Megaherbivores such as elephants and hippos like to graze on lowland tropical grasses, but they also browse the twigs, shoots, leaves, and fruits of bushes and trees. There’s more to this diet than meets the eye, though.
These animals, particularly the elephants which have a big appetite, help keep woody plant encroachment in check. But now that many of these megaherbivores have been killed, either by poaching or habitat loss, the vegetation of Africa’s savannas is changing — and it all strikingly shows in hippo teeth, a new study found.
Ask and nature will provide
There is not one, but three forms of photosynthesis — the essential biological process that harnesses energy from sunlight into chemical energy. These types are known as C3, C4 and CAM.
Tropical grasses, such as those commonly found in an elephant’s ecosystem, are C4 plants because of these employ a unique enzyme to process CO2 into sugars during photosynthesis. Other C4 plants include corn and sugarcane. Also, in the elephant’s surroundings are C3 plants, which use a different enzyme, like trees, shrubs, herbs and various flowering plants.
The two classes of plants, C3 and C4, are in direct competition for resources, sunlight no exception, and megaherbivores like elephants act like nature’s gardeners, keeping everything in balance. Since the 1960s, however, elephant populations have plummeted which has led to a significant advance of C3 plants. Grasslands, which many animals depend on for food and shelter, have been retreating, on the other hand.
Aerial photographs and ground surveys have confirmed that C3 plants are beginning to take over in the parts of Africa, but these have never been very reliable because you need a lot of data recorded at fixed time intervals to glimpse the big picture. Many African parks, like Uganda’s Queen Elizabeth National Park, have little to any such observations recorded.
We’re surrounded by data, though. While there are many things humans have failed to record, nature never sleeps and there is so much we can learn about past environments by studying proxies like minerals, fossils or remains. For instance, coral build their skeletons from calcium carbonate, which is a mineral extracted from the seawater. Inside the mineral are oxygen isotopes as well as trace metals which can be used to determine the water’s temperature at the time the coral grew. Millions-of-years-old sediment cores trap tiny fossils and chemicals and we can use them to learn what past climates and faunas looked like. Patterns in tree rings can also tell us if a particular year in its life was dry or wet, among many other things.
In science, most of the time, it’s all about asking the right question. In this case, the right question would be ‘what proxy can we use to build a timeline of vegetation growth in Africa’s savanna?’ That would be heavy carbon isotopes, researchers at the University of Utah might say. Because C3 and C4 plants use metabolic processes which treat carbon isotopes differently, namely C4 plants store more heavy carbon isotopes in their biomass, we have a reliable marker at our disposal.
Going up the food chain, these isotopic signatures become incorporated into the bodies of the animals which consume plants and get preserved in durable tissue, like teeth.
Kendra Chritz and colleagues at the University of Utah worked closely with officials from the Queen Elizabeth National Park to get ahold of hippo teeth from the 1960s, 2000s, and a lucky sample in between from 1991. This gave them the confidence they needed to build a nice timeline as they had both pre and post poaching samples to work with. There are only about 150 elephants left in the park, down from over 4,000 in the 70s. Thousands of hippos were poached in the intervening four decades, too.
The results published in Scientific Reports show that the 1960s hippos had a diet comprised of 80 percent C4 plants but later hippos only ate 65 percent C4. This suggests that in the span of only a couple of decades, C3 plant encroachment had progressed fast enough to alter the diet of the animals in the park. The proportion of C3 plants in later hippo diet came as a total surprise given initial studies of hippo diet surmised the animals almost exclusively ate grass. And few researchers have suggested otherwise,” Chritz says. “It appears that they’re actually quite flexible in their diets and adaptable to environmental change.”
“We’ve built a record that shows just how drastic the loss of megaherbivores in a park can be on a very short timescale,” she says. “Within ten years, we see a big change in what’s happening in this once diverse grassy area of the park. This is a window into the future of what could to happen in East African savannas as elephants continue to be poached at the currently unprecedented rate.”
Poachers kill a hundred elephants every day in Africa. Today, there are only 350,000 elephants left in fifteen African countries or 30 percent fewer than only a decade ago. The University of Utah study shows that losing elephants might have far-reaching consequences for the local ecosystem.
“There’s a balance you have to reach. The most important thing right now is to work hard at fighting poaching,” Chritz notes.
“Not purchasing ivory and knowing which products you might use that are made from ivory is the best thing you can do to protect elephants,” she adds.