How much logging can tropical forests withstand?

It is universally agreed in conservation circles that when forests are razed to make room for roads or agriculture, the consequences for biodiversity are dire. But there are other forms of timber harvest, like selective logging, for which the consequences are either mixed or uncertain.

In selective logging, only a certain amount of timber may be harvested from a given area. Some studies have reported that selective logging results in decreased biodiversity, as you might expect, while others report that selective logging actually leads to increases in biodiversity. To try to see the forest for the trees, graduate student Zuzana Burivalova from ETH Zurich in Switzerland gathered up 48 studies that assessed the impacts of selective logging on biodiversity in tropical areas of South America, Africa, Eastern Asia, and the Pacific Islands.

What they found, perhaps surprisingly, was that species richness is slightly higher for mammals, amphibians, and invertebrates in “lightly logged forests” than in “primary forests” that remain undisturbed. But as logging intensity goes up, diversity for those three taxonomic groups goes down.

Biodiversity for mammals would be halved at a logging intensity of 38 cubic meters per hectare, and 63 cubic meters per hectare for amphibians. Burivalova and her colleagues suspect that the reason that mammals are so sensitive to logging is the increased hunting and poaching pressure that is associated with logging. As roads are carved through forests, it brings humans into closer contact with wildlife that would previously have been inaccessible. For amphibians, on the other hand, they surmise that they might suffer because logging creates warmer and drier microclimates. That’s a problem for species that require cooler, wetter habitats. In addition, logging might directly kill more amphibians than other types of animals because they’re far less agile than birds or mammals.

Invertebrate biodiversity, like mammals and amphibians, also suffered from increases in logging intensity, but at a far more gradual pace than for mammals or amphibians.

Birds, on the other hand, show the opposite trend. Their species richness increases with logging intensity. However, a closer analysis revealed a more complicated picture. As logging increases, the diversity of bird species that are forest specialists decreases, while the overall level of bird diversity rises thanks to generalist species. In other words, birds that are uniquely adapted to their forest habitats suffer from logging, but they are replaced with a larger amount of bird species that can survive in a wider range of conditions. Birds are also highly mobile; they may be able to nest in primary forests, while still using selectively logged forests for foraging. A patch of forest used mainly for foraging rather than for nesting might therefore appear to contain a higher level of avian biodiversity.

selective logging How much logging can tropical forests withstand?

Left: Biodiversity changes as logging increases. Right: For birds, while total species richness in logged forests increases, that’s driven by habitat generalists rather than specialists.

Taken together, Burivalova’s findings suggest that a small amount of logging may be desirable, but that it needs to be carefully controlled and kept to a relative minimum. This isn’t the only study to arrive at that conclusion. Other research has indicated that “reduced-impact logging” helps to maintain plant diversity, soil quality, and carbon accumulation. Burivalova offers a maximum logging intensity of 10 cubic meters per hectare as an upper limit to allow all taxonomic groups to remain resilient. – Jason G. Goldman | 08 August 2014

Source: Burivalova Z., Sekercioğlu C.H. & Koh L.P. (2014). Thresholds of Logging Intensity to Maintain Tropical Forest Biodiversity., Current biology : CB, PMID: http://www.ncbi.nlm.nih.gov/pubmed/25088557

Header image: shutterstock.com

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