Valliere, J.M. et al. 2017. High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought. Global Change Biology 23: 4333-4345. https://doi.org/10.1111/gcb.13694
Earlier this year, I came across the best quote on nitrogen pollution I had heard during my five years of PhD research into nitrogen effects on natural ecosystems. The quote is from the ecologist Mark Sutton, who’s had a long and successful career studying the effect that nitrogen pollution has on nature. When explaining the severity and stealth of the nitrogen impact, he uses a colorful analogy: “It’s like the godfather of pollution: you see the results but you don’t see the godfather.”1
Nitrogen deposition – the most commonly used term for human-driven increases in nitrogen availability in natural ecosystems – is one of the main threats to biodiversity in areas of high conservation value. The excess nitrogen is introduced into the environment mainly through the combustion of fossil fuels, and the excessive use of mineral fertilizers in agriculture. Nitrogen is an essential nutrient for plants, and in our gardens we might worry that our plants don’t get enough of it. But in nature, too much of it can lead to problems – not only for the plant itself, but for the whole ecosystem.
Too much pressure: when nitrogen deposition and climate change happen at the same time
The first and most well-documented consequence of nitrogen deposition is a shift in the performance of different plant species. Plants that grow quickly, such as many grass species, will get a disproportionate boost compared to their slower-growing competitors, and can start shading and eventually outcompeting their neighbors2. The result is most often a plant community with fewer species3. What happens after this reduction in plant diversity is difficult to map, given how massively interlinked species interaction networks can be in nature. For example, we know very little about how animal diversity responds to these nitrogen-induced changes in the plant community, especially at higher levels of the food chain4.
One big reason to start thinking more about nitrogen deposition is the possibility that its effects can worsen when compounded with the impacts of climate change5. Research into environmental change effects has most often looked at different effects of change in isolation – for example how drought affects species diversity, how warming affects insect herbivory, or how nitrogen deposition affects plant communities. But in nature all these different impacts might be acting simultaneously, and we know little about how their joint impacts could look.
When (science) life gives you lemons…
While setting up an experiment in 2011, researchers at the University of California Riverside and the U.S. National Park Service were given an unexpected opportunity to investigate such joint impacts6. Their initial plan was to study how nitrogen deposition affected southern California’s shrublands of native coastal sage scrub (Fig 1). These shrublands are biodiversity hotspots, and happen to also be situated in areas that experience some of the highest levels of nitrogen deposition in the United States. The high levels of nitrogen deposition threaten to convert these sensitive shrublands to grasslands dominated by invasive plant species, as non-native grasses often outcompete native plant species when nitrogen levels increase. Incidentally, the set-up of the experiment coincided with the onset of a prolonged period of extreme drought, and the researchers decided to take this chance to learn more about how the effects of nitrogen and drought would interact.
Two wrongs don’t make a right
The drought dramatically reduced the area of land covered by native shrubs in the experiment, from an average 70% cover at the onset of the drought, to 30% at the culmination of the experiment five years later. But the researchers saw that in places where they had added nitrogen, there was a markedly greater decrease in shrub cover. In other words, it looked as though nitrogen addition was, in fact, worsening the negative impact of drought on these sensitive ecosystems. The herbaceous vegetation also changed, as non-native species benefited more from nitrogen addition than native species did.
As mentioned above, the coastal sage scrub shrublands of southern California are hotspots for biodiversity7. Their plant communities, dominated by low-growing, aromatic shrubs, are hosts to many species that occur only in this very specific habitat. One example is the California Gnatcatcher (Fig 2), a songbird whose habitat has been severely fragmented and is therefore the target of local conservation efforts. Other examples are two butterfly species: El Segundo blue (Fig 3) and Palos Verdes blue. Both species are endangered, and both are only encountered in the coastal sage scrub shrublands. Changes in the vegetation of the shrublands, as observed by the researchers in this experiment, will undoubtedly have consequences for all the animals that count this specific ecosystem as their home.
A window of opportunity for invaders
The researchers conclude that nitrogen deposition can accelerate the loss of native shrubs when these shrublands undergo extended periods of drought. Additionally, when the shrublands are subjected to simultaneous nitrogen deposition and drought, non-native, invasive plant species seem to get a ‘window of opportunity’ thanks to the dieback of shrubs. The results illustrate the threats that these sensitive and biologically valuable shrublands face when they have to respond to both climate change and nitrogen deposition at once. The study is yet another indicator that it’s high time we follow Mark Sutton’s advice, and pay more attention to nitrogen – that tricky ‘godfather of pollution’.
1Interview by Marta Daniela Santos, Centre for Ecology, Evolution and Environmental Changes, University of Lisbon, Portugal. Link: http://ce3c.ciencias.ulisboa.pt/outreach/press&events/ver.php?id=798
2Hautier, Y., P. A. Niklaus, and A. Hector. 2009. Competition for light causes plant biodiversity loss after eutrophication. Science 324:636–638. https://doi.org/10.1126/science.1169640
3Bobbink, R., K. Hicks, J. Galloway, T. Spranger, R. Alkemade, M. Ashmore, S. Bustamante, S. Cinderby, E. Davidson, F. Dentener, B. Emmett, J. Erisman, M. Fenn, A. Gilliam, A. Nordin, L. Pardo, and W. De Vries. 2010. Global assessment of nitrogen deposition effects on terrestrial plant diversity : a synthesis. Ecological Applications 20:30–59. doi:10.1890/08-1140.1
4WallisDeVries, M.F. & Bobbink, R. 2017. Nitrogen deposition impacts on biodiversity in terrestrial ecosystems: Mechanisms and perspectives for restoration. Biological Conservation 212: 387-389. https://doi.org/10.1016/j.biocon.2017.01.017
5Tylianakis et al. 2008. Global change and species interactions in terrestrial ecosystems. Ecology Letters 11: 1351-1363. DOI: 10.1111/j.1461-0248.2008.01250.x
6Valliere, J.M. et al. 2017. High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought. Global Change Biology 23: 4333-4345. https://doi.org/10.1111/gcb.13694
7Frost, Garrison. Audublog, National Audubon Society, February 05, 2015. Rare coastal sage scrub habitat provides a home for threatened gnatcatcher and many other species. Link: http://ca.audubon.org/news/rare-coastal-sage-scrub-habitat-provides-home-threatened-gnatcatcher-and-many-other-species