Blog written by Joel Abraham. Read the full paper here.
Droughts are expected to increase in frequency and severity as a result of human-caused climate change, which makes understanding how droughts impact ecosystems essential. Unfortunately, studying droughts can be a major challenge. Because droughts are unpredictable, studying them often requires quick thinking and improvisation.
So, when my co-author Carla Staver and I heard in 2015 that a severe drought was happening at her long-term field site in Kruger National Park in South Africa, we decided to capitalize on the opportunity to learn more about how droughts work. A lot of savanna animals eat mostly plants (they’re herbivores) so we thought they might be negatively affected by drought, but their responses to drought represent a big – and important – unknown. You can, to a large extent, approximate drought conditions for plants in an experimental setting. But herbivores, especially the large mammals that inhabit African savannas, are too big and move around too much to study in a lab. This meant that a large-scale approach was needed to capture their responses.
Kruger happens to be the perfect place to work at large scales. It’s a massive savanna reserve –about the size of New Jersey – making it one of the largest protected areas in the world. What makes this especially interesting is that the 2014-2016 drought affected different parts of Kruger differently – some places were hit hard, whereas others were pretty much normal. This meant that we could compare what animals were doing in a range of different drought conditions.
To do our work, we took advantage of the fact that animal dung is a great way to understand what animals are up to in tropical savannas. To quote a well-known children’s book, everybody poops, and every species poops in slightly different ways. Zebra dung (my favorite) is formed into beautiful, sleek pellets the shape of kidney beans, whereas elephants poop out what are essentially massive piles of mulch – largely undigested twigs, leaves, and grass. Also, animals can only poop in places they’ve been, so counting where animal dung is and how much is there actually gives you a pretty good sense of how animals are distributed. You can also use dung (nice and fresh!) to figure out what animals are eating, via lab techniques to analyze the plant contents of dung. Armed with this dung-based approach, we counted animal dung at more than 150 sites throughout Kruger and collected nearly 600 dung samples over the course of two years to see how animal behavior changed during the drought. This whole process – counting and collecting thousands of piles of animal dung in total – made me fall in love with just how powerful poop can be!
Herbivores responded to the drought in different ways. Animals capable of eating both grass and trees (we call these ‘mixed feeders’; elephants are a great example) ended up eating more trees than usual during the drought, particularly where the drought was worse. But, animals that eat only grass (‘grazers’, including zebras) couldn’t switch what they were eating, and so instead moved to parts of Kruger where the drought wasn’t as bad. Meanwhile, tree eaters (aka ‘browsers’, like giraffe) didn’t change their behavior much at all. What’s so interesting about these results is that they’re actually not all that different from what these animals do during normal years. During normal dry seasons, mixed feeders switch to eating more trees as grass gets used up. Similarly, it is widely known that grazers can travel really far distances even in normal conditions – think the mass migrations of wildebeest in the Serengeti. So, these drought behaviors are actually just amplifications of behaviors that these animals already have!
These results have some important consequences for savannas going forward, too. Other research by my lab mate Maddy Case is showing us that the effects of droughts on trees can be severe, too, halting tree growth and causing widespread die-offs in some tree species. The increased reliance of some animals on trees during drought will only make things worse for trees, which could prove very bad as droughts become more severe and more frequent.
Also, we found that grazers relied on movement to survive drought. But moving only helps you if there are places to go. This drought was patchy, and in a large park like Kruger, there’s lots of space to move. But many savanna reserves are small and cut off from other parks by fences, which prevents animals from moving. These fences, which we have erected to protect animals, may very well pose a problem as droughts become more frequent.
Our results also challenge long-held beliefs about the flexibility of animal behavior. Here we found that animal behaviors are limited; animals could only use their existing dry-season behaviors to respond to the drought. This contrasts with the commonly held belief amongst ecologist and evolutionary biologists that behavior is quite flexible, and that it is easy for animals to modify their behavior. Instead, our findings reveal that animals are fundamentally creatures of habit, and that we might need to rethink how we view some animal behaviors. Not that behavioral constraints are an entirely new concept. As the saying goes, you can’t teach an old dog new tricks. Or, in this case, you can’t teach a hungry grazer to eat trees.