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，但它的可用性限制了大多数陆地生态系统的生产力。这是因为只有反应形式的氮（从惰性氮气进入生物圈，通过固氮细菌和光照氧化）可用于生物体合成生长所需的蛋白质。然而，人类活动大大增加了活性氮的可用性，（主要是通过化石燃料燃烧过程中产生的一氧化二氮和氮基肥料中的氨化合物）3，自20世纪中期以来，其沉积到陆地生态系统的速度翻了一番4，并解除了对它们生产力的限制5。大多数关于活性氮大量增加对生态系统结构的影响的研究都集中在植物群落上6，7，并且开始出现清晰的模式8，9。最初，氮可用性的增加导致植物生产力10和生物量的增加，以及越来越密集，均匀的植被11。这导致了对其他资源的激烈竞争，随着土壤氮含量的增加，能够科普土壤化学变化的快速生长的物种开始主导植物群落，通常导致其整体物种丰富度的减少。这些变化已经在欧洲的植物群落中得到了证明12，13，那里的沉积水平最高14，并且最近被证明在大的空间尺度上正在形成物种发生和丰度的模式15。虽然很明显，氮沉降是植物群落变化的一个强大而普遍的驱动力，但其影响如何传播到消费者的食物链目前还不太清楚6。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，草食性物种的世代时间短，生态位窄，通常以特定的植物或植物的一部分为食，因此可能是对氮沉积引起的变化最敏感的消费者之一19。对昆虫监测的少量分析表明，对沉积物的反应更为复杂，一些物种受益，另一些物种减少22 -25。驱动这些变化的机制仍然不太清楚7，随着人为氮沉降的预计增加和扩大26，更全面地了解氮沉降如何以及为什么会影响昆虫种群是必不可少的。考虑到目前关于昆虫减少的报道27 -29的不确定性，这一点尤其如此，这些昆虫减少似乎发生在所有栖息地的常见物种30，31，无论是在保护区32和更广泛的农村30，符合扩散的影响，大规模的压力，如气候变化和氮沉积25。目的和目标我的目的是评估氮沉积的作用，作为一个驱动器的变化，以昆虫食虫动物群落，并对这些变化背后的拟议机制进行实验测试（重点是被认为最容易受到氮沉积影响的草原栖息地33）。然后，我将尝试测试这些机制的更广泛预测，并研究它们如何在更大的空间尺度上影响食草昆虫群落和种群。

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.