Change in the Litter Dwelling Arthropods after the Establishment of Rubber and Oil Palm Plantations in the Forest Landscape

Abstract

Background: The forest conversion into agroecosystems is perceived as a threat to biodiversity conservation. Aims: The study aimed to assess the impact of tropical forest conversion into agricultural land on the communities’ patterns of litter arthropods. We expected that over time the abundance of litter dwelling arthropods and their diversity in plantations would reach similar conditions as in secondary forest. Study Design: The fieldwork was conducted on two study sites: 1– rubber landscape (secondary forests, 7-, 12- and 25-year-old rubber plantations) and 2– oil palm landscape (secondary forests, 13-, 20- and 39-year-old oil palm plantations). Three sampling areas were defined on each land-use type and age class, for a total of 24 sampling stands. Place and Duration of Study: The study was conducted in the department of Grand Lahou and La Mé Station with samples collected, respectively, in May 2013 and August-September 2017. Methodology: Over a 40–50 m transect, litter invertebrates were sampled by using the pitfall traps. Five pitfall traps were realized per stand along the transect, either a total of 120 pitfall traps for the two sites combined. Results: The abundance of litter dwelling arthropods increased with the ageing of rubber plantations whilst in the oil palm plantations, diversity and the abundance tended to decrease with the increasing age. The biological material showed that oil palm landscapes contained higher abundance and taxonomic richness of invertebrates compared to rubber landscapes, and this due to the strong understory vegetation. The high similarities in taxonomic composition detected between the land use types would be due to specimens’ hierarchical level or identification based on orders. Apart from the rubber plantations, the non-saturation phenomenon of curves was registered for the rest of the land use types, indicating that several orders of the litter dwelling arthropods were not collected in the studied sites. The omnivorous represented the dominant trophic group detected across the two study sites, reflecting their strong capacity for adaptation to biotic and abiotic factors acting on their habitats. Conclusion: The forests conversion into plantations was characterized by a change in the abundance, taxonomic richness and functional diversity of litter invertebrates. Additionally, the mean taxonomic richness and total abundance of litter dwelling invertebrates are slightly higher in the oil palm landscape compared to rubber tree landscapes. Our findings suggest the adoption of conservation agriculture in the rubber plantations by reducing the rubber tree density per hectare, and optimizing the understory vegetation.