暖化試驗對作物產量、害蟲族群、及生物防治效果的影響：以大豆─蚜蟲─瓢蟲系統為研究範例

Abstract

氣候暖化影響生物群聚的例子近年來不斷的增加，在未來暖化的程度將更趨顯著下，我們需要了解暖化影響生物群聚的機制，以幫助預測未來暖化對生態系統造成的衝擊。本研究探討暖化影響生物群聚(含三個營養階層)的機制，主要著重在暖化如何影響各營養階層，及營養階層間的交互作用。我們探討3種溫度處理(18.5℃(控制組，為研究區域農夫種植大豆時的月均溫)、21.5℃(暖化3℃)、24.5℃(暖化6℃))下的大豆產量、蟲害(大豆蚜)、及生物防治效果(七星瓢蟲)；而每溫度下設3種營養階層處理，以反映不同的營養階層結構：1) 大豆；2) 大豆和大豆蚜；3) 大豆、大豆蚜、及七星瓢蟲。結果顯示暖化可顯著影響大豆的一些形質，且此影響會取決於營養階層結構，例如：1) 若營養階層僅含大豆，暖化可直接增進大豆發育速率、繁殖生物量分配的比率及種子產量；2) 若營養階層含大豆和大豆蚜，暖化會增加蟲害(蚜蟲)，進而減少大豆產量 ；3) 若營養階層含大豆、大豆蚜、及七星瓢蟲，暖化會增加瓢蟲生物防治的成效(控制蚜蟲族群)，進而增加大豆的產量，展現出顯著的”營養瀑布”(瓢蟲至大豆)。大豆產量提高主要原因在累積較多的繁殖器官生物量，而非先累積了較高的營養器官生物量，而產量變化主要反映在種子數上而非種子粒重及種子碳氮元素比。對照種子產量的結果，暖化對大豆生長發育，及大豆防禦的影響和營養階層結構無關。另外，植物的三種物理或化學防禦對暖化或是營養階層處理有不同的反應：葉表絨毛密度隨暖化增加；葉片韌度不受暖化影響，但會隨蚜蟲感染處理而降低；而葉片總酚含量則在各實驗處理間無差異。這些結果顯示暖化很有可能經由影響營養階層及(或)其間的交互作用，進而影響作物產量、害蟲族群動態、及蟲害生物防治的成效。

As climate warming has been increasingly reported to affect communities, there is a need to understand its underlying mechanisms in order to help predict the outcome of future warming, which is projected to become more severe than the current one. This study investigated the mechanisms through which warming may affect a sub-tropical tri-trophic agricultural community, focusing on warming effects on each trophic level and trophic interactions. In specific, this study examined how experimental warming would affect plants (the soybean Glycine max), herbivores (the aphid Aphis glycines), predators (the ladybug Coccinella septempunctata), and their interactions in three environmental chambers (18.5, 21.5, 24.5°C). The chamber at 18.5°C served as control, reflecting the average monthly temperature when local farmers grew soybeans in our study region. The temperature of 21.5 and 24.5°C represented a 3 and 6°C warming by the year of 2100, respectively, based on IPCC predictions. Each chamber included three treatments, representing systems with different trophic structure: 1) soybeans; 2) soybeans and aphids; 3) soybeans, aphids, and ladybugs. Our results showed that the impact of warming on soybeans was strong and could be trophic-structure dependent in some cases. For example, warming impact on seed production was trophic-structure dependent: 1) In the system with soybeans only, warming increased soybean developmental rate, reproductive investment (i.e. reproductive / vegetative biomass), and seed production. 2) In the system with soybeans and aphids, warming increased the top-down control of plants by herbivores (aphids) and reduced seed production relatively. 3) In the system with soybeans, aphids, and ladybugs, warming increased the top-down control of aphids by ladybugs, yielding a stronger trophic cascade (from predators to plants) and higher soybean production. Soybean seed yield changes were mainly due to a higher investment in reproductive mass, but not the vegetative part, and in seed number, rather than the weight per seed or seed C/N ratio. Contrary to the effect on seed production, warming impact on many soybean growth, developmental, and defensive traits were trophic-structure independent. As for plant physical and chemical defense, three soybean defensive traits responded differently in various treatment combinations: leaf trichome density increased under warming; leaf toughness was not affected by warming but decreased in aphid-only treatment (Tro2); soybean total phenolics remained constant across temperature or trophic structure treatments. These results above suggest that climate warming will likely affect crop production, pest population dynamics, and biocontrol effectiveness, via warming effects on specific trophic levels and/ or trophic interactions.