Utrecht University: Light competition drives the effects of herbivores and nutrients on plant diversity


Nutrient enrichment by human activity and loss of herbivory are two major causes for diversity loss in plant communities worldwide. Increased competition for light is one of the most often suggested forces linking the impacts of nutrients and herbivory on diversity. Hautier: “When an area is being fertilized, not all plant species respond in the same way. Some species are fast growers. When they have the chance, they get larger quickly and start shading the other plant species that get excluded. On the other hand, herbivores remove plant biomass, which gives other plants access to light and a chance to grow and survive.”

Strong debates
Although these ideas have been around for decades, empirical tests of the role of light in natural systems were based on indirect evidence and have led to strong debates. According to Hautier, testing these ideas empirically is just not that easy. Hautier: “Until now, researchers have been manipulating the competition for light indirectly, for example by tying back vegetation. But if you do that, you change more than just the amount of light that the plants are exposed to. It is also challenging to directly add light to a natural setting. You need lamps that do not produce heat, a waterproof installation and electricity close to the plants.”

Novel experimental setup
The researchers from the Netherlands, Finland and Germany stood up to the challenge and used a novel experimental manipulation: they added supplementary light using modern LED-lamps to understory plants in a natural grassland.

The experiments were conducted in Germany at the Global Change Experimental Facility (GCEF), Firstly, the researchers fertilized areas of grasslands. As expected, this resulted in a local decrease in plant diversity. But when they used the LED-lamps to add light to those areas, the diversity returned. Secondly, the researchers used sheep to investigate the effect of herbivory. When the animals were introduced to areas that were previously ungrazed, plant diversity was restored. At the same time, the addition of light to areas that were maintained without herbivores resulted in an increase in plant diversity.

The researchers also found that the story was not quite that straightforward. Hautier: “In fertilized areas without herbivores, the plant diversity was initially restored when we added the light. But after a while, the positive effect of the supplemented light disappeared and diversity went down again. This highlights two things: that light is a major driver, but not the only one, and that herbivores can outpace the effects of fertilization on competition for light.”

Implications for management and policy
How do these findings affect grassland management and conservation policy? Anu Eskelinen, Hautier’s colleague who is the lead author of the paper, emphasizes the important role of herbivores. Eskelinen: “Our results highlight the importance to preserve native herbivores, to use sustainable livestock grazing as a management strategy, and to employ rewilding (i.e. returning herbivores to areas from where they have been extirpated) as a tool to revert lost plant diversity. The key is that we now know one important mechanism, alleviation of light competition, by which herbivores protect plant diversity”.

Hautier: “We are obviously not going to use lamps to add light to our grasslands. We know the danger of fertilizing. Although adding herbivores to fertilized grasslands is beneficial for plant diversity, the diversity in grazed, fertilized grasslands is still lower than in unfertilized lands. So if we want more diversity, we have to stop fertilizing.”

International research team
Besides Yann Hautier, assistant professor at Utrecht University, the research team included Anu Eskelinen, associate professor at Oulu University, Stan Harpole, a professor from Helmholtz Centre for Environmental Research UFZ and German Centre for Integrative Biodiversity Research iDiv, Maria-Theresa Jessen, a postdoctoral researcher from UFZ and iDiv, and Risto Virtanen, a senior researcher from the Botanical Museum of Oulu University. The main funders of the study are the Academy of Finland and the Helmholtz Centre for Environmental Research UFZ.