Tropical rain forests are disappearing at an alarming rate, threatening the world’s richest ecosystems and crucial carbon stores in favor of immediate needs such as fuel and raw materials.
Conservation efforts focus mainly on preserving pristine and undisturbed old forests, yet selectively logged forests—where timber is not clear-cut, but instead selectively harvested—now make up about a third of rain forests worldwide.
“The ecological value of logged forests has been underestimated; they are not as broken as they look,” said Yadvinder Malhi, an ecosystem ecologist from the University of Oxford who was involved in a large-scale biodiversity survey of forests and agricultural land in the state of Sabah, Malaysia, on the island of Borneo.
The results, which were published in December in the journal Nature, showed that logged forests can be buzzing with life and ecological functions and therefore have an important role to play in conservation.
Logged forests in the tropics are often seen as “degraded,” a label that is used to justify clearing them for agriculture such as oil palm harvesting. “That label rightly draws attention to the necessity of old-growth forests for biodiversity but can be dangerous because it suggests logged forests are of low ecological value,” said Malhi.
Ecologists have previously been unable to calculate exactly how ecosystem processes are affected by logging because of a lack of field data from the tropics, including species population counts.
To address this lack, Malhi and a team of researchers spent a decade collecting an extensive data set, including 36,000 measurements of tree growth rates and population data on 248 bird and mammal species from cameras, cage traps, and point counts.
“Their data [are] unusually rich,” said Natalia Ocampo-Peñuela, a conservation ecologist from the University of California, Santa Cruz who was not involved in the study. “It means they can go beyond showing the value of logged forests to quantifying that value.”
A Cascade of Sunlight
To quantify ecosystem function, the team calculated how much energy was produced and consumed through different levels of the feeding network and in adjacent sections of logged forest, old forest, and oil palm plantation.
All life on Earth is ultimately powered by energy in the form of sunlight, initially supporting plants and other photosynthesizing organisms, which in turn underpin the rest of the food chain by providing food for insects, birds, and mammals. “In our study we use this cascade of captured sunshine as an indicator of ecosystem vibrancy,” said Malhi.
For each area, researchers measured the productivity of the vegetation—the amount of biomass in terms of leaves and roots—and then calculated how much food energy was being consumed by all the birds and mammals on the basis of their population densities. That gave them an estimate of the fraction of captured sunshine moving through birds and mammals in the rain forest.
The surprise, said Malhi, was that the birds and mammals in the logged forests consumed twice as much food in the form of leaves and insects as those in the old forests. “We expected that the logged forests would just be holding on, but they are equally if not more ecologically vibrant.”
The reason for this increased energy, said Malhi, is simply that the logged forests become more edible—the plants put their energy into rapid growth rather than protecting themselves with chemical defenses as they would in a mature forest.
That increase in plant growth can sustain more insects and in turn more birds and mammals. The disturbed canopy also allows more light onto the forest floor, explained Malhi, meaning larger animals like elephants and deer can access lower-level vegetation. “In some ways you can liken the logged forests to a salad bowl—the leaves are richer in nitrogen and protein,” said Malhi.
“That doubling of energy is a remarkable indicator of the conservation potential of logged forests,” said Pieter Zuidema, a forest ecologist from Wageningen University and Research in the Netherlands who was not involved in the study. “This could kick-start a new avenue of investigation into ecosystem energetics in human-modified forests throughout the tropics, where logging intensities and management strategies differ and energy flow might vary.”
These results don’t mean logged forests are superior, cautions Malhi. “There are lots of reasons why old-growth forests are irreplaceable—including their biomass, carbon stores, and rare species.”
“Old forests can’t be beaten in terms of their biodiversity,” said Ocampo-Peñuela. “But mosaics of different land covers are becoming more common, and that means the landscape needs better protection as a whole, whilst balancing society’s needs.” She added that conservationists are increasingly seeing the need for this kind of holistic approach, but data on how different land types can complement each other are missing from the global conversation.
For Malhi, the findings are a ray of hope in the concerning trend of global deforestation, showing how resilient nature can be in the right circumstances. “We’re not implying that it’s fine to log more old-growth forests; this is about changing the perception that logged forests are somehow ruined ecosystems.”
—Erin Martin-Jones (@Erin_M_J), Science Writer