PS 41-24 - Indirect effects of introduced ungulates on native foliar arthropod herbivory

Background/Question/Methods

Ungulates are important agents of natural selection in different ecosystems. Although the negative effects of introduced ungulates on plant communities have been well documented, few studies have assessed their indirect effects on successive trophic levels. As herbivory by ungulates can alter the nutritional, chemical and structural characteristics of the remaining plant tissues, modifying them as food and refuge for other organisms, indirect effects on arthropod herbivory are expected. The aim of this study was to assess the indirect effect of browsing by introduced ungulates on herbivory by native foliar arthropods, using a very palatable and common pioneer native plant species from northern Patagonian temperate forests, Aristotelia chilensis (Elaeocarpaceae). We grow from seed plants from 7 populations (4 without and 3 with historic presence of introduced ungulates, intact and invaded sites respectively), in a common garden experiment. At the young sapling stage, 126 plants were subjected to herbivory simulation treatments: (i) control - no damage, (ii) mechanical damage (50% of the aerial biomass removed) + addition of ungulates saliva ungulates and (iii) mechanical damage + addition of plant hormone (MeJA: jasmonic acid). After the treatments, all plants were taken to the field, where they were exposed to native arthropod herbivory.

Results/Conclusions

We evaluated the proportion of leaves with signs of herbivory in response to (i) sapling origin (fixed factor with two levels) and (ii) herbivory treatment (fixed factor with three levels), considering the population as random factor using Generalized Linear Mixed Models. We found that herbivory by native arthropods was significantly lower in plants from populations invaded compared to those from intact sites. Likewise, herbivory by native arthropods was significantly lower in plants receiving the mechanical damage + MeJA treatment compared to the other two treatments. We did not find a significant interaction between the two mentioned explanatory variables. Thus, our results support the existence of indirect effects of introduced ungulates on the plant-arthropod herbivore interaction. All plants, regardless of their origin, expressed an induced response after mechanical and hormone damage that made them less susceptible to damage by foliar herbivores. In turn, plants from sites with a history of herbivory by introduced ungulates were less susceptible to damage, including the control treatment, suggesting the existence of constitutive defenses. Overall, our work demonstrates that ungulates are important structuring agents modulating species interactions, even in upper trophic levels.