暖化將如何影響本土及外來種紋白蝶於二種寄主植物上的競爭關係?

Abstract

氣候暖化會直接或間接地影響植物-植食者的交互關係。過去已有許多研究探討暖化如何透過直接或間接的效應，進而影響植物與植食者。但較少有研究檢驗暖化如何同時透過直接與間接機制，進而影響植物-植食者的交互關係。除了暖化因素以外，入侵物種也會對現有的植物-植食者交互關係產生重要的衝擊，比如入侵植食者與原生植食者可能會競爭食物資源。然而，原生物種與入侵物種的競爭關係將如何被暖化所影響，則是有待釐清的重要科學議題。 為探討暖化如何同時直接或間接地影響植物與植食者的關係，以及探討暖化是否會改變現有原生種與入侵種的競爭狀態，本研究採用台灣的兩種紋白蝶(粉蝶科)-代表原生物種的緣點白粉蝶(Pieris canidia)與代表入侵物種的菜粉蝶(P. rapae)，以及這2種幼蟲皆會利用的十字花科寄主植物-葶藶(Rorippa indica)與結球甘藍(Brassica oleracea var. capitata)，置於3種溫度設定(18.5, 21.5, 24.5°C)的生長箱內進行實驗。18.5的生長箱代表控制組的基準溫度，反應台灣北部紋白蝶野外密度進入高峰期的月均溫。而21.5與24.5°C則分別代表聯合國預測未來100年氣後暖化3 與6 °C的情境。每台生長箱包含4種實驗處理-a) 寄主植物加上4隻緣點白粉蝶幼蟲、b) 寄主植物加上2隻緣點白粉蝶幼蟲與2隻菜粉蝶幼蟲、 c) 寄主植物加上4隻菜粉蝶幼蟲，以及d) 僅放置寄主植物於實驗設施內。 結果顯示兩種寄主植物產量與品質特徵皆會受暖化影響，但兩種植物改變的趨勢並不完全一致；例如葶藶與結球甘藍皆會在暖化下增加葉片數量，但葶藶會隨暖化減少葉片的碳氮比(植食者重要的食物品質指標)，而結球甘藍的碳氮比則不隨暖化改變。而兩種植食者在暖化下的表現，會取決於寄主植物的種類：在葶藶上，兩種紋白蝶皆隨暖化而縮短幼蟲期長度、增加體重與體型；而暖化可能藉由間接效應，造成植食者有較大的體型 (例如增加植物的含氮量，間接助益紋白蝶)。在結球甘藍上，暖化會加速幼蟲的發育，然而會減小緣點白粉蝶的體型。此外，暖化對兩種紋白蝶種間競爭的影響，也會因不同種類的寄主植物而異。葶藶上原生種的緣點白粉蝶，在基準溫度下擁有種間競爭的優勢；然而，暖化會消除此爭優勢。結球甘藍上的入侵種紋白蝶(菜粉蝶)擁有種間競爭優勢，而此優勢在暖化下仍可維持，與葶藶上的結果不同。 本研究顯示暖化可同時透過直接與間接的機制改變植食者(例如紋白蝶)的表現，比如暖化可直接影響植食者的生長與發育，亦可透過影響寄主植物的品質、或影響植食者間的競爭關係，進而間接地影響植食者的表現。本研究亦顯示，暖化可以改變原生種與入侵種的現有競爭狀態，鑑於物種入侵的全球普遍化，這代表暖化可能會透過此途徑影響世界上許多動植物的群聚。

Climate warming can affect plant-herbivore interactions directly or indirectly. Numerous studies have demonstrated direct or indirect warming effects separately, but relative few studies examine direct and indirect warming effects simultaneously on plant-herbivore interactions. Besides warming, invasive species also significantly affect plant-herbivore interactions, such as via their competition against native herbivores for food resource. However, little is known about how the competition between native and invasive species might be affected by climate warming. To understand how warming will directly and indirectly affect plant-herbivore interactions, and whether warming will change the competitive status between native and invasive herbivores, this study examined two competing butterfly species of Taiwan (the native Pieris canidia and the invasive P. rapae) on their two common host plants- variableleaf yellowcress (Rorippa indica) and cabbage (Brassica oleracea var. capitata) in 3 environmental chambers (18.5, 21.5, 24.5°C). The chamber at 18.5 °C served as control, reflecting the average monthly temperature when both butterflies became abundant in the field; the one at 21.5 or 24.5 °C simulated 3 or 6 °C warming, respectively. Each chamber included four treatments, allowing us to examine the intra- and inter-specific competition: a) 4 P. canidia, b) 2 P. canidia and 2 P. rapae, c) 4 P. rapae larvae on a host plant, and d) a host plant only. As for the results in plant traits, warming affected the quantity and quality of both host plants in a similar or different fashion. For example, warming increased the leaf number in both R. indica and B. oleracea plants. However, warming reduced the C/N ratio (an important plant quality index for herbivore performance) in R. indica but not in B. oleracea. As for the performance of each herbivore species, warming effects were host plant dependent. In the R. indica plant system, warming increased the developmental rate and body size of both Pieris species. The larger body size could be contributed by an indirect warming effect (i.e. warming increased plant nitrogen content and consequently, indirectly benefited Pieris). In the cabbage plant system, warming, however, increased the developmental rate of both Pieris and reduced the body size of P. canidia.. As for herbivore competition, warming effects was host plant dependent. In the R. indica plant system, the native P. canidia, facing stronger intraspecific than interspecific competition, had a competitive advantage over the invasive P. rapae under ambient temperature. However, the advantage for the native Pieris disappeared under warming. In the cabbage plant system, the invasive P. rapae had a competitive advantage over the native P. canidia. The advantage for the invasive Pieris sustained under warming, differently from the pattern in Rorippa indica plant system. This study shows that warming may affect performance of herbivores (i.e. Pieris) directly via growth and development, and indirectly via host plant quality or the competitive outcome against other herbivores. This study also suggests that warming could shift the current competitive status between native and invasive herbivores (e.g. P. canidia vs. P. rapae), likely changing many plant and animal communities worldwide.