Madagascar's lemurs are disappearing ... and they're taking a precious commodity with them: their poop. And that's pretty bad news for the island's rainforests, new research warns. 

The researchers observed the eating (and pooping) habits of lemurs during a three-year study. Image: Onja Razafindratsima/Rice University

Researchers from Rice University in Texas in the US have been studying the important symbiotic relationship between the African island's endemic and increasingly threatened primates and the tree populations in the forests they call home. What they've discovered is that the animals play the crucial role of forest 'gardeners', unwittingly ensuring that seeds are effectively dispersed and so helping to maintain forest health and biodiversity.

Professor Amy Dunham, along with graduate student Onja Razafindratsima, conducted a three-year study that focused on three fruit-eating lemur species – the red-fronted brown lemur, the red-bellied lemur and the southern black-and-white ruffed lemur – and the seed-dispersal services they provide for trees in a rainforest on the southeastern part of the island.

The dispersal mechanism is pretty simple: a choice fruit is gobbled up, the lemur leaps from tree to tree and the indigestible seeds are distributed throughout the rainforest in its scat. But what's quite remarkable is just how good the lemurs are at depositing the seeds in just the right spot for sprouting. The researchers' observations, experiments and mathematical models show that seeds of a common canopy tree have a 300% higher chance of sprouting and becoming a sapling when dispersed by lemurs than by simply falling to the ground. 

“Lemurs fill an important role as the gardeners for these trees. By ensuring that some seeds land in spots suitable for germination and survival, they increase the ability of these trees to replace themselves,” says Dunham.

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Since radio collars were not used, the team enlisted the help of locals with extensive knowledge of the forest to help track 24 lemur groups. Image: Onja Razafindratsima/Rice University.

Aside from ensuring seeds find a good sprouting site, a bit of assistance from the lemurs also likely ensures that the seeds land a good distance away from their parent trees, where survival is low. “Seeds away from the parent tree survive better because there’s less competition among seedlings,” Razafindratsima explains. “If they’re close by the parent, they may also share the same natural enemies, like soil pathogens and seed predators, so there’s higher mortality.”

The red-fronted brown lemurs turned out to be particularly talented gardeners, dropping seeds well away from the parent plant and in places where there were gaps in the canopy.

But with 90% of Madagascar's 101 or so lemur species now threatened with extinction, the island's trees could soon lose their handy seed distribution service. This is a particular threat for those tree species that rely exclusively on lemurs for seed dispersal (as the largest fruit-eaters on the island, lemurs are able to swallow seeds that may be too large for smaller dispersers like birds or bats). But the loss of lemurs would also have consequences for the entire rainforest system.

"[If some dispersers are lost], this could have potentially important consequences for the tree populations ... and for the rainforest structure as a whole. If some species suddenly lose their dispersers ... but others dispersed by birds or the wind are doing fine, it may change population trajectories and alter which tree species are dominant in a community," explains Dunham.

By understanding more about how lemurs help to boost tree populations, the researchers hope to contribute to growing efforts to protect them. "What got us interested is that frugivorous lemur populations are declining across the island, and we know very little about how these seed dispersers actually affect tree populations. Once we understand that better, maybe we’ll have a better idea of how the community might change if the lemurs disappear," Dunham says. 

The new research appears online in the journal Ecology.

Top header image: Andrea Schieber, Flickr