(originally published on ConservationBytes.com)

Last year we reported experimental evidence that the dilution effect was the phenomenon by which greater biodiversity imparts disease resistancein plant communities. Our latest paper shows the mechanism underlying this.

In my ongoing collaboration with the crack team of plant community ecologists led by Shurong Zhou at Fudan University in Shanghai, we have now shown that nitrogen-based fertilisers — in addition to causing soil damage and environmental problems from run-off — reduce a plant community’s resistance to fungal diseases.

This means that prolonged use of artificial fertilisers can lead to the extinction of the most resistant plant species in a community, meaning that the remaining species are in fact more susceptible to diseases.

Continuing the experimental field trials in alpine meadows of the Tibetan Plateau, we tested the biodiversity resilience of an isolated plant community to increasing concentrations of nitrogenous fertilisers. In this diverse and pristine ecosystem, we have finally established that extended fertilisation of soils not only alters the structure of natural plant communities, it also exacerbates pathogen emergence and transmission.

This is one of the ways in which monocultures common in food cropping succumb to sudden outbreaks of severe disease. Having more species in an ecosystem provides a sort of insurance policy against disease for any given species. This — the ‘dilution effect’ — means that when there are many species in a given area, the chance of passing a pathogen from an infected individual to a neighbour of the same species is lower, so the entire community benefits from an overall lower prevalence and severity of diseases.

In our latest research published today, we determined one of the main mechanisms of this disease-dilution effect. Adding fertilisers allows certain species to outcompete others, leaving the overall biodiversity of a system lower and more susceptible to disease. In other words, while some species benefit from adding nitrogen, the overall effect at the community level could be worse because the surviving species end up being more diseased.

These experiments provide powerful information about how species diversity maintains ecosystem function, and how agriculture and other human interventions can accelerate ecosystem degradation.