Conversion of rainforest to both traditional and industrial oil palm systems changes the biodiversity, web-building, and prey capture of understory spiders (Liberia, West Africa)

Abstract

Oil palm (Elaeis guineensis) is a tropical crop that produces palm oil: the most traded vegetable oil worldwide. It is principally grown in Southeast Asia, but West Africa – oil palm’s native range – is rapidly becoming a hotspot of cultivation. Oil palm in West Africa is cultivated using both traditional (i.e., by local people) and industrial (i.e., by corporations) approaches. Little is known about the relative ecological impacts of these different oil palm cultivation styles on native rainforest ecosystems. Working in Liberia within the framework of the Sustainable Oil Palm in West Africa (SOPWA) Project, we investigated the impacts of traditional and industrial approaches to oil palm cultivation on spiders—important meso-predators that provide pest control services—inhabiting understory vegetation. We sampled spiders in three systems: (1) rainforest, (2) fallowed farmland with wild-growing oil palms, which local people manage and harvest (‘country palm’), and (3) industrial oil palm farms. We assessed differences in the abundance of all spiders, adults only, and juveniles only; spider species richness, and spider species-level community composition. Through focussed samples of orb web-building spiders, we also assessed differences in spider web-building (the average web area, total web area, standard deviation of web area) and prey capture (the average captured prey, total captured prey, standard deviation of captured prey) across systems. We found that rainforest had more species in total and on average than country palm and industrial oil palm, and that country palm had fewer spider individuals than rainforest and industrial oil palm. All systems supported distinct spider communities. Our focussed studies on orb web-building spiders indicated that the average web area, total web area, and standard deviation of web area were all higher in industrial oil palm than forest and country palm, which led to higher average and total prey capture. Our findings indicate that conversion of rainforest to country palm and industrial oil palm in Liberia has negative effects on spider biodiversity, with clear “winning” and “losing” species, and levels of spider-associated ecosystem functioning. Our findings also show that industrial oil palm farms can support relatively abundant and speciose spider communities, which may provide important pest control services that could boost oil palm productivity. Future work is needed to identify effective management strategies to conserve spiders and associated pest control services in West African country palm and industrial oil palm systems.