Biodiversity Erodes Fast in Amazonian Forest Fragments
6 Dec, 2006 11:11 am
Amazonian rainforests contain some of the most biologically-rich ecosystems on the planet and are being cleared and fragmented at truly alarming rates. I lead an international team of scientists that is attempting to understand how habitat fragmentation affects complex Amazonian rainforests?specifically, the tree communities that form the foundation of forest architecture and biodiversity. We have discovered that the tree communities in fragments change far faster and more dramatically than we might ever have imagined.
We summarize our findings in an article just published in the leading journal Proceedings of the National Academy of Sciences USA. Our analyses are based on nearly 32,000 trees of 1170 species that have been monitored since 1980 in fragmented and intact forests in central Amazonia. The forest fragments we study range from one hectare (the size as two football fields) to one hundred hectares in area, and were created when local ranchers felled and burned the surrounding forest to make cattle pastures.Because rainforest trees can live for centuries or even millennia, none of us expected the tree communities to change very rapidly. But in just two decades—a wink of time for a thousand year-old tree—the ecosystem has been seriously degraded.
Ecological changes near fragment margins drive many of these changes. When a fragment is created, hot, dry winds from the surrounding pastures blow into the fragments. This greatly stresses the rainforest trees, some of which simply drop their leaves and die standing. In addition, powerful winds build up when the surrounding landscape is denuded of trees, and the winds slam into the fragments, toppling or snapping many trees. Many trees are linked together by heavy climbing vines (called lianas), so that when one tree falls, it sometimes drags down several other trees with it.
Because of such changes, trees die fast in fragmented rainforests. The biggest trees—many of which are ancient and slow-growing—are especially vulnerable.
The new trees that regenerate in fragments are often completely different from the species that died. We evaluated twenty-two characteristics of the “winning” and “losing” species in fragmented forests, in order to understand how the tree community is changing. The winners are mostly fast-growing pioneer and generalist species that favor forest disturbance, whereas the biggest losers are slow-growing trees specialized for life in the dark forest understory, that need animals such as birds or bats to disperse their seeds and pollen.
Fragmentation also dramatically changes forest structure and dynamics. The forest fragments are highly unstable, with major pulses of tree mortality caused by periodic windstorms or droughts. Fragments become dominated by small, fast-growing species that tend to die young, rather than by tall, slow-growing giants that dominate in undisturbed rainforest.
Our findings have important implications. Rainforest trees are involved in complex arrays of ecological interdependencies with many pollinators, seed dispersers, herbivores, and parasites, and some of these relationships are remarkably specialized. Most fig and orchid species, for example, each rely on a single species of insect for pollination. If the tree communities change, it is almost inconceivable that the animal, plant, and fungal species that rely on the trees will not change as well.
Forest fragmentation could even affect global warming. Small, fast-growing trees that proliferate in fragments contain less biomass, and thereby store less carbon, than do the original rainforest trees they replace. Carbon from the dead trees becomes carbon dioxide, the most important greenhouse gas. Hence, loss and fragmentation of the Amazon not only imperils the world’s most biodiverse ecosystem—it also has far-ranging effects on us all.
Laurance, William, et al. Proceedings of the National Academy of Sciences USA. 27 Nov 2006
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