Plant invasions are a global concern. However, less is known about how invasive plants directly and indirectly influence native plants across spatial gradients, and how this might subsequently affects associated arthropods. To answer these questions, we examined an invasive plant (Bidens pilosa var. Radiata; hereafter Bidens), a native plant (Rorippa indica; hereafter Rorippa), and associated Pieris butterflies at low (100m) and medium (1000m) altitudes in subtropical Taiwan. Pieris are pollinators of Bidens but herbivores of Rorippa. We conducted four experiments: 1)To examine the Bidens-Rorippa competition across altitude, we set up a laboratory experiment and measured plant performance in Bidens-Bidens, Rorippa-Rorippa, and Bidens-Rorippa pots. Plants, collected as seeds from low or medium altitude, were paired in a pot based on origin (altitude). 2)To identify the interspecific competition mechanisms, we performed laboratory factorial experiments examining above-ground (shading/non-shading) and below-ground (with/without/partial root contact) factors. 3)To investigate the indirect effect of Bidens on Rorippa via attracting Pieris, we manipulated Bidens-Rorippa distance in the field and examined the distance effect on Pieris oviposition on Rorippa. 4)To test whether the impact of Bidens on Rorippa generates a cascading effect on Rorippa’s herbivores, we raised and monitored Pieirs canidia on Rorippa with or without Bidenscompetition.
Results/Conclusions
We found that the invasive Bidens suppressed the native Rorippa at low altitude only (Exp.1), contributed by the below-ground effect of Bidens on Rorippa (e.g., soil chemicals) (Exp.2). The distance between Bidens and Rorippa didn’t affect Pieris oviposition on Rorippa (Exp.3), suggesting no indirect effect of Bidens on Rorippa via pollinator/herbivore (Pieris) attraction. Furthermore, Bidens impact on Rorippa subsequently reduced P. canidia performance (Exp.4), indicating a cascading effect of plant invasion on a higher trophic level. In conclusion, the invasive Bidens suppressed the native Rorippa at low altitude, likely via below-ground mechanisms. This altitude-dependent, negative impact on Rorippa could subsequently cascade up to associated pollinators/herbivores (Pieris butterflies). As plant invasions are ubiquitous and occur across altitude or latitude, the spatial variation of their impact on different trophic levels deserves further investigation.