入侵植物(大花咸豐草)如何在不同海拔影響本土植物(葶藶)和相關蝴蝶?

Abstract

入侵植物是全球重要的生態議題，然而我們對於入侵植物在不同海拔是否會對本土植物造成直接和/或間接的影響，以及入侵植物對本土植物的影響是否會進一步影響到更高階的營養階層仍不清楚。本研究探討以上議題利用台灣可發現的物種：入侵植物大花咸豐草(Bidens pilosa var. radiata)，本土植物葶藶(Rorippa indica)，和紋白蝶。大花咸豐草是紋白蝶(Pieris)的蜜源植物，葶藶則是紋白蝶的寄主植物。本研究進行以下實驗：（1）野外調查—為了解大花咸豐草與葶藶在野外可能的直接與間接交互關係如何隨海拔變化，我們在低、中海拔（100、1000 m）各三個地點測量兩植物的間距是否和葶藶的生長表現，及葶藶上的紋白蝶卵數有相關性。（2）操控實驗一—為了解大花咸豐草如何在不同海拔下直接影響葶藶的生長，我們自低、中海拔收集兩種植物的種子，並依以下三組處理種植於模擬低海拔(25 oC ~ 20 oC）和中海拔溫度(20 oC ~ 15 oC）的人工氣候室：兩株大花咸豐草、一株大花咸豐草及一株葶藶、兩株葶藶種在一起。(3) 操控實驗二—為進一步檢測大花咸豐草直接影響葶藶的地上與地下部機制，我們利用類似實驗一的方法將大花咸豐草與葶藶共植，操控並探討大花咸豐草如何在不同海拔透過地上部(遮蔽)和地下部競爭(根部接觸)影響葶藶的生長。(4) 操控實驗三—為了解大花咸豐草是否會透過吸引紋白蝶而間接的影響葶藶，我們在低、中海拔擺放等距的植株，檢驗距離大花咸豐草越近的葶藶是否會有較多的紋白蝶卵，進而造成葶藶的傷害。(5) 操控實驗四—為探討大花咸豐草對葶藶的影響是否會進一步影響到更高階的生物，我們將台灣紋白蝶 (Pieris canidia)（取食葶藶的草食動物）飼養在有或沒有受到大花咸豐草競爭影響的葶藶上，並監測台灣紋白蝶的生長表現。野外調查結果暗示大花咸豐草與葶藶在野外可能有直接與間接的交互關係，然而這些關係可能隨著月份和海拔改變。進一步的研究室操控實驗結果顯示，大花咸豐草在低海拔會直接抑制葶藶的生長(實驗一)，可能的機制是直接透過地下部非物理性的接觸(實驗二)。雖然大花咸豐草似乎不會透過紋白蝶而間接影響葶藶(實驗三)，然而大花咸豐草對葶藶的負面影響會延伸到葶藶的草食動物上，降低台灣紋白蝶的生長表現(實驗四)。這些結果顯示入侵植物可在不同海拔對本土植物造成不同的直接影響，並且進一步影響到更高階的營養階層。

Plant invasion changes invaded communities and has become a global concern. However, little is known about how invasive plants directly and indirectly influence native plants across spatial gradients, and how this might create cascading effects on the associated arthropods. To answer these questions, we examined an invasive plant (Bidens pilosa var. radiata; hereafter B. p. r.), a native plant (Rorippa indica; hereafter R. i), and associated Pieris butterflies across altitude (100 and 1000 m). Pieris are pollinators of Bidens but herbivores of Rorippa. We conducted following field and laboratory studies: 1) Field survey - To explore the field relationship between Bidens and Rorippa (e.g., direct and indirect effect of Bidens on Rorippa), we surveyed plant height vs. distance across altitude. 2) Experiment 1 - To examine the direct effect of Bidens on Rorippa across altitude, we conducted a laboratory competition experiment and measured plant performance in the Bidens-Bidens, Rorippa-Rorippa, and Bidens-Rorippa treatment groups. Plants, germinated from seeds collected from low and medium altitude, were cultivated in a greenhouse simulating low- and medium-altitude temperature (25-20 and 20-15 oC [day-night], respectively). 3) Experimental 2 - To explore the competition mechanisms, we performed an above-ground (shading) x below-ground (root contact) experiment in laboratory, with detailed methods similar to those in Experiment 1. Soil chemicals were analyzed. 4) Experiment 3 - To investigate the indirect effect of Bidens on Rorippa via Pieris across altitude, we examined whether the distance between these two plants would affect Pieris oviposition on Rorippa, using potted plants with fixed interval. 5) Experiment 4 - To understand whether the impact of Bidens on Rorippa would have a cascading effect on Rorippa’s herbivores, we raised and monitored Pieirs canidia on Rorippa with or without grown with Bidens competition (low altitude condition only). Our field survey results implied direct and indirect effects of Bidens on Rorippa, which varied with seasons and altitude. The laboratory competition experiment further showed that the invasive Bidens directly suppressed the native Rorippa at low altitude condition (Exp.1), contributed by the below-ground effect of Bidens (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 Pieris. Furthermore, Bidens impact on Rorippa reduced P. canidia performance (Exp.4), indicating a cascading effect of plant invasion. In conclusion, the study reveals that the negative impact of invasive plants (e.g., Bidens) on native ones (e.g., Rorippa) can act through below-ground mechanisms and vary with altitude. In addition, the negative impact of invasive on native plants may cascade up to a higher trophic level (e.g., native plants’ herbivores). These cascading effects may be common in various systems and deserve further investigations.