The ever-expanding global population is already placing an intense strain on natural resources, and one of the major facets of this is land-use change. Loss of natural habitats to human land-uses, particularly agriculture, is predicted to be the major driver of biodiversity loss in the 21st century (Sala et al 2000), and this is particularly true in tropical forest ecosystems.
Tropical forests contain the highest diversities of plants and animals anywhere on the planet (Myers et al 2000). With many located in some of the fastest-developing economies, preserving this biodiversity is an ongoing challenge. One of the richest of all tropical forest regions, south-east Asia, is experiencing one of the most rapid rates of loss, and could lose 75% of its original forest extent and 30-40% of its biodiversity by 2100 (Laurance 1999, Sodhi et al 2004, Wilcove et al 2013).
As well as preventing a mass extinction of enormous proportions, maintaining a sustainable approach to tropical forest land-use is critical to preserve the supporting ecosystem services forests provide to humans. These include soil stability, carbon sequestration, weather and climate modulation, pest control, flooding control, and many others.
In south-east Asia, two land use changes are simultaneously occurring in tropical forest. These activities; selective logging and clearance for plantation agriculture, present very different prospects for sustainability.
Logging and biodiversity
Selective logging involves removal of large, high-value timber trees, which are felled and dragged out of the forest, leaving smaller or worthless trees behind. Selectively logged forests account for at least 20% of the global tropical forest biome (Edwards et al 2014a) and the great majority of surviving lowland forest in south-east Asia (Meijard and Shiel 2008). These forests have generally undergone one or two rounds of tree removal, which leaves a ragged, open and structurally simplified forest cloaked in vines and shrubby growth, with greatly reduced above-ground biomass (Berry et al 2010).
Nevertheless, the great majority of biodiversity found in untouched, primary forest persists after logging, with 75% of birds and dung beetles surviving (Edwards et al 2011), although abundances do often decline, and complex changes to community composition occur. Furthermore, after logging forest retains similar levels of (Edwards et al 2013, 2014c).
If logged forest is allowed to rehabilitate after production ceases, biomass accumulates five times as quickly as in primary forest (Berry et al 2010), sequestering atmospheric CO2, and potentially allowing re-harvesting on a 50-100 year cycle as trees regenerate. However, the length of this cycle may well preclude economic sustainability. Selective logging experiences rapidly diminishing returns, and under existing economic conditions logging concessions generate no further income until the forest has regenerated and is ready for further harvesting. This, together with little or no legal protection for these forests, leaves them open to the second land-use change: conversion to plantation agriculture.
In search of sustainable palm oil
Agriculture is the number one cause of forest loss in the tropics generally, and in south-east Asia in particular. The most rapidly-expanding crop is Oil Palm (Elaeis guineensis), which is grown in plantations to provide palm oil for processed food, cosmetics, plastics etc.
80% of the world’s palm oil is grown in Indonesia and Malaysia, most of it at the expense of remaining primary and logged forest (Fitzherbert et al 2008, Koh and Wilcove 2008). Rates of forest loss in this region are among the highest in the world (Sodhi et al 2004). The palms are grown in monocultures, often with herbicide suppression of ground flora and minimal buffers around rivers, so it is perhaps unsurprising that after conversion of forest to oil palm, species richness in all taxa which have been studied declines rapidly and dramatically (Edwards et al 2014b). The same pattern occurs with functional diversity, and crucial ecosystem functions, such as flood prevention, pest control pollination and erosion containment are also lost.
Attempts have been made to improve the sustainability of oil palm plantations, partly driven by increased consumer awareness of the ecological costs. The Round Table on Sustainable Palm Oil (RSPO) is one example of an international voluntary agreement on sustainable palm oil which aims to encourage improved landscape-scale planning, such as identification of areas of “High Conservation Value” to be avoided during land conversion (RSPO 2007). “Wildlife-friendly” improvements to oil palm plantations have also been mooted, such as encouragement of trunk epiphytes, retention of forest fragments within oil palm plantations, and of other species with the oil palm (Fitzherbert et al 2008).
However, there is little evidence to suggest that these measures significantly improve the biodiversity value of oil palm (Koh 2008, Edwards et al 2010, but see Nájera and Simonetti 2010), and NGOs such as Friends of the Earth and WWF argue that the RSPO and other voluntary certification schemes lack legal power, and fail to target the increasing demand for palm oil in developing and developed nations (eg. Friends of the Earth 2009).
Reaching a compromise
In order to prioritise sustainability in south-east Asian land-use, a compromise will have to be found between the expansion of oil palm agriculture and the retention of tropical forests. Calls by NGOs to halt and reverse agricultural expansion are impractical in the face of continued population growth and the need to increase standards of living, together with the lack of economically viable alternatives to palm oil.
Conversely, the generally poor recognition of the values of logged forest and landscape-scale planning by both governments and oil palm growers urgently needs addressing in order to avert a biodiversity disaster.
Solutions include greater regulation and enforcement, particularly a legally binding international replacement for conventions such as RSPO. Financial incentives to retain logged forest, such as international payments for ecosystem services and emission under programmes like REDD+, will also be necessary to pay the opportunity cost of not converting it to oil palm (Edwards et al 2014a) as well as the option of NGOs becoming oil palm growers in order to raise revenue for purchase of private reserves of logged forest (Koh and Wilcove 2007).
Above all, more joined-up thinking, dialogue between regulators, producers, NGOs and scientists will be needed to promote clever solutions to this global problem.
Berry, N.J.; Phillips, O.L.; Lewis, S.L.; Hill, J.K.; Edwards, D.P.; Tawatao, N.B.; Ahman, N.; Magintan, D,; Khen. C.V.; Maryati, M.; Ong; R.C.; and Hamer, K.C. (2010). The high value of logged tropical forests: lessons from northern Borneo. Biodiversity Conservation 19, 985-997
Edwards, D.P.; Hodgson, J.A.; Hamer, K.C.; Mitchell, S.L.; Ahmad, A.H.; Cornell, S.J.; & Wilcove, D.S. (2010). Wildlife-friendly oil palm plantations fail to protect biodiversity effectively. Conservation Letters 3, 236–242.
Edwards, D.P.; Larsen, T.H.; Docherty, T.D.S.; Ansell, F.A.; Hsu, W.W.; Derhé, M.A.; Hamer, K.C.; and Wilcove, D.S. (2011). Degraded lands worth protecting: the biological importance of Southeast Asia’s repeatedly-logged forests. Proceedings of the Royal Society B: Biological Sciences 278, 82-90
Edwards, D.P; Magrach, A.; Woodcock, P.; Ji, Y.; Lim, N.T-L.; Edwards, F.A.; Larsen, T.H.; Hsu, W.W.; Benedick, S.; Khen, C.V.; Chung, A.Y.C.; Reynolds, G.; Fisher, B.; Laurance, W.F.; Wilcove, D.S.; Hamer, K.C.; and Yu, D.W (2014b). Selective-logging and oil palm: multitaxon impacts, biodiversity indicators, and trade-offs for conservation planning. Ecological Applications 24, 2029-2049
Edwards, F.A.; Edwards, D.P.; Larsen, T.H.; Hsu, W.W.; Benedick, S.; Chung, A.; Vun Khen, C.; Wilcove, D.S.; and Hamer, K.C. (2014c). Does logging and forest conversion to oil palm agriculture alter functional diversity in a biodiversity hotspot? Animal Conservation 17, 163-173