Functional Fragments

Mention fragmentation, and many ecologists fall to pieces. Roads, farms and settlements that cut woodlands and grasslands into disconnected chunks can have a big impact on ecosystems, isolating populations and hastening species loss. But a new long-term study from Kenya finds that even small forest fragments can sustain much of their ecological function, suggesting that even chopped-up forests could be candidates for conservation.

The results were a bit surprising, says Matthias Schleuning of Germany’s LOEWE Biodiversity and Climate Research Centre (BiK‐F), the lead author of the PLoS ONE study. “What we didn’t expect was the fact that the intensity of ecosystem functions such as decomposition, pollination or seed dispersal remain stable or have in some cases even increased in fragmented forests, in which selective logging of trees occurs. This means that the overall functionality of these forests has been preserved, despite moderate human disturbance.”

For nine years, Schleuning’s team studied the Kakamega rainforest in Western Kenya. It is a biodiversity hotspot, with more than 400 bird and more than 320 ant species. The forest is under threat, however, and its area has shrunk by more than one-half over the last century. The Kakamega has become a series of isolated fragments, surrounded by sugarcane, corn and other farm fields. And loggers continue to fell the remaining trees, often using “selective” logging techniques that leave less valuable trees standing.

To see if the fragments were still providing important ecological functions, the researchers studied eleven sites, recording both the diversity of animals and conducting experiments designed to measure function. For example, to measure decomposition, the researchers laid out leaf samples and watched to see how long it took them to decay. To measure the intensity of army ant raids, they buried hundreds of traps in the ground and then waited for the tiny troops to fall in. They also studied things like the predation of seeds, pollination, and seed dispersal.

They found that selective logging – which tended to leave larger chunks of connected forest – had just a moderate effect of animal diversity. As a result, mobile species – such as birds and ants – continued to provide important services. In fact, the scientists noticed that selective logging had a positive effect on pollination by insects, seed dispersal by birds and raids by army ants. In contrast, ecosystem function appeared to be more impaired in fragments that lost much of their animal biodiversity.

“The mechanisms of the two types of human intervention are different,” Schleuning says. “Fragmentation tends to have an indirect influence on ecosystem functions. The fragmentation of changes the biological diversity, which causes a threat to the preservation of their ecosystem functions. In contrast, selective logging impacts directly on ecosystem functions and has, at the moderate intensities in our study area, hardly any effect on the diversity of the animal species we examined.”

The next step, he says, is to do longer-term studies that see how functionality changes over time. But the results should stimulate a rethink of tropical forest conservation, the authors write. “The classical approach is to preserve large, intact forest areas, such as the Amazon or Congo basins,” Schleuning says. “Our study shows that it can also make sense to protect the many isolated rainforests that have been influenced by humans.” David Malakoff | November 29, 2011

Source: Schleuning M., Farwig N., Peters MK. et al. (2011) Forest Fragmentation and Selective Logging Have Inconsistent Effects on Multiple Animal‐Mediated Ecosystem Processes in a Tropical Forest. PLoS ONE 6 (11): e27785. doi:10.1371/journal.pone.0027785

Image copyright: N. Farwig, BIOTA‐E02