Cross-boundary subsidy cascades from oil palm degrade distant tropical forests

Matthew Scott Luskin,Kalan Ickes,Justin S Brashares,Christine Fletcher,S Joseph Wright,Matthew D Potts,I-Fang Sun

doi:10.1038/s41467-017-01920-7

Abstract

Native species that forage in farmland may increase their local abundances thereby affecting adjacent ecosystems within their landscape. We used two decades of ecological data from a protected primary rainforest in Malaysia to illutrate how subsidies from neighboring oil palm plantations triggered powerful secondary ‘cascading’ effects on natural habitats located >1.3 km away. We found (i) oil palm fruit drove 100-fold increases in crop-raiding native wild boar (Sus scrofa), (ii) wild boar used thousands of understory plants to construct birthing nests in the pristine forest interior, and (iii) nest building caused a 62% decline in forest tree sapling density over the 24-year study period. The long-term, landscape-scale indirect effects from agriculture suggest its full ecological footprint may be larger in extent than is currently recognized. Cross-boundary subsidy cascades may be widespread in both terrestrial and marine ecosystems and present significant conservation challenges.

Highlights

Native species that forage in farmland may increase their local abundances thereby affecting adjacent ecosystems within their landscape
We studied wild boar reproduction and forest tree dynamics over two decades in a 130 km[2] forest reserve in Peninsular Malaysia surrounded by oil palm plantations
We evaluated whether oil palm subsidies, by increasing wild boar abundance, indirectly caused negative impacts on forest trees in the Pasoh Research Forest (PRF)

Summary

Introduction

Native species that forage in farmland may increase their local abundances thereby affecting adjacent ecosystems within their landscape. Just as the spatiotemporal availability of agricultural resource subsidies affect ecotone food webs, the movements of subsidized animals can extend the ecological impacts of cultivation into food webs in far away and seemingly unaltered areas[12] These secondary food-web impacts (cascades) can take many forms, but may degrade otherwise protected ecosystems (Fig. 1). Monoculture oil palm cultivation is an ideal system in which to study the ecological impacts of agricultural subsidies because plantations continuously produce fruit for 20–25 years, are cleared and replanted over a 4- to 6-year period, during which time fruit is completely absent[19]

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Results

Conclusion