Research Brief: Herbivorous animals crucial to healthy ecosystems
Large or small, herbivorous animals such as red deer, marmots, mice, snails and insects, play a central role in grassland ecosystems. According to a University of Minnesota study in collaboration with the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), without these herbivores, ecosystems become unstable.
The study, conducted in the Swiss National Park in eastern Switzerland, revealed that food webs and nutrient cycles disintegrate without vertebrates and invertebrates, leading to the collapse of the ecosystem.
"Few experiments examine the sequential loss of herbivores in a way that reflects real-world declines in biodiversity; this study does exactly that in a robust way," said University of Minnesota Associate Professor Joseph Bump.
The results, co-authored by Bump and published in the journal Nature Communications, revealed that when vertebrates disappear, invertebrates take over.
Until now, little was known about how the loss of animals of different body-size, ranging from the large red deer to the small aphids, affects ecological networks and therefore the functioning of an ecosystem. During the five-year experiment, WSL scientists and international partners investigated for the first time under real conditions what happens when various herbivores are selectively excluded from grassland ecosystems.
“Another take-home message from this study is that international collaboration enhances scientific research,” said Bump.
In coordination with the park administration, the researchers installed fences in the Swiss National Park from 2009 to 2013. These fences excluded herbivores of different body-sizes from the grasslands: first large mammals like red deer, then smaller ones like marmots, hares and mice, and finally invertebrates like snails, grasshoppers and aphids.
The experimental set-up reflects real-life observations: when animals die out, the largest disappear first, followed by the smaller ones.
When large mammals were missing, there were more interactions between the remaining biological communities and their abiotic environment, such as soil chemistry, than when they were present. For example, the exclusion of large grazing mammals benefited fast-growing plant species adapted to making good use of soil nutrients at the expense of plants that tolerate heavy grazing. An ecosystem where large mammals – in this study red deer – are present, does not function inferior than without, it simply functions differently.
By contrast, when all animals – all mammals and above ground-dwelling invertebrates – were excluded, interactions between plants and soil bacteria soil nutrients decreased. The connection between above- and below-ground networks became weakened.
"We assumed that large animals in particular have a big impact on the system, but our results show that small invertebrates are also very important for the functioning of the system," said Anita Risch, lead author of the study and head of the WSL's Plant-Animal Interactions research group.
The better the biological communities interacted and were connected with their environment, the better the ecosystem functioned. Proxies for ecosystem functioning in the National Park included nutrient availability, soil respiration and the number of plant species present. In contrast, when interactions were low, the ecosystem also functioned poorly. It becomes unstable and can may be less resilient to changing environmental conditions.
The results of the experiment show the importance of the invertebrate community for ecosystem functioning, especially when larger mammals are absent. However, recently the number of invertebrate species and individuals appears to have dwindled in Switzerland and Central Europe, and not just on intensively farmed arable land.
"We are worried that invertebrates are present in increasingly small numbers, even in protected areas,” said Martin Schütz, co-author of the study. Researchers warn of a loss of invertebrates, agreeing that increased efforts are needed to protect invertebrates, because they are of immense importance for coupling and functioning of our ecosystems.