生態学：伐採された森林でも生態学的活力が大きい

Selectively logged forests in Borneo can be highly diverse and functional ecosystems, despite sometimes being classified as degraded systems, a study in Nature suggests. Although protecting pristine, relatively undisturbed forests is crucial, this study demonstrates the value of also preserving ecosystems that are often labelled as degraded and are thus converted for agriculture.

Selectively logged and other types of structurally altered forests are becoming the prevailing vegetation cover in many tropical regions. Logged tropical forests in Borneo would be considered degraded ecosystems under typical measures of carbon stocks or forest structure, and consequently would be allowed to be converted for agricultural purposes such as oil palm plantations. By contrast, more ‘pristine’, so-called old-growth forests are prioritized for protection. While old-growth forests do have value in terms of carbon stocks and biodiversity, logged ecosystems still contain complex energetic food webs.

Yadvinder Malhi and colleagues estimate primary productivity of vegetation across old-growth forests, logged forests and oil palm plantations. They also survey mammal and bird species across these landscapes to calculate food energetic pathways: how photosynthetic energy cascades from sunlight to be distributed among organisms. The authors find that relative to energy flow in old-growth forests, 2.5 times more total energy flows through these pathways in logged forests. Open forest structure in logged areas is likely to be the reason for this amplified energy flow, as it makes vegetation more accessible at ground level, and light availability favours fast-growing plants with fewer chemical defences and higher nutrient content, which makes food more palatable for animals. This ecosystem also seems to largely retain the same level of resilience as in old-growth forest, as a diverse range of species indicates how an ecosystem will respond to the loss of individual species. In oil palm plantations, the proportion of energy consumed by mammals and birds shows a large decline compared with logged forests, alongside increased ecosystem vulnerability.

Even heavily logged forests can be vibrant and diverse ecosystems with high ecological function, and conversion of these logged forests into oil palm plantation results in the collapse of most energetic pathways. The authors suggest that the meaning of the term ‘degradation’ in this context may need changing to protect forests that still have the capacity to maintain functional ecosystems.