What if elephants stopped shaping the savanna?

African savanna elephants are the biggest animals on land. They eat lots of plants every day.

They push over trees, dig for water and carry seeds far away. What if they stopped?

An African savanna is a wide, grassy land dotted with trees. Elephants are some of its busiest workers. They feed on trees and sometimes push them over, they dig for water in dry riverbeds, and they carry seeds a long way in their stomachs.

So what if elephants stopped shaping the savanna? In this thought experiment you can explore three paths: the trees, the water holes and the seeds. Real studies give us clues, but the answers are ideas, not certainties.

Animals that change their surroundings in ways that affect other species are often called ecosystem engineers, and South African National Parks scientists note that elephants are often given that name because they push over trees and dig holes in dry riverbeds. The African savanna elephant does this in at least three ways: its feeding alters tree cover, its digging opens water to other animals, and its long journeys move seeds across the landscape.

This lab removes those effects in imagination. Each path draws on a real study, from Kruger National Park in South Africa and from the deserts of northwestern Namibia, and labels clearly where the evidence ends and informed speculation begins.

Elephants eat grass, leaves, bark and even small trees.

They use their trunks and tusks to dig for water. Other animals drink there too!

Elephants eat a lot. San Diego Zoo Wildlife Alliance says they eat about 75 to 150 kilograms of plants a day and spend about 16 hours a day eating. African elephants eat grasses, leaves, shrubs and small to medium-sized trees. Their trunks are strong enough to pick up large tree trunks.

In dry places, elephants also dig for water. In the deserts of Namibia, they dig wells in dry, sandy riverbeds, and animals such as springbok, jackals and baboons drink from them too.

A savanna elephant eats roughly 75 to 150 kg of vegetation a day, about 4 to 6 percent of its body weight, and spends around 16 hours a day feeding, according to San Diego Zoo Wildlife Alliance. As grazer-browsers they take grasses, leaves, shrubs and small- to medium-sized trees, using their tusks to dig for water and food.

In Namibia's arid Kunene region, Ramey and colleagues (Pachyderm, 2013) describe elephants digging wells up to 1 m deep with their feet and trunks in dry sandy riverbeds, where groundwater nears the surface. Numerous other species, including springbok, jackals and baboons, use these wells.

Now imagine the elephants stop pushing trees and digging.

More bushes and trees might grow. Some dry riverbeds might have no water holes. Some seeds might not travel so far.

In our imaginary savanna, elephants are still there, but they stop shaping the land. No more feeding on big trees, no more wells in the riverbeds, no more long seed journeys.

What might change? Trees and bushes might spread in some places, leaving less open grass. In dry seasons, springbok and baboons might lose some places to drink. And some trees might no longer get their seeds carried far away. These are ideas: scientists know elephants do these jobs, but exactly how a savanna would change without them would depend on fire, rain and many other things.

Removing the engineering in imagination suggests three changes, each an idea rather than a prediction. Tree cover might increase where elephant feeding had held it back, though Shannon and colleagues concluded that tree deaths in Kruger come from a combination of factors, including fire, drought and disease, rather than elephants alone. Species that drink from elephant wells in desert riverbeds might lose a water source in the dry season. And trees whose seeds elephants carry tens of kilometers might lose their longest-distance disperser.

The size of each effect would vary with rainfall, fire, other herbivores and time, which no single study can capture.

An elephant is very big and very strong. Its trunk can lift a whole tree trunk.

Seeds ride inside it and come out far away. And it can dig down to water in dry riverbeds.

Elephants are so big and strong that they can do things few other animals can. Their trunks can lift large tree trunks, and they spend about 16 hours a day eating. In one park, scientists found elephant feeding marks on most of the large trees.

They also travel far. A study of elephant dung in South Africa found they can carry seeds up to 65 kilometers, about 30 times farther than savanna birds carry them. And their digging reaches water that smaller animals cannot get to on their own.

Elephants are the largest living land animals, strong enough to lift large tree trunks and eating 75 to 150 kg a day, much of it from woody plants. In southern Kruger, 60 percent of 3,082 large trees surveyed showed elephant use, with marula (Sclerocarya birrea) actively selected (75 percent of marula trees used).

They are also exceptional seed dispersers: a dung study reported by the University of Cape Town found seeds carried up to 65 km, 30 times farther than savanna birds take them, suggesting elephants help maintain the genetic diversity of savanna trees.

Scientists count trees, test water and follow seeds.

They found that elephants matter a lot. But rain, fire and other things matter too.

In Kruger National Park, researchers checked 3,082 large trees. About 60 percent had been fed on by elephants, and 4 percent had died because of elephant feeding. But the scientists concluded that fire, drought and disease also help decide which trees die, not elephants alone.

In Namibia, scientists tested the water in elephant wells and found it had far fewer bacteria than nearby pools and streams. Elephants dig wells even right next to flowing water, perhaps to reach cleaner water.

Shannon and colleagues (Journal of Tropical Ecology, 2008) surveyed 3,082 trees at least 5 m tall along 22 transects in southern Kruger: 60 percent showed elephant use and 4 percent had died as a direct result of elephant foraging. Trees farther from permanent water were more likely to be used and pushed over. They concluded mortality is likely driven by fire, drought and disease together with elephants.

Ramey and colleagues (Pachyderm, 2013) found significantly lower coliform bacteria counts in fresh elephant wells than in the nearest surface water, and concluded that well-digging is an attempt to reach less-contaminated water. Wells seldom last more than a day or two before collapsing.