Funktionelle Vielfalt von Hymenopteren entlang eines Gradienten agroforstlicher Nutzung in Indonesien

Abstract

The rate of deforestation of primary tropical forests due to agricultural expansion is higher in Southeast Asia compared to all other world's tropical regions. Deforestation in tropical forests is a major threat for global biodiversity, because estimated 50-90 % of the world's species are living in tropical forests. Therefore, human dominated landscapes and forested land-use systems such as agroforestry become increasingly important as a storage reservoir of global biodiversity. Biodiversity is related to ecosystem functioning, with complementarity in resource use as a major mechanism. Pollination is essential for plant-derived ecosystem services such as food production, and pollinators are one of the major functional groups for human well-being. Agroforestry systems often include coffee (Coffea sp. L.) and cacao (Theobroma cacao L.) as important cash crops, traditionally crown under a canopy of shade trees and with fruit set depending on pollination. Because of the variety of planted shade and fruit trees, agroforestry systems resemble primary forests more than any other agricultural habitat type and have great but little explored potential to offer substitute habitats for many tropical forest species. My aim was to evaluate the importance of agricultural systems for conservation of trap-nesting Hymenoptera and the pollinator community. I analyzed a land-use gradient in a tropical forested landscape, from primary forests, agroforestry systems differing in diversity of the shade trees and openland such as grassland and fallow land. I assessed the relative importance of different strata for species richness with trap nests, which offer standardized nesting resources for different guilds of above-ground nesting Hymenoptera and their antagonists. Furthermore, I linked pollinator richness to habitat modification and final crop yield and analyzed species-specific differences in resource use. I also assessed the contribution of agroforestry system for the maintenance of high densities of a pest predating wasp species over the course of one year. This study took place in Central Sulawesi (Indonesia) in the surrounding of the village Toro at the margin of a primary tropical forest (Lore Lindu National Park). The results show that total bee species richness in the herb layer increased with increasing land-use intensity and was highest in openland. However, species richness estimation and additive partitioning of biodiversity revealed higher overall bee species richness, presumably due to high management diversity, in agroforestry systems compared to primary forests and openland. Agroforestry systems offer higher floral diversity in the understorey, due to the richer herb layer and cash crops, than primary forests and provide more nesting sites for cavity-nesting bee species than openland. The canopy in primary forests was far more important for trap-nesting bee and wasp richness than the understorey. This pattern reversed with intensification in agroforestry systems where highest richness was found in the understorey. Food resource availability and microclimatic conditions appeared to make the canopy in primary forests a more suitable habitat for most species and might cause underestimation of species richness when only lower strata are sampled. In addition, agroforestry habitats showed high beta diversity due to high land-use diversity and between-plot community dissimilarity, showing that agricultural management can be crucial for conservation plannings on a landscape scale. I used standardized pumpkin plantations in different habitat types to test the effect of pollinator diversity on crop yield. Bee pollinators showed species specific differences in flower visiting traits, such as height, time and body size-related flower visiting behaviour. I provide the first empirical evidence of the relation between spatiotemporal and behavioural complementarity in resource use of a pollinator community causing increased crop yield. Furthermore, I show that the native bee community can sustain pollination services in absence of managed honey bees. Using the highly abundant wasp species Rhynchium haemorrhoidale (F.) (Eumenidae, Hymenoptera), I show that agroforestry systems can maintain high insect densities of a pest predator, even under intense land-use management. Tropical regions experience lower climatic fluctuations between the seasons compared to temperate zones. However, R. haemorrhoidale undergoes seasonal changes in density and body size, due to a complex combination of potential reasons (e.g. climate, food supply, parasitism) independent of habitat type. In conclusion, agroforestry systems can play a crucial role in the conservation of the understorey pollinator communities and densities of pest predators, whereas the importance of the canopy for species richness is increasing with decreasing land-use intensity and is highest in primary forests. Furthermore, I show that the ecosystem service of pollination and crop yield strongly depends on the bees' functional diversity. Low-intensity land-use and a complex shade-tree canopy appear to increase local and regional Hymenoptera richness and to sustain important ecosystem services, showing the necessity to include such agricultural systems in landscape-wide conservation programs.