Attractive tree ISO lemur to start a family

Science News, July 31, 1999 by S. Milius

A much discussed, rarely demonstrated kind of relationship between plants and animals has turned up between certain trees and the lemurs of Madagascar, report German researchers.

Twenty tree species in the island's dry forests depend largely on lemurs to eat their fruit and disperse their seeds, according to Jorg U. Ganzhorn of the Institute of Zoology in Hamburg and his colleagues. In the August CONSERVATION BIOLOGY, they warn that the new generation of these dependent species looks sparse in forest fragments where lemurs are disappearing.

Conservationists have fretted for years about the possibility that the loss of fruit-eating animals could wipe out the plant species that they disperse. Fruit that plops to the ground under maternal branches rarely gets a good start in life. It faces such menaces as deep shade and seed-damaging grazers that know where to find a good thing. Seeds in fruit eaten by the right animal, however, can hitchhike in its gut to more promising spots.

Demonstrating ecological effects of fruit eaters has been difficult, Ganzhorn explains, because so many diets overlap. In parts of Africa, a fruit that doesn't get gobbled by monkeys could still be spread by antelopes, bats, birds, or elephants. Some New World floodplain trees even get dispersed by fish.

Also, a tree can live so long that ephemeral creatures like people might not notice that the loss of seed dispersers is changing a forest. "It looks all bright and sunny," Ganzhorn says, yet the nonreproducing trees are just "living dead."

One famous proposed example of a disperser affecting a plant turns out to be less than clear, Ganzhorn notes. In the 1970s, scientists reported that no new generation of Calvaria trees had sprouted in Mauritius in the last couple of centuries. They speculated that without the dodo, the tree seeds no longer germinate. More recent work, however, has located some postdodo saplings.

Madagascar appears to be the perfect place to look for a new example of disperser dependence because the island has few fruit-eating mammals and birds, Ganzhorn notes. He has watched animals and collected droppings year-round. Lemurs seem to be major dispersers, especially the brown lemur. With the possible exception of the bush pig, it was the only animal that eats and disperses seeds greater than 11 millimeters long.

Madagascar's booming human population is wiping out wild habitat. The researchers compared sapling numbers at plots in extended forests with plots in forest fragments where much of the largest wildlife had disappeared. In the fragments without the brown lemur, many fewer saplings of the 20 tree species dependent on lemurs were sprouting than in the larger areas. Saplings from trees dispersed by other animals thrived in both locations.

"It's the first place we've been able to make the connection," Ganzhorn says.

Kent H. Redford of the Wildlife Conservation Society in New York welcomes the study as "amongst the best we've got" on disperser dependence. For a bulletproof case, Redford would require studies that add and remove animals from plots. He says that he also wants to know more about other forest interactions, such as what rodents and ants do to the seeds that lemurs leave behind.

Despite his questions, Redford finds value in correlational studies such as this one. "There's a tension between our work as scientists and our work as conservationists," he says. "If we wait for proof that convinces us as scientists, the chances are there will be no work for us as conservationists."