| Summary: | 碩士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 103 === 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.
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