CONSERVATION in many parts of the tropics relies on preserving fragments of
forest left over after farmers have cleared the rest. But such efforts may be
wasted on areas of 100 hectares or less, which new research suggests are too
small to remain as self-regenerating ecosystems.
Julieta Benitez-Malvido of the National University of Mexico in Morelia
studied 11 fragments of forest ranging in size from 1 to 100 hectares near
Manaus, Brazil. She found that the density of shade-tolerant
seedlings鈥攚hich represent the majority of trees鈥攄ecreases
dramatically towards the forest edge, and is up to 40 per cent less in the
corners of a fragment than in the centre. She concludes that below a certain
size, rainforest segments cannot contain enough seedlings to fully regenerate
the plant species within them (Conservation Biology, vol 12, p
380).
鈥淲hen I started the project, I thought seedling density would be higher
closer to forest edges, as the edges are lighter,鈥� says Benitez-Malvido. In
fact, the opposite was true. One of the key factors was the change in climate
within the forest, with a hotter, drier environment near the edges preventing
germination of shade-tolerant species. They were also hindered by having to
compete with light-tolerant species that have higher growth rates.
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The reduced number of animal species in small forest sections is also an
important factor. Around 80 per cent of tropical trees are pollinated by
animals, which also help to disperse the seeds and fruits. Fig trees, for
example, are pollinated by wasps and their seeds dispersed by bats, rodents,
monkeys, iguanas and ants. Fewer primates, birds and other animals means there
is less 鈥渟eed rain鈥濃�攕eeds dispersed in faeces.
Benitez-Malvido concludes that even 100-hectare fragments do not nurture
enough seedlings to be self-sustaining. But she admits that preserving fragments
of 100 hectares or less may be the only option in some areas. 鈥淎t least these
fragments can serve as germplasm pools for many animals and plants,鈥� she says.
鈥淲e need to learn how to manage these patches to preserve them, and if possible
to increase their area.鈥�