Degraded lands worth protecting: the biological importance of Southeast Asia's repeatedly logged forests

Abstract

Southeast Asia is a hotspot of imperiled biodiversity, owing to extensive logging and forest conversion to oil palm agriculture. The degraded forests that remain after multiple rounds of intensive logging are often assumed to be of little conservation value; consequently, there has been no concerted effort to prevent them from being converted to oil palm. However, no study has quantified the biodiversity of repeatedly logged forests. We compare the species richness and composition of birds and dung beetles within unlogged (primary), once-logged and twice-logged forests in Sabah, Borneo. Logging had little effect on the overall richness of birds. Dung beetle richness declined following once-logging but did not decline further after twice-logging. The species composition of bird and dung beetle communities was altered, particularly after the second logging rotation, but globally imperiled bird species (IUCN Red List) did not decline further after twice-logging. Remarkably, over 75 per cent of bird and dung beetle species found in unlogged forest persisted within twice-logged forest. Although twice-logged forests have less biological value than primary and once-logged forests, they clearly provide important habitat for numerous bird and dung beetle species. Preventing these degraded forests from being converted to oil palm should be a priority of policy-makers and conservationists.

Figure 1.

(a) Map of Sabah, Malaysian Borneo. The black region represents the Danum Valley Conservation Area (DVCA) and the arrow indicates a mist net and point count site at Tawau Hills National Park. (b) Map of the study area within the Yayasan Sabah (YS) logging concession. Circles indicate mist net sites, squares indicate point count sites, and black horizontal bars indicate pitfall trapping sites. Shading: dark grey, unlogged forest (DVCA); medium grey, once-logged forest; pale grey, twice-logged forest; white, oil palm outside the YS concession.

Figure 2.

Observed species richness, constructed using sample-based rarefaction curves for the three forest types. (a) All birds, (b) understorey birds and (c) dung beetles. The x-axis is scaled to show the number of individuals and scales differ between figures. Black circle, unlogged; grey triangle, once-logged; white diamond, twice-logged; dotted lines, 95% CI for unlogged forest.

Figure 3.

Non-metric multidimensional scaling (MDS) ordination of community structure among unlogged (black circle), once-logged (grey triangle) and twice-logged (open diamond) forest types. (a) All birds (stress = 0.08), (b) understorey birds (stress = 0.15) and (c) dung beetles (stress = 0.08).

Figure 4.

The prevalence of Red-Listed birds (Birdlife International 2009) in unlogged (black bar), once-logged (grey bar) and twice-logged (white bar) forest. (a) Mean abundance and (b) mean species richness. Letters denote significant differences at the p < 0.001 level using pairwise comparisons.

Figure 5.

Population-level responses of bird and dung beetle species to two logging rotations. The percentage of species recorded in unlogged forest that were absent (black), declined significantly (dark grey), revealed no significant change (light grey) and increased significantly (white) following once-logging (1L) and twice-logging (2L).