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

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.