Article: Davies, A. B., & Asner, G. P. (2019). Elephants limit aboveground carbon gains in African savannas. Global change biology, 25(4), 1368-1382. https://doi.org/10.1111/gcb.14585
The ins and outs of carbon budgeting
Knowing that carbon dioxide drives climate change, a major question scientists have been trying to answer is how the amount of carbon dioxide in the air is changing now and what will happen in the future. In order to do this, scientists take an approach that most of us are familiar with from our daily lives: they make a budget. Similar to how you might make a budget for your personal spending, scientists make a budget of carbon for the entire planet. Just as you have to identify spending that is making money go out of your bank account and the income going into your bank account, scientists try to identify all the sinks that are taking carbon out of the atmosphere and all the sources that are adding carbon into the atmosphere. Some of these sources and sinks we already know a lot about. Burning oil and coal are major sources. The ocean and plants act as sinks.
Despite knowing a lot about these sources and sinks, there are still some instances where the numbers don’t quite add up, which suggests there is some missing source or sink. One of those things that might be missing from the budget is the impact of large animals. A previous envirobite explored how cows can be an important source of methane, another greenhouse gas. However, it’s also possible that wild animals are having a climate impact in ways that are not just a matter of them directly emitting gases. As big animals move and eat, they can leave a big mark on the landscape and vegetation around them. A recent study, led by scientists Andrew Davies and Gregory Asner, were interested in determining if elephants were changing the plants around them enough to impact the carbon budget in South Africa.
Some pretty fly methods
While there are ways to measure the size and amount of plants covering a landscape on the ground, it can be a really slow and difficult job. Because of that, the scientists took a different approach for this project, they measured plants from a plane! Specifically, they used a method called “Light Detection and Ranging” (LIDAR). In this method a machine sends a laser light down towards the ground and then measures how the light reflects back up. Based on those reflections, it is possible to know the 3D structure of the plants on the ground. Scientists can then calculate how tall the trees are, how many leaves the trees have, and how many trees there are.
For this study, Andrew Davies and Greg Asner used LIDAR airborne methods to study how the amount and size of plants were changing in a national park in South Africa where the elephant population has been increasing. There has also been a decrease in the amount of woody plants, which some thought might be because of the elephants. As big animals, elephants eat a lot of plants (more than 300 pounds a day for some!) and also have the ability to knock down trees. However, given that there were also changes in rainfall and wildfire events at the same time, the scientists needed to figure out if the elephants were what was causing the decrease in woody plant cover. The scientists took measurements over six years at this site and looked at areas where elephants were allowed to freely roam, as well as some experimental areas where they excluded all plant-eating animals and other areas where they only excluded the large plant-eating animals like giraffes and elephants. This set up helped to separate the potential impact of elephants from the impact of other animals.
Are elephants a big deal?
Yes and no. The science team found that the ways in which elephants were impacting the carbon budget was not a simple story. In general, there was an increase in the amount of plants over the course of the study period meaning the plants took up more carbon through photosynthesis. This occurred in areas with and without elephants, suggesting that elephants were not drastically changing the carbon budget. However, a more detailed look revealed that elephant behavior might have an impact on plant growth.
Once young male elephants reach a certain age, they leave the herd. When researchers looked at the locations of these male elephants separate from the herds, they found that areas where more of these male elephants spent their time experienced decreases in plant cover and were not able to draw in as much carbon through photosynthesis compared to areas with fewer male elephants. So while the herds did not drastically change the carbon budget of the landscape, the activity of the male elephants separate from the herds was limiting carbon uptake.
This study reveals some of the complex ways that nature can work, revealing how large animals can change the environment in which they live. While we do know a lot about the carbon cycle, there are still many unanswered questions. The impact of elephants on the carbon cycle is one of those questions that highlights the importance of tackling some of these “big” scientific unknowns that are still out there.