Disturbance Can Benefit Some Rare Forest Plants

"Forest destruction is a reality and finding that forests subject to human use can support endemic plants is good news," says Michael Kessler of the Albrecht-von-Haller- Institut fur Pflanzenwissenschaften in Gottingen, Germany, who reports this work in the June issue of Conservation Biology.

Kessler studied 650 plants that were both relatively common and easy to identify at 16 forest sites in the Bolivian Andes. The plants ranged from being widespread to endemic. At each site, Kessler compared areas with varying levels of human disturbance, including mature forest with little or no disturbance; moderately disturbed forest (where the original canopy layer was intact but there had been some logging, grazing or burning); and severely disturbed forest (such as secondary forest in areas that had been clearcut).

To his surprise, Kessler found that moderately disturbed forest had more endemic plant species than adjacent mature forest. However, severely disturbed forest had fewer endemic plants and was dominated by bracken ferns and other opportunistic pioneers that colonize cleared areas.

Why would moderate human disturbance be good for endemic plants? Endemics are more vulnerable to invasion by non- native species and apparently do not compete as well as widespread species. Kessler hypothesizes that endemics depend on natural disturbances (such as tree falls, flooding and landslides) to keep widespread species from dominating them, and so can also benefit from some human disturbance.

"My observation suggests that moderate use of tropical forests may be compatible with the conservation of endemic plant species," says Kessler. However, he cautions that not all endemic plants benefit from moderate disturbance, and that endemics do not benefit from severe disturbances such as clearcuts or extensive removal of canopy trees. Moreover, preserving undisturbed forests remains important because they often have more biodiversity than disturbed forests.