German researchers have found that a plant community with a higher variety of species results in a lusher growth. Estonian researchers who studied local grasslands discovered that this correlation does not hold. Their study ultimately raises the question of the value of life, as the abundance of plant species does not necessarily lead to a higher biomass productivity.
"The key message from our research is that we should think about diversity in much broader terms than the number of plant species per square meter," Katrin Heinsoo, senior research fellow in ecophysiology at the Estonian University of Life Sciences (EMÜ), says.
Heinsoo says that the meaning of the term biodiversity has become blurred. "Do we mean the diversity of life forms, the diversity of ecosystems, or the number of species per square meter when we talk about biodiversity? These are very different ideas that have mostly merged together," she explains.
She and her colleagues study different ecosystem benefits that are associated with the plant biodiversity. Heinsoo says that there is a substantial amount of qualitative study on what was traditionally referred to as ecosystem services. For example, the maintenance of grasslands via mowing or grazing is financially subsidized, based on assumptions that are made after comparing different ecosystems and their management ways.
However, less information is available about the relations between plant biodiversity and ecosystem services in practice, she says.
The team recently published an article on the subject: "How is plant biodiversity inside grassland type related to economic and ecosystem services: An Estonian case study."
They were specifically interested in the correlation between the number of plant species per 100 square meters and ecosystem benefits. The team examined for this two Estonian semi-natural grassland habitats: calcareous (alvar) grasslands on dryer chalky soils and mesic meadows, the most widely spread semi-natural grassland type in the north temperate zone. "These habitats are well preserved throughout Estonia and we have an extensive network of observers," Heinsoo says. It turned out that there was no clear correlation between species richness and the productivity of the site in these grasslands.
Estonian grassland vs German test field
Heinsoo says that it is not necessarily true that the greater the number of species per square meter, the greater the economic benefits. "Of course, the more species there are, the more likely it is that we will find medicinal plants or a variety of pollinators," she continues. "The ecology becomes more resilient as a result of diversity, because the system is better able to cope with unpredictable fluctuations."
However, in their JENA experiment, German scientists sought to demonstrate that the greater the number of plant species per square meter, the greater the productivity. "The German scientists assumed that the combination of several species would improve biomass production, because different species have slightly different growth requirements, allowing them to occupy the same space more efficiently than a single species," she explained.
Heinsoo and her team disagrees with the conclusion. When plants grow in close proximity, sunlight only reaches a limited number of leaf layers, she says. She also mentions that the JENA research was conducted on small test-plots with a carefully selected species composition. "This is precisely the claim we addressed in our study and no correlation was found between the number of species and the biomass of our most common grassland types," she explains.
When studying Estonian grasslands, a reverse correlation was discovered. "When we look at the different types of grassland in Estonia, the calcareous alvar grasslands have the most plant species richness," Heinsoo says. The lowest species diversity, on the other hand, develops in floodplain meadows, where plants like meadowsweet and cabbage thistle can suffocate all other species.
Is life as such a value?
Heinsoo and colleagues discovered in their study that having a more species-rich grassland does not inevitably entail conserving some endangered species. "I would not give the impression that adding a 25th or 26th plant species per square meter would automatically make it more endangered or rare, this is not the case," she says.
Species are added to the meadow at random, including those that have already grown there as well as those that have spread from nearby areas.
The team also evaluated the correlation between species diversity and soil characteristics. It should not be assumed, Heinsoo says, that in semi-natural grassland more species will result in more humus. Firstly, livestock eat away the above-ground part of the plants. "Secondly, our sample was almost exclusively grassland, which means that biomass is removed from it annually, so humus is mainly produced when the rind thickens," she said.
Heinsoo strongly suggests the more plant species there are on the ground, the more probable the underground biota will be species-rich: both in terms of ecological groups and species. A more variegated grassland also offers more options to pollinators.
She advises farmers and landowners to keep their grasslands as semi-natural as possible, avoiding the dominance of single species. "The more plant species there are in my meadow, the more life there is in it," she says. It is more about whether or not you value life, she says. "If I value the lives of all species and specimens, then all the more reason for me to provide habitats for them," Heinsoo says.
Katrin Heinsoo and colleagues published their article in the journal Agriculture, Ecosystems & Environment.
Editor: Kristina Kersa