All living organisms in an ecosystem are connected. Researchers at WSL are investigating in grasslands why such networks get out of equilibrium when species disappear or fertilisers are heavily applied.
If you go hiking in the Swiss National Park, you will walk on a loose network of paths through forests, over rocks and across grasslands. You won’t see any livestock grazing on the grasslands because cows, sheep and goats have not been allowed in the Park since it was founded in 1914. The vegetation is, however, still quite short in some places. In principle, the former pastures below the tree line should be covered with forest if humans and their livestock are no longer keeping them open. Why is this not the case? What is keeping the vegetation so short? Are the plants growing on the grasslands today the same as those growing on the former cattle pastures before the National Park was founded, or are they different?
These are the kinds of questions Anita Risch and Martin Schütz are asking. The ecologists at WSL are investigating how the living organisms in an ecosystem are connected, how food webs work and what happens if one or other species disappears from the ecosystem. For the past 25 years they have been studying the former cattle pastures in the National Park. “We initially assumed that it was mainly large mammals such as red deer and chamois that were, instead of the livestock, eating the plants”, says Anita. But this alone does not explain why the vegetation is taller in some places and shorter in others. Herbivores that influence the vegetation differently from large mammals must, it seems, be involved.
The system needs all of them
To test this hypothesis, Anita and Martin came up with the idea of excluding the herbivores from the grasslands according to their body size, starting with red deer and other large mammals, then smaller ones such as marmots, hares and mice, before finally excluding invertebrates like insects and snails. To do this, they developed special fences (exclosures), which they were allowed to install in the National Park in 2009 after thorough preparation and coordination with the Park administration. Working in the Park was not just physically tough. Snow between May and September, i.e. during the period of data collection, regularly destroyed the fine-mesh invertebrate exclosures. “Maintenance involved a lot of work, particularly as we had to carry all the replacement material to the sites,” says Martin.
During the five years when the exclosures were in place in the National Park, the researchers collected large amounts of data – not just about the plants themselves, but also about the soil under the vegetation. They wanted to find out, for example, how the herbivores influenced the soil organisms and the nutrient cycles in the soil. Since the experiment stopped in 2013, Anita and Martin have, together with other researchers, published many of their findings. They found out, for instance, that the large mammals increased the plant diversity in the National Park. Without these animals, fast-growing plants take over on the grasslands, suppressing other plant species, and the diversity decreases. This, in turn, has an impact on ground-beetle species that hunt by sight. If the vegetation is too dense, they no longer see their prey and the number of these predatory beetles falls.
The soil temperature and the availability of soil water also depend on animals that feed aboveground. Without them, the vegetation becomes denser and less sunlight reaches the ground. The soil cools down by as much as one degree, becomes moister and more plant material can grow. The most surprising result so far is that, should large mammals be missing, the invertebrates take over their function in the system. They consume almost as much plant material from some vegetation types as the large mammals. Since they also eat plants that the large mammals avoid – such as prickly thistles or poisonous monkshood – they influence the vegetation differently from red deer and chamois. Should, however, no invertebrates be present, the whole ecosystem will collapse, and food webs and nutrient cycles will fall apart.
Fertilisers disturb the food web
During the exclosure experiment in the National Park, Anita and Martin started a similar experiment not far away in Val Müstair. Among the thirty experimental plots amidst the avalanche barriers above Lü, several have also been fenced. The plots are part of the international research project Nutrient Network (NutNet), in which seventy teams from nineteen countries on six continents are taking part. This project is also looking into what happens when large wild herbivores are absent on grasslands, as well as into the influence of fertilisation on the 25 m2 plots. Each research team uses the same methods to collect the same data on their sites in order to investigate changes in the ecosystem’s species diversity, productivity and nutrient cycles. The aim is to identify global patterns in how very different grassland ecosystems react to the exclusion of wild animals and the application of fertilisers. The collected data is stored centrally at the University of Minnesota, USA, where the idea to set up a research network was developed.
Anita and Martin have been part of this network since 2008. At the end of May each year, they fertilise their plots with nitrogen, phosphorous and potassium. At the beginning of July the vegetation is cut by hand. The quantity of cut plant material indicates how much biomass was produced during the vegetation period – a simple way of measuring the influence of browsing and fertilisation. Even though the project is far from finished, many findings are available. They are alarming: if nutrients are applied to grasslands where there are no large herbivores, plant diversity decreases markedly. On the sites in Val Müstair today, red fescue (Festuca rubra), a common and widespread grass species, is practically the only plant still growing. The interactions in the food web there have been greatly disturbed.
The experiment is not just based on theoretical considerations: the number of large wild herbivores has, since the end of the last Ice Age, diminished worldwide. This is probably connected with the emergence of modern humans. In addition, since industrialisation, the stocks of nitrogen and phosphate worldwide have increased by 100 % to 500 % due to the overuse of artificial fertilisers in agriculture. Applying fertilisers in areas where large herbivores are absent leads to species-poor and instable ecosystems, which are scarcely capable of still reacting to changing environmental conditions.
Exchanging ideas, experiences and data
Global networks like NutNet enable a look beyond the fence. “Through exchanging with researchers from other continents, we can learn to understand our own system better,” maintains Anita. “Many hypotheses in botany are based, for example, on the conditions prevailing in the Northern Hemisphere. If these hypotheses are tested in the Southern Hemisphere, it quickly becomes apparent that some of them do not apply there.” This insight helps to throw preconceived ideas overboard and to understand better how everything is interconnected.
The personal contact between the researchers is very important, and not just during working hours. “Often joint projects take shape over a beer after work,” says Martin. At one of the annual meetings of the NutNet Network, the two of them got to know the Australian, Jennifer Firn, from the Queensland University of Technology in Brisbane. She found the exclosure experiment in the National Park very interesting and suggested that Anita and Martin collaborate with her. Perhaps the exclosures will soon be used in Australia – the research proposals have been submitted. In this case, however, it will not be red deer and marmots that need to be excluded from the vegetation, but rather kangaroos and wombats. (Lisa Bose, Diagonal 2/18)