| Summary: | 碩士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 107 === Climate warming has had tremendous impact on agriculture, a lot of which can be attributed to altered biotic interactions under warming. In agricultural systems, biotic interactions often take on the form of tri-trophic systems consisting of plants, herbivores, and predators. Such systems, however, are often made more complex with the presence of ants, which can play multiple ecological roles. While ants have certain benefits to agriculture, they might also facilitate agricultural pests such as aphids. However, ant–aphid interactions, typically described as mutualism, actually range from mutualism to exploitation, and are contingent upon a variety of biotic and abiotic factors. Moreover, due to their ability to influence herbivores (aphids), the outcome of such interactions may generate trophic cascades on plants (crops). On account of their ecological significance, many studies have investigated ant-aphid interactions and ant–plant cascades. Nonetheless, it remains unclear how ant density affects ant-aphid interactions and thereby plant performance (via trophic cascades), and how these effects are mediated by climate warming. To help fill these knowledge gaps, this study examined 1) the effect of ant density on ant–aphid interactions (i.e., aphid population size), 2) the effect of ant density on plants via trophic cascades (i.e., plant growth, defense, and reproduction), and 3) the impact of climate warming on the aforementioned processes. We designed a tri-trophic system (soybean, aphid, ladybug) with the addition of ants, and conducted a 3x3 factorial experiment including temperature (control, + 3°C, + 6°C) and ant density treatments (0 ants [control], 15 ants, 30 ants). We recorded the number of soybean aphids (Aphis glycines), along with the growth, defense, and reproductive traits of soybean plants (Glycine max). The study system included seven-spotted ladybugs (Coccinella septempunctata) as aphid predators and tropical fire ants (Solenopsis geminata) as the aphids’ ant mutualists. Our results show that ant density, overlooked in previous studies, played an important role in ant–aphid interactions and ant–plant cascades. Higher ant density increased aphid number (mutualism), which likely resulted in trophic cascades that reduced leaf number, increased leaf toughness, and possibly reduced pod number and seed number. However, these trends became less obvious under warming. This was probably because warming generated a strong, positive direct effect on aphid population, which may have reached the carrying capacity of soybeans, thus obscuring the effect of ant density. Surprisingly, higher ant density increased plant height, possibly due to some direct ant effect instead of trophic cascades. In addition, warming had a strong direct impact on plant growth, though not reproduction; these effects ranged from positive to negative. Since the effect of biotic interactions (mutualism, trophic cascades) diminished under warming, it is likely that these direct warming effects became the primary influence on plant performance. To conclude, this study showed that warming reduced the effects of biotic interactions (mutualism and trophic cascades), suggesting that abiotic effects (warming) may become the dominant factor in determining crop performance.
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