Grazers as Managers

Livestock grazing regulates ecosystem multifunctionality in semiarid grassland (2018) Ren et al., Functional Ecology, DOI: http://doi.org/10.1111/1365-2435.13215

The Crux

Grassland is the most common type of land cover on our planet, and about one third of the Earth’s population depends on these ecosystems to survive. Grasslands are not only the most common terrestrial ecosystem, but they are also some of the most vulnerable to degradation.

Sustainable grazing practices on these ecosystems are incredibly important when trying to maintain or improve the health of the many organisms that call grasslands home. The challenge with maintaining ecosystem multifunctionality is that trade offs between different functionalities are common, making it difficult to accomplish every goal at once. When managers try to maximize the productivity of a system this may reduce the overall diversity, as this often involves promoting the dominant plant species at the expense of every other species. The authors of this study were interested in determining the effects of grazing on environmental multifunctionality, so they analyzed what impact grazers had on multiple traits of the grassland ecosystem.

Did you know: Ecosystem Multifunctionality

The exact definition of Ecosystem Multifunctionality is still a matter of debate, but for the purpose of this summary we are going to say that it is exactly what it sounds like: the various services and functions that an ecosystem provides to different parties. These services can be from a purely agricultural standpoint, whereby a group is interested in the health of an ecosystem so that they can feed as many head of cattle as possible; however, an ecosystem can also be important from a cultural point of view.

A great example of cultural importance is the large mammals in the African savannah. When you think of Africa you immediately think of elephants, lions, zebra, and gazelles. Maintaining and protecting the ecosystems that these animals inhabit provides these animals with a place to live and reproduce, but it also brings a lot of tourism to the countries that house these animals.

inner steppe

The Inner Mongolia steppe, where this study took place. (Image Credit: robinxiawen, Creative Commons CC0)

How it Works

The authors used data collected over a period of 11 years from the Inner Mongolia steppe, where they broke up a 160 hectare area into either flat or sloped types, with each type being further broken up into 7 areas which received different types of grazing pressures (number of sheep in a given plot, the more sheep the higher the pressure). Because the flat and sloped sites responded to the different grazing pressures in similar ways, the results were pooled together, giving us a 160 hectare area that experienced 7 different levels of grazing pressure.

All of the plots contained similar plant species, with the relative abundance (what percentage of the population a given plant species makes up) of each plant species being about the same in each plot.

To measure ecosystem multifunctionality the authors used five variables: plant aboveground biomass, plant-tissue nitrogen content, plant-available nitrogen, plant-available phosphorous, and soil organic carbon. These five were chosen because they are all important parts of a functioning ecosystem, in addition to being important for both plant and soil health. They combined these variables together using statistical methods to get a measure of ecosystem multifunctionality.

What They Found

As expected, higher grazing pressure reduced ecosystem multifunctionality. In addition, increasing grazing pressure resulted in increasing loss of plant species richness. Interestingly, low levels of grazing actually resulted in an increase in ecosystem multifunctionality, but this multifunctionality quickly decreased with increases in grazing pressure.

Any Problems?

Nothing significant. The study is extremely impressive in its length as well as in its level of detail, but all of this data is from one location (the Inner Mongolia steppe). Although unlikely, these results may only be relevant to that specific grassland ecosystem. Unfortunately, this kind of study is expensive, both in the amount of work it took and the money involved in this research, so replicating it will be difficult and time consuming.

So What?

This study has shown that we need to assess multiple aspects of an ecosystem to understand what effects grazing has on grasslands. Traditionally, environmental managers consider one aspect of the ecosystem’s health, but as we mentioned before this may come at the expense of another part of the ecosystem. In the future, using information gained from studies like this will allow for a compromise when it comes to utilizing a native ecosystem and getting the most out of it, both for farming and cultural significance.

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