impala acacia_2014_11_26
Where impala lunch (and what they lunch on) has a bigger impact on Africa's savannahs than you might imagine. Image: David Dennis, Flickr

Let’s get one thing straight: impala do not like eating around giant thorns. Particularly when they are the fearsome nearly 2.5 inch (6 centimetre) long thorns sported by a species of acacia tree common to East Africa – Acacia etbaica, we’re looking at you.

But what these African antelopes like even less is risking getting eaten by leopards and African wild dogs, their main predators. An unfortunate side effect of avoiding them, however, is that impala are forced to deal with the acacia’s massive thorns more often.

While a seemingly unrelated bummer, these kind of incredibly nuanced relationships among big carnivores, impala and plants are shaping the East African savannah, with similar scenarios playing out in ecosystems across the planet. Not only are such relationships vulnerable to human influence, but sometimes humans are already a subtle part of them.

Humans have been manipulating our environments for probably as long as we have been around – and East Africa is no exception. For thousands of years, people on the savannah have protected their cattle from lurking predators by fencing livestock at night in large corrals known as 'bomas'. After a month or two, they move on, leaving behind open – and well-fertilised – grassy areas, which impala quickly colonise. 

Constantly browsed down by impala, fewer trees are able to grow in these areas, maintaining their openness. As a result, abandoned bomas may remain a feature of the landscape for decades or even a century.

Thorn of Plenty

A study of the Kenyan savannah recently published in the journal Science examined whether impalas’ fear of becoming lunch drives where impala get their own lunch – including around former bomas – and if these preferences even determine where certain plants grow or don’t grow. 

The study, led by ecologist and former PhD student Adam T. Ford, now at the University of Guelph in Ontario, Canada, found that impala definitely lingered in open areas, including former bomas. They also avoided densely wooded thickets where hidden predators were more likely to pounce. But eschewing dangerous thickets also meant foregoing the much shorter-thorned (and therefore much more appealing) Acacia brevispica trees that grow more abundantly there than near the bomas, where the thornier Acacia etbaica grows.

The fact that the delectable, shorter-thorned acacia thrives in areas frequented by big, hungry meat-eaters is no accident. Actually, it’s why this species can thrive as it does.

“African wild dogs and leopards were creating a safe haven for this tree by scaring impala,” says another study author, ecologist Jacob R. Goheen of the University of Wyoming. 

With or Without You

These results underline just how crucial large carnivores are to the current functioning of East African savannahs.

Acacia Thorns Species 2014 11 26
Will the shorter-thorned Acacia brevispica (left) give way to a thorny landscape dominated by the extra-spiky Acacia etbaica (right)? Images: Adam T. Ford

Of course, humans are starting to meddle with this system once again. East Africa’s large carnivores are dwindling (and it isn’t from overindulging on impala). Humans are breaking up their habitat, poaching them and shooting them as pests. The IUCN Red List of Threatened Species lists leopards as Near Threatened and African wild dogs as Endangered.

So what would the savannah look like if these predators eventually disappeared?

“Over the short term we don’t think there will be a change in what impala eat and where,” says Ford. Relearning the evolutionary lesson that open areas are safer would take a while for the antelopes. 

But in the long term, a low-predator landscape would stop reinforcing that lesson, eventually giving impala a free meal ticket to a short-thorned acacia buffet. Without carnivores’ protection, the less-thorny acacia could give way to a landscape of giant thorns. 

Even if that (hopefully) never happens, we now understand how it could. This study highlights “the intricate pathways by which humans can influence ecosystems,” warns Goheen.

Top header image: Willem van der Horst, Flickr