On the importance of elephant poop

In the last century or so, Asian elephants have lost some 95 percent of their habitat and 90 percent of their population, and there are now fewer than 50,000 Elphas maximus elephants. One consequence of the loss is that some plant species are losing a key seed disperser. Elephants eat the plant’s fruit and defecate the seeds, often far away from the parent plant.

But elephants aren’t the only frugivores in the forest. Perhaps some other species could pick up the slack, such as monkeys or bovids.

Domestic cattle and buffalo are the most likely contenders, Nitin Sekar and colleagues at Princeton University and the Indian Institute of Science in Bangalore note in the August Ecology. “Across much of the tropical world, domestic bovids are arguably inheriting ecosystems from megaherbivores and wild ungulates,” they write. “They are an undeniable ecological force.”

Sekar and his colleagues compared the seed dispersal capabilities of cattle and buffalo with that of Asian elephants from the Buxa Tiger Reserve in India. They looked at three tree species: the elephant apple, wild guava and Artocarpus chaplasha, a relative of the jackfruit. For each species of tree, the researchers looked at how much fruit the three mammals would eat and how much of the fruit seeds would pass undigested through the animal (because a digested seed won’t germinate). They then took those seeds and planted them, to see how many would germinate and how quickly. And the team calculated how far the seeds could travel, based on how long they stayed inside the animals’ digestive tracts and the distance over which those animals traveled while the seed passed through the animal.

By most measures, elephants are better for dispersing the tree seeds than the bovids, the researchers found. Bovids had difficulty even eating the elephant apples; the fruit was too tough. Cattle had a better time with A. chaplasha, and both bovids managed to eat and defecate the seeds of the wild guava. But the seeds of all three species were almost always more likely to germinate if they had passed through an elephant than through a cow or buffalo.

The owned livestock also had a very different pattern of movement from the free-roaming elephants. Each day, the cattle and buffalo would leave their owner’s home, travel a few kilometers, and return home. They tended to revisit the same spot day after day, shifting every several days. The result of that is that seeds may be dropped not so far from where they are consumed. Elephants, in contrast, have more variation in their movements. They may only move a very short distance in day — or they might roam more than 10 kilometers.

Simulating potential seed movement revealed that elephants can carry seeds some 50 kilometers from a tree, but the bovids could take them less than 5 kilometers. And 20 percent of the seeds that elephants disperse travel farther than the ones carried the farthest by bovids. “Elephants are more effective than domestic bovids as dispersers” of the seeds of these three tree species, the researchers conclude.

The disappearance of seed dispersers is often overlooked when people are considering potential drivers of biodiversity loss, another group of researchers noted in Biological Conservation in 2012. But it should probably get more attention, especially if there is any hope that plants will be able to migrate in response to climate change. If plant species lose the ability to spread their seeds (or that ability is curtailed), then that will probably reduce the chance that they will be able to keep up with the movement of favorable climes.