In July, Functional Ecology published the Special Feature: A Mechanistic Understanding of Global Change Ecology. We have invited the authors of the papers to write about their paper. In this post, Carly Stevens  writes about her paper Atmospheric nitrogen deposition in terrestrial ecosystems: Its impact on plant communities and consequences across trophic levels: why she wrote it, what she found and where the big gaps are.

A botanist travels up the foodchain

When I was invited to write a paper for the special feature on a mechanistic understanding of global change ecology, I decided to write about impacts of nitrogen deposition on invertebrates and mammals. I am a botanist so writing a paper on the impacts on nitrogen further up the food chain was not the obvious thing to do, and there were times when I felt it was rather a foolish paper to have proposed. However, having worked on the impacts of atmospheric nitrogen deposition for most of my career, I was very aware that, although we have a growing knowledge of the impacts on vegetation and soils, our knowledge of the impacts on consumers is low. I really felt that we needed to know where the gaps are. I also supervise a student based at Rothamsted Research, Tom David, who is investigating the impacts of nitrogen deposition on pollinators and is a co-author on this paper..

What do we know?

I started by looking at primary consumers and I was actually pleasantly surprised; there were a reasonable number of papers out there, not always on nitrogen deposition specifically, but papers in relation to nitrogen addition in some form. There was also a great paper by Nijssen, Wallis, DeVries, and Siepel (2017) which identified the main mechanisms for impact. However, although there were more papers than I was expecting, it was still a small number compared to how many papers there were on plants. There were also a great many knowledge gaps as many papers only investigated one or two species. It quickly became apparent that, via a range of different mechanisms, nitrogen deposition has a great potential to have negative impacts on insect herbivores.

Moving on from herbivores, the number of papers available to review dropped very rapidly. Studies on secondary consumers, whether vertebrate or invertebrate, were few and far between. There is an urgent need to more research in this area.

Moving target

We botanists are lucky, the things we need to identify stay still and in some cases can even be picked and taken away for later identification, so I have the utmost respect for people trying to identify things that zip past at top speed and do their level best to avoid you! Luckily I didn’t need to do practical work for this manuscript. We did include some new analysis, but this was using an existing dataset to analyse plant traits and determine if the traits most important to pollinators were changing. These results showed that plants pollinated by large bees were negatively associated with nitrogen deposition,  and low pH (which can be caused by nitrogen deposition) was associated with lower nectar production, reduced occurrence of plants pollinated by long‐tongued insects and a reduction in plants with larger flowers.

What we don’t know

Overall, undertaking this review showed that we know very little about the impacts of N deposition on primary and secondary consumers, and that for many groups knowledge is patchy at best. But the evidence we have suggests that there are likely to be negative impacts, and we need to know more.

Further reading

If you are interested in learning more about nitrogen deposition the European Nitrogen Assessment will tell you loads about nitrogen, the problems it causes as a pollutant and what we need to do to reduce nitrogen pollution.  To learn more about nitrogen deposition and insect herbivores Throop and Lerdau (2004) is a great place to start.

Read other posts on the mechanisms of global change ecology Special Feature here!