No return of diversity?

In the past few decades, there has been an increasing concern and therefore research into the eutrophication of natural ecosystems caused by high levels of nitrogen addition. Some studies, such as Foster and Gross 1998 and Clark and Tilman 2008 (among others), have shown that nitrogen fertilization leads to a decrease in the diversity of communities. However, the long-term consequences of these actions are not well understood. How quickly do the nitrogen concentrations reduce? Do communities revert to pre-fertilization levels of diversity and composition when fertilization is ceased? Or do the communities essentially enter an alternative stable state? These are some questions that long-term ecological research can help answer and lucky for us Isbell et al. 2013 approaches these questions in an important fertilization experiment by David Tilman (1987).

At Cedar Creek in Minnesota, Tilman started fertilizing plots in 1982 at 8 different rates. He ceased fertilizing in half of those plots in 1991 and has continually fertilized the other half. This has provided a great opportunity to explore the consequences of chronic nitrogen addition and compare that with the rates of recovery of plots that have not been fertilized in 2 decades, thus also exploring whether the plots switch to alternative stable states due to land-use history. Isbell et al. found that mid and high levels of fertilization greatly reduced diversity. They also found that continual lower levels of nitrogen fertilization reduce plant diversity as much as shorter-term high nitrogen application, meaning that even such lower levels of nitrogen accumulation can have major impacts on communities. There was recovery of diversity for some treatments, such as the cessation of fertilization for mid-nitrogen treatments. However, there was not a return of diversity for plots that experienced high levels of nitrogen addition, despite the soil nitrate levels recovering to pre-fertilization levels. These plots were mainly dominated by a grass species, Elymus repens. This indicates that temporary high levels of nitrogen addition can cause the community to enter into an alternative stable state.

This study is pretty awesome because they return to a long-term study on the effects of nitrogen fertilization on grasslands and test not only the chronic effects but also for new consequences like alternative stable states post restoration of the disrupting factor. This study impressively extends from previous studies and is quite relevant at multiple perspectives. It has important pertinence for ecosystem responses to atmospheric nitrogen deposition and has very interesting applied implications for restorations. For example, while perhaps understated, we definitely face obstacles trying to mitigate or correct the adverse effects of human impacts on the environment, this study demonstrates that just ceasing activities is not enough and we may need to actively restore habitats that we may have pushed into alternative stable states.

Isbell, F., D. Tilman, S. Polansky, S. Binder, and P. Hawthorne. 2013. Low biodiversity state persists two decades after cessation of nutrient enrichment. Ecology Letters 16: 454-460.

Kane Keller

About Kane Keller

Kane is a community ecologist who studies how mutualisms and intraspecific variation in the mutualist species can influence community and ecosystem functioning.
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