Two of the most far-reaching human influences on grasslands are an increase in nutrients, and changes in the species and numbers of large grazing animals. One urgent issue, for example, is the long-term effect of more nitrogen released into the atmosphere by livestock farming and traffic, which eventually ends up in nature reserves.
To assess the impact of an increase in nutrients and changes in grazing on plant biomass in grasslands, the researchers used data from the global research network Nutrient Network (NutNet). NutNet was established in 2005 and studies how grasslands worldwide respond to changes in fertilisation and grazing. Members carry out the same experiments everywhere: they add nitrogen, phosphorus and/or potassium to grassland, in every possible combination. They allow some experimental areas to be grazed and others not. Every year, data is collected on the diversity and biomass of plants. NutNet has already experiments in 100 locations in 20 countries. This research used data from 58 locations on six continents. Prof Dr Harry Olde Venterink and Dr Judith Sitters of the Department of Biology at VUB are members of NutNet and each manages a NutNet experiment, one in Brazil and one in the Netherlands.
The research first of all shows that an increase in nutrients increases plant growth in grasslands. NutNet then examined what happens to the extra biomass. Is it also eaten away? The grasslands studied contained large wild animals, such as deer or gazelles, or domesticated species such as cattle or sheep. The results show that, in many cases, the animals are unable to keep up with the biomass increase. In fact, a ‘top-down’ control of grazers on vegetation is only present where there is an extra-large number of grazing animals, such as grasslands where cattle are kept. This is not the case in natural grasslands, where this excessive one-sided vegetation growth leads to a decline in biodiversity. The increase in biomass also increases the risk of natural fires.
Dr Sitters explains: “At a global level, grasslands suffer from eutrophication, increased nutrient supply from human activities such as agriculture and traffic. These grasslands are important ecosystems that are essential for storing carbon and preserving biodiversity. This research shows it is important to preserve wild grazers in order to mitigate the negative effects of eutrophication. Unfortunately, we see that in many grasslands, the diversity of wild grazers is also declining, or they are replaced by one type of livestock such as cattle. This also reduces biodiversity in the landscape.”
Harry Olde Venterink