At first glance, they seem to be abandoned lands. However, according to the Food and Agriculture Organization of the United Nations (FAO), the grasslands constitute one of the largest ecosystems of the world and contribute to the livelihood of 800 millions of people. Apart from providing services and products such as food, fodder and energy, they are the habitat of a number of species and contribute to water and carbon storage, among other benefits.
Two studies published in Nature magazine analyze the different factors that affect the number of species in these habitats and that in a direct or indirect way impact on their productivity – for instance, for grazing livestock. According to the results, the availability of soil nutrients and the presence of herbivorous determine to a large extent the levels of biodiversity of grasslands and their stability.
For the first study, an international cooperative of researchers, where Argentine scientists participate, studied 41 grasslands in five continents. The group examined the variation of the relationship between diversity of species and its stability when the soils are supplemented with fertilizers that provide substances such as nitrogen and phosphorous, among others.
“When the availability of these nutrients increases, the species that use them produce more biomass and they grow. This produces a shadow for the rest of the plants that do not use them, and thus many species disappear”, Pedro Daleo, CONICET associate researcher at the Marine and Coastal Research Institute (IIMyC, CONICET-UNMdP) and one of the authors of both studies, comments.
These results surprised the scientists. So far, it was believed that the most stable ecosystems were those with most number of species and that their loss, after an increase of nutrients, would lead to a reduction of stability. However, this study proved that the reduction in stability was not caused by the reduction of the number of species but by the decrease in the asynchrony of different communities of organisms.
What is asynchrony? “Not all species behave in the same way when temporal or spatial changes occur”, Daleo explains. “During a humid year, some are going to respond better and others worse”, Daleo explains. If all species were synchronized and responded positively or negatively, the changes in temperature and precipitations, for instance, would generate great variations in the biomass”. Asynchrony refers to the balance achieved by the communities that react in different ways to the changes in the environment, therefore maintaining the biomass’ balance and stability over time.
So, the loss of stability of grasslands in a nutrient imbalance’ scenario would be more related to the loss of balance between the species than to the reduction of the number per se.
“Asynchrony has been a long-term question in ecology, that has primarily been studied grasslands that were planted with 1, 2, 4 or 8 species, to determine whether the production over time is variable or constant. That is an important service provided by grasslands, for example those used for grazing”, Elizabeth Borer, professor at the University of Minnesota, USA, and one of the authors of the studies, comments.
In this sense, the fertilization reduces the positive effect of the diversity over stability. When fertilizers are added, the biomass production increases every year; however, at the same time, stability is not preserved because all the species respond in the same way, thus loosing asynchrony and the response capacity in the face of adverse situations.
For Eric Seabloom, also professor at the University of Minnesota, “Fertilization has two primary impacts on natural grasslands: the first is destabilization of grasslands over time, which we described in this paper. But probably the biggest impact is loss of species, generally when you add fertilization you get a lot of extinctions”, he comments.
In this regard, the second investigation analyzed the relationship between herbivores and nutrients in grasslands. For this research, 40 grasslands in 6 continents were studied and it was proved that the nutrients and herbivorous allow controlling the local diversity of plants through light limitation.
“Plants compete among themselves for light. Thus, both nutrients as herbivores affect the stability when they intervene in that competition”, Oscar Iribarne, CONICET principal researcher, tenured professor of the UNMdP and author of the study, states.
In the case of the first ones, the addition of nutrients to the soil makes plants grow more and species could disappear as competition for light increases. Regarding herbivores, as they feed on plants, they increase the levels of light at ground level. This would favor diversity because the species would have more light.
These results allow establishing new strategies for the preservation of these ecosystems in a situation where boosting food production has become more and more important.
“We have strong evidence that greater diversity leads to greater stability of biomass production over time”, Borer concludes.
This investigation was conducted within the framework of “Nutrient Network: A Global Research Cooperative”, a cooperative in which more than 150 scientists – 4 Argentine – of more than 15 countries participate.
For Iribarne, one of the main achievements of this joint work is that “it allows analyzing grasslands of the entire world, with different features and varying degrees of human intervention”, he says.
- By Ana Belluscio
- About the research
- Oscar Iribarne. Principal reseacher. IIMYC.
- Pedro Daleo. Associate researcher. IIMYC.
- Juan Alberti. Associate researcher. IIMYC.
- Jesús Pascual. Doctoral fellow. UNMdP