Too much of a good thing? Understanding the effects of anthropogenicnitrogen deposition on insect herbivores.

氮(N)是地球上最丰富的元素之一，但其有效性限制了大多数陆地生态系统的生产力1，2这是因为只有活性形式的氮(从惰性氮气进入生物圈，通过固氮菌进入生物圈，以及在光照下氧化)可供生物合成蛋白质以供生长。然而，人类活动显著增加了活性氮的有效性，(主要是通过化石燃料燃烧过程中产生的氧化亚氮和氮基化肥中的氨化合物)3，自20世纪中期以来，活性氮在陆地生态系统中的沉积速度翻了一番4，并取消了对其生产力的限制。5到目前为止，大多数关于活性氮素生态系统结构大幅增加的影响的研究都集中在植物群落6，7，并开始出现明显的模式8，9最初，氮有效性的增加导致植物生产力和生物量的增加，以及日益密集、均匀的植被11。这导致对其他资源的激烈竞争，随着土壤氮素水平的增加，能够应对土壤化学变化的快速生长物种开始主导植物群落，往往导致其总体物种丰富度下降。这些变化在沉积水平最高的欧洲植物群落中得到证明14，最近被证明正在形成大空间尺度上的物种出现和丰度模式15。很明显，氮沉积是导致植物群落变化的一种强大而普遍的驱动力，但榴弹炮效应会沿着食物链向上传播到消费者那里，但目前人们对它们的了解要少得多。昆虫是几乎所有陆地生态系统结构和功能的关键16，反过来又具有巨大的经济重要性17，18。此外，由于它们的世代时间短，生态位狭窄--通常以特定的植物或植物部分为食--食草动物很可能是对氮沉积引起的变化最敏感的消费者之一。19.虽然传统上认为它们基本上是氮素受限的物种，因此可能会从氮素供应的增加中受益，对昆虫监测数据的少量分析表明，对沉积的反应更为复杂，一些物种受益，另一些物种下降22-25。驱动这些变化的机制仍然很难理解7，随着人为氮沉积的预计增加和扩大26，更全面地了解氮沉积可能如何以及为什么影响昆虫种群是至关重要的。考虑到目前关于昆虫数量下降的不确定性27-29尤其如此，这似乎正在发生在栖息地32和更广泛的农村地区的常见物种30，31，与扩散的大规模应激源的影响一致，如气候变化和氮沉积25。目的和目的我的目的是评估氮沉积作为昆虫生物群落变化的驱动因素的作用，并对这些变化背后的拟议机制进行实验测试(重点放在被认为最容易受到氮沉积影响的草原生境33)。然后，我将尝试测试这些机制的更广泛的预测，并研究它们如何在更大的空间尺度上影响昆虫食草动物群落和种群。

Nitrogen (N) is one of the most abundant elements on earth, yet its availability limits the productivity of most terrestrial ecosystems1,2. This is because onlyreactive forms of nitrogen(enteringthe biosphere from inert nitrogen gas through nitrogen-fixing bacteria and oxidation in lighting) are available to living organisms for synthesising proteins for growth. Human activity,however, has significantly increased the availability of reactive nitrogen,(mostly through the production of nitrous oxidesduringfossil fuel combustion, and ammonia compoundsfrom N-based fertilisers)3, doubling its rate ofdeposition into terrestrial ecosystems since the mid -20thcentury4, and lifting limits on theirproductivity5.So far, most studies of the effects of this massive increase in reactive nitrogenon ecosystem structurehave focused on plant communities6,7, and clear patterns are beginning to emerge8,9. Initially, an increase in nitrogen availability leads to an increase in plant productivity10andbiomass, and increasingly dense, homogeneous vegetation11. This resultsinintense competition for other resources, and as soil nitrogen levels increase, fast-growing speciesable to cope with changing soil chemistrycome to dominate plant communities, often causinga reduction intheir overallspecies richness.These changes have been demonstrated across plant communities in Europe12,13,where deposition levels are highest14,and have recently been shownto be shaping patterns of species occurrence and abundance at large spatial scales15. While it isclear then thatnitrogen deposition represents a powerful and pervasive driver of changes to plant communities, howits effects propagate up the food chainto consumersiscurrentlymuch less well-understood6.Insects are key to the structure and function of almost all terrestrial ecosystems 16, andin turn of enormous economic importance17,18. Furthermore, with their short generation times and narrow niches -often feeding on specific plants or parts of plants, herbivorous species are likely to be among the most sensitive consumers to changes caused by nitrogen deposition19.While they are traditionally regarded as being mostly nitrogen-limited20,21, and therefore likely to benefit from an increase in nitrogen availability, what little analysis there has been of insect monitoring datasuggests a more complex response to deposition,with some species benefiting, and others declining22-25. The mechanisms driving these changes remain poorly-understood7, and with the projected increase and expansion of anthropogenic nitrogen deposition26, a more comprehensive understanding of how and why nitrogen deposition may beaffecting insect populations is essential.This is particularly true giventhe current uncertainty aboutreported insect declines27-29, which appear to be occurringnowin common species across habitats30,31, both within protected areas32and the wider countryside30, consistent with the effects of diffuse, large-scale stressors, like climate change and nitrogen deposition25.Aims and ObjectivesI aim to assess the role of nitrogen deposition as a driver of changes to insectherbivorecommunities, and perform experimental tests oftheproposedmechanisms behind these changes(focusing on the grassland habitats which are thought to be most vulnerable to the effects of N deposition33). I will then attempt to test the broader predictions of these mechanisms and examinehow they areaffecting insect herbivore communities and populations at largerspatial scales.