RESUMO
Temporal fluctuation in nutrient availability generally promotes the growth of exotic plant species and has been recognized as an important driver of exotic plant invasions. However, little is known about how the impact of fluctuating nutrients on exotic species is dependent on the availability of other resources, although most ecosystems are experiencing dramatic variations in a wide variety of resources due to global change and human disturbance. Here, we explored how water availability mediates the effect of nutrient pulses on the growth of six exotic and six native plant species. We subjected individual plants of exotic and native species to well watered or water stressed conditions. For each level of water availability, we added equivalent amounts of nutrients at a constant rate, as a single large pulse, or in multiple small pulses. Under well watered conditions, nutrient pulses promoted exotic plant growth relative to nutrients supplied constantly, while they had no significant effect on natives. In contrast, under water stressed conditions, water deficiency inhibited the growth of all exotic and native species. More importantly, nutrient pulses did not increase plant growth relative to nutrients supplied constantly and these phenomena were observed for both exotic and native species. Taken together, our study shows that the impact of fluctuating nutrient availability on the growth of exotic plant species strongly depends on the variation of other resources, and that the positive effect of nutrient pulses under well watered conditions disappears under water stressed conditions. Our findings suggest that the variation in multiple resources may have complex feedback on exotic plant invasions and, therefore, it is critical to encompass multiple resources for the evaluation of fluctuating resource availability effects on exotic plant species. This will allow us to project the invasive trajectory of exotic plant species more accurately under future global change and human disturbance.
RESUMO
Plants are often exposed to multiple herbivores and densities of these attackers (or corresponding damage intensities) often fluctuate greatly in the field. Plant-mediated interactions vary among herbivore species and with changing feeding intensity, but little is known about how herbivore identity and density interact to determine plant responses and herbivore fitness. Here, we investigated this question using Triadica sebifera (tallow) and two common and abundant specialist insect herbivores, Bikasha collaris (flea beetle) and Heterapoderopsis bicallosicollis (weevil). By manipulating densities of leaf-feeding adults of these two herbivore species, we tested how variations in the intensity of leaf damage caused by flea beetle or weevil adults affected the performance of root-feeding flea beetle larvae and evaluated the potential of induced tallow root traits to predict flea beetle larval performance. We found that weevil adults consistently decreased the survival of flea beetle larvae with increasing leaf damage intensities. In contrast, conspecific flea beetle adults increased their larval survival at low damage then decreased larval survival at high damage, resulting in a unimodal pattern. Chemical analyses showed that increasing leaf damage from weevil adults linearly decreased root carbohydrates and increased root tannin, whereas flea beetle adults had opposite effects as weevil adults at low damage and similar effects as them at high damage. Furthermore, across all feeding treatments, flea beetle larval survival correlated positively with concentrations of carbohydrates and negatively with concentration of tannin, suggesting that root primary and secondary metabolism might underlie the observed effects on flea beetle larvae. Our study demonstrates that herbivore identity and density interact to determine systemic plant responses and plant-mediated effects on herbivores. In particular, effects are species-specific at low densities, but converge at high densities. These findings emphasize the importance of considering herbivore identity and density simultaneously when investigating factors driving plant-mediated interactions between herbivores, which advances our understanding of the structure and composition of herbivore communities and terrestrial food webs.