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).

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