Can Codependent Species Survive Forest Breakup?

John Pickrell
for National Geographic News
November 12, 2003
Scientists already know that dividing ecosystems into chunks can fatally limit an animal's territory or obstruct migration routes, sending species spiraling towards extinction. Now, new research says that forest fragmentation can also harm plants and animals in less obvious ways, severing the relationships both need to survive.

A new study, published this week in the Proceedings of the National Academy of Sciences, shows that habitat fragmentation dangerously disrupts the life cycle of the tropical African tree Leptonychia usmabarensis. The tree, found only in one mountain range, relies on birds to eat its fruit and disperse the seeds it produces as few as five times each century.

Researchers studied some of the many, isolated chunks of montane forest studding Tanzania's colonial-era and later tea plantations. They found that trees there encountered substantially fewer seed-dispersing birds than did trees in continuous tracts of forest.

"Even substantial populations of plants or animals may be vulnerable to extinction if key [partners] that they depend on, disappear from remnant forest fragments," researchers wrote.

Disperse to Survive

Norbert Cordeiro, an ecologist with the Tanzania Wildlife Research Institute in Arusha and the University of Illinois in Chicago, said, "For some tree species, seed and seedling survival is greatly increased when seed dispersers deposit seeds away from parent trees." Not only is there great competition between neighboring seedlings, but piles of rotting fruit beneath parent trees can attract rodents, insects, and microbes that may destroy the seeds all together, he said.

Plants have hit upon many ingenious ways to disperse their seeds by wind, water, and other means. Floating coconuts are one example. More common ones are the tasty, seed-bearing fruits that many trees produce. Animals eat or remove them, excreting or dropping seeds away from the parent tree.

In wet tropical forests, 80 to 90 percent of tree species may attempt to disperse their seeds in this manner, producing smelly, energy-rich, or colorful fruits (such as the avocado or the mango) to attract birds, monkeys, and other animals, said Cordeiro.

Leptonychia and other trees in the moist forests of Tanzania's East Usambara Mountains are no exception. These mountains belong to one of the world's 25 global biodiversity hotspots, or most species-rich regions. One in five plants found in the mountain ecosystem there grow nowhere else. These forests have persisted for 30 million years or more, giving ample chance for relationships of mutual benefit between key animals and plants to evolve.

Less Regeneration

But starting with 19th-century colonial tea plantations, deforestation and agricultural development in Tanzania have reduced and fragmented the mountain range's original 950 square kilometers (366.8 square miles) of forest to an area less than half that size today.

To test how fragmentation might have affected tree regeneration in East Usambara's forests, Cordeiro and study co-author Henry Howe of the University of Illinois in Chicago, compared seed dispersal success of Leptonychia trees found in four isolated forest patches less than 35 hectares (86 acres), with those trees found in continuous tracts of forest covering 3,500 hectares (8,649 acres).

The pair found that fewer birds were available to eat the tree's bright red, waxy fruit and disperse its seeds in smaller patches of forest.

Three of the tree's four most common fruit-eating visitors—the striped-cheek greenbul (Andropadus milanjensis), Shelley's greenbul (Andropadus masukuensis), and the green-headed oriole (Oriolus chlorocephalus)—removed 62 to 75 percent less fruit from the trees than in the continuous forest patch. A fourth species, the olive thrush (Turdus olivaceus), was totally absent in small forest patches.

Trees in isolated patches were lucky to disperse as few as a quarter of the seeds that their relatives in the continuous forest were able to spread. These forest fragment trees were more likely to have dense clumps of seedlings growing near the parent tree.

Cordeiro noted that Leptonychia isn't even designated as an endangered species. "If this is happening to a tree common in the landscape, what could be happening to much rarer trees?" he said.

The idea that fragmentation could break up critical partnerships between species "has been floating around for a good while," commented Daniel Simberloff, an ecologist at the University of Tennessee in Knoxville. But, until now, there has been no direct evidence, he said.

"Seeds of this tree are dispersed by a number of non-specialist dispersers, not by one species that seeks out this tree," said Simberloff. "Thus, this is a species that might have been less expected to suffer from fragmentation than many others, since there will always be some possible seed-dispersing species around."

Trees dependent on a single animal species for seed dispersal could be at even greater risk of extinction, he said.

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