Two wrongs do not make a right. But when it comes to the biodiversity of plants in grasslands, they just might. That is because two apparently negative impacts often controlled by humans — the use of fertiliser and the grazing of plant species by herbivores — combine to the benefit of biodiversity. This is according to an innovative international study involving Yvonne Buckley, Professor of Zoology at Trinity College Dublin, Ireland.
The findings, just published in the online edition of leading journal Nature, are important in a world in which humans are changing both the distribution of herbivores and the supply of nutrients like nitrogen or phosphorus that act as fertilisers. The findings will have major implications for our understanding of the complex interplay among nutrients, herbivores and plant growth, which impacts our capacity to feed a growing human population and protect threatened species and ecosystems.
In a globally collaborative study, scientists used the Nutrient Network, or ‘NutNet’, to help predict how grasslands around the world will respond to a changing environment. NutNet is a grass-roots campaign supported by scientists who volunteer their time and resources. There are now 75 sites around the world that are run by more than 100 scientists. Professor Buckley will soon establish the first Irish site to add to NutNet’s growing catchment area.
In this study, NutNet scientists gathered data from 40 sites that spanned six continents. They set up research plots with and without added fertiliser, and with and without fences to keep out local herbivores such as deer, kangaroos, sheep and zebras. Every year since 2005, they have measured the amount of plant material grown, the amount of light reaching the ground, and the diversity of different plant species growing in the plots.
“We found that fertiliser reduced the diversity of plant species in the plots because species less able to tolerate a lack of light were literally overshadowed by fast-growing neighbours,” said Professor Buckley.
“But whenever herbivores increased the amount of light that struck the ground, by eating plants, the total plant species diversity increased. What was especially interesting – and convincing – was that these results held true from Australia to Europe, China and the US, and whether the herbivores involved were rabbits, sheep, elephants or kangaroos.”
The findings add a key piece to the puzzle of how human impacts affect prairies, savannas, alpine meadows and other grasslands. Biodiversity plays an important role in how resilient communities of plants and animals are in the face of change. So by showing how fertilisation, grazing, and biodiversity are linked, the research moves us one step closer to understanding what we can do to help keep grassland ecosystems and all of the services they provide healthy and thriving in a changing world.
“Global patterns of biodiversity have largely defied explanation due to many interacting, local driving forces,” added Henry Gholz, Program Director in the National Science Foundation’s Division of Environmental Biology, which funded the coordination of this research.
“These results show that grassland biodiversity is likely largely determined by the offsetting influences of nutrition and grazing on light capture by plants.”