It’s easy to understand how the clear-cutting of vast tracts of tropical forest might be bad. After all, the loss of all those trees is bound to also take out many of the animals that made that forest their home. So selective logging—in which just at most 20 trees are removed from a single hectare of land (10,000 square meters, about the size of two football fields)—would seem to be a no-brainer improvement.
But a new study published today in Current Biology is adding to evidence that this type of timber removal can still be destructive. Zuzana Burivalova of ETH Zurich and colleagues found that taking out just three or four trees in a hectare of tropical forest can halve the number of mammal species present. Logging six or seven trees can do the same to amphibians.
Burivalova and colleagues looked at 48 published studies of species diversity in forests before and after they were logged. With this information, they built a computer model that let the researchers calculate the effects of selective logging on wildlife in tropical forests in three regions: Africa, Central and South America, and Southeast Asia.
The model showed them that for mammals, amphibians and invertebrates, species richness (the number of different species) dropped as logging intensity rose. But it didn’t take all that much logging to have a big effect on mammals and amphibians.
Extrapolating from the published data, the researchers found that amphibian diversity dropped by 50 percent once logging reached 63 square meters per hectare; for mammals, it took the removal of only 38 square meters of forest from that two-football-field-sized parcel to cut diversity in half. The number of different bird species, however, actually rose on logged land, the researchers calculated.
So what’s up with this seemingly counterintuitive result? After all, selective logging brings up images of one guy with a chainsaw removing a single tree from a forest with pinpoint precision. But this type of logging can take out multiple trees at a time, and it requires more than just one guy. There’s the guy’s buddies, their trucks and equipment, and the roads to get them where they need to go. The logging can leave behind bare patches of forest that are open to the heat of the sun. And it can let in people who are even more dangerous to wildlife than loggers—hunters.
Just which factors are driving the changes in richness of the various types of animals isn’t exactly clear, but the researchers have some ideas. Mammals, for instance, are probably taking a hit from poaching. Amphibians may be more susceptible to the changes in microclimates created by logging—those spots that become unbearably hotter and drier when the heat of the sun is suddenly able to reach the ground.
And the story for birds may not be as great as it seems. That’s because the types of birds in logged forests frequently changes and some species are lost, while others move in. Specialists that may have depended on the fruit or nectar of specific trees or plants may disappear, for instance, replaced by species that are generalists and can survive on a more varied diet.
There was a level below which species appeared to be resilient to logging, according to the model—10 square meters per hectare, or about one tree or less in that space. And the researchers note that there are measures that can be taken to reduce the impact of logging on forest creatures, such as leaving behind hollow trees that provide homes for animals but aren’t all that useful to the timber industry.
Burivalova and colleagues caution that their results are not applicable to every forest—some will be more resilient than others. But they warn that the conservation value of this type of logging may be overestimated, and efforts to harvest tropical trees sustainably may not be as successful as hoped.
Plus, the researchers note, it would be helpful if the timber harvesters would consider biodiversity, and not just which species are the most desirable and profitable, when choosing which trees to cut down.