Robinia pseudoacacia and Ailanthus altissima) is influenced by the amount of herbivore regulation experienced by each. We hypothesized that Ailanthus would out-compete Robinia under natural herbivore pressure due to the higher level of herbivore regulation of Robinia. Under reduced herbivore pressure, we hypothesized that Robinia would out-compete Ailanthus. We examined 39 plots at the Blandy Experimental Farm in Virginia. Each plot contained three sapling trees, representing one of four possible combinations of pure and mixed stands of Robinia and Ailanthus (9-10 replicates each) allowing the strength of both intra- and interspecific competition to be measured. Twenty plots were sprayed with pesticide (carbaryl) to reduce herbivory on both species. We measured leaf area damage and relative growth of each tree from June through July.
The results of this study did not support the ERH prediction that differential herbivore regulation accounts for the competitive advantage of the nonnative Ailanthus over the native Robinia. Robinia experienced more herbivore damage (primarily from the locust leaf miner, Odonatata dorsalis) than Ailanthus (damaged primarily by the Ailanthus web worm, Atteva aurea) under natural herbivore pressure, which was expected based on the ERH. Also consistent with the ERH, the carbaryl spray treatment significantly reduced herbivore damage in Robinia but not Ailanthus. We found that Robinia responded to herbivore protection by adding more biomass during the growing season, but Ailanthus growth did not differ between sprayed and unsprayed treatments. Contrary to the ERH, however, reducing herbivore damage did not make Robinia more competitive against Ailanthus. Relative diameter increase, height increase, and relative biomass increase did not differ significantly for either Robinia or Ailanthus trees among plots of different composition and the composition x spray interaction was not significant. These results suggest that Robinia and Ailanthus are relatively equal competitors despite a difference in herbivore regulation, or that if unequal competitors, any differences are not due to a competitive advantage caused by Ailanthus experiencing less herbivore regulation.