Decreasing predation rates and shifting predator compositions along a land-use gradient in Madagascar's Vanilla landscapes.
Abstract
Land-use change is the main driver of deforestation and land degradation resulting in the loss of biodiversity and ecosystem functioning in north-eastern Madagascar. Vanilla, the region's main cash crop, is grown in agroforestry systems and may provide an opportunity for the conservation of biodiversity and ecosystem functioning. We used dummy caterpillars to assess predation rates and predator communities along a land-use gradient including unburned old-growth and forest fragments, herbaceous and woody fallows after shifting cultivation with fire usage, as well as rice paddies. The studied vanilla agroforests were either forest-derived or fallow-derived. Besides land-use type, we considered the effects of land-use history (unburned/burned), plot-level parameters and the landscape composition to conclude on management recommendations. Old-growth forest and forest fragments exhibited highest predation rates, which decreased with land-use intensity. Overall, predation was higher in unburned land-use types than in more open, previously burned habitats and rice paddies. High stem and vegetation densities were positively related to predation rates, but decreased with land-use intensity. High forest cover in the surrounding landscape led to higher predation rates, while local structural parameters remained more important. The predator community was arthropod-dominated across all land-use types with ants responsible for between 33% and 69% of all predation events. Overall predator composition in old-growth and forest fragments differed from all other land-use types. Predation by Gryllacrididae (Orthoptera) was lower in all land-use types, including forest-derived vanilla, than in old-growth forest and forest fragments, where they were important contributors to total predation. Vertebrate predation was low throughout. Synthesis and applications: Forested habitats feature higher predation rates and different predator compositions than other land-use systems. Maintaining or restoring tree- and understorey-rich vanilla agroforestry represents a viable tool in landscape conservation programmes as it has the potential to contribute to the conservation of predation as an important ecosystem function in both forest- and fallow-derived agroforests. However, vanilla agroforestry has limited value in conserving forest-specialized predator communities. While the establishment of tree-rich agroforests on former fallow land is favourable for conservation ecosystem functioning, further forest transformation should be avoided.