Indirect interactions and community processes: impacts of hemi-parasitic plants on the structure of insect herbivore communities.

黄铃虫是一种寄生植物，通过根连接从其他植物(寄主植物)那里窃取资源。水分和营养物质从寄主转移到黄拨鼠体内，减少了寄主的生长。这可能会使寄主不太适合同样生活在寄主植物上的昆虫，特别是那些以与黄拨鼠相同的水和养分供应为食的昆虫。之前在温室里的实验表明，以感染了黄铃虫的寄主植物为食的昆虫死亡的可能性是以未受感染的寄主植物为食的昆虫的两倍，但没有人在有许多不同昆虫和植物的自然群落中测量这些影响。除了使寄主植物不太适合昆虫食用外，黄铃虫还可以减少昆虫的寄主植物数量-它更喜欢感染草，所以在黄铃虫多的地方，草的数量会减少，食草的昆虫也可能会减少。这还没有得到测试，科学家也不知道是草量的变化还是寄主植物适应性的变化对昆虫产生了最大的影响。我们计划找出黄鼠是如何影响生活在同一生态群落中的昆虫食草动物的数量和类型的。我们预计以草为食的昆虫种类将会减少，但当黄铃虫出现时，以未受黄铃虫攻击的寄主植物为食的昆虫将会增加。我们认为黄铃虫将通过两种方式造成这些影响：(1)通过改变生活在群落中的植物物种的平衡；(2)通过改变受感染植物对昆虫的适应性。我们将调查这些影响中哪一个是最重要的。我们还将测试个别昆虫物种在有和没有黄斑潜蝇存在的植物上生长和繁殖的能力，并找出昆虫是否避免以受感染的植物为食。

Yellow-rattle is a parasitic plant that steals resources from other plants (host plants) via root connections. Water and nutrients are diverted from the host into the yellow-rattle, reducing the growth of the host. This may make the host less suitable for insects which also live on the host plant, particularly those that feed on the same water and nutrient supply as the yellow-rattle. Previous experiments in greenhouses have shown that insects which are fed on host plants infected with yellow-rattle are twice as likely to die than those feeding on uninfected hosts, but no one has measured these effects in natural communities where there are many different insects and plants. As well as making host plants less suitable for insects to feed on, yellow-rattle can also reduce the number of host plants for the insects - it prefers to infect grasses so in places where there is a lot of yellow-rattle the amount of grass decreases and grass-feeding insects may also decrease. This has not been tested yet, nor do scientists know whether it is the changes in the amount of grass or the changes in the suitability of the host plants which have the biggest effect on insects. We plan to find out how yellow-rattle affects the number and type of insect herbivores that are living in the same ecological community. We expect that grass-feeding species of insects will be reduced in number, but insects feeding on host plants that are not attacked by yellow-rattle will increase when the yellow-rattle is present. We think yellow-rattle will cause these effects in two ways: (1) by changing the balance of plant species living in the community and (2) by changing the suitability of the infected plants for insects. We will investigate which of these effects is the most important. We will also test the ability of individual insect species to grow and reproduce on plants with and without yellow-rattle present and find out whether insects avoid feeding on infected plants.