Tropical agroforestry supports insect pollinators and improves bean yield

Pollination services contribute to crop productivity worldwide, but insect
pollinators are declining in most agricultural landscapes, mainly due to
agricultural expansion and an increase in intensive agricultural
practices. To reduce the negative effects of agricultural expansion and
intensification, farmers can adopt ecological interventions, such as
diversifying agricultural systems through agroforestry. However, there
have been variable results on the effectiveness of agroforestry in
enhancing pollinators and pollination services. Furthermore, most of the
information has been generated in temperate regions, while the impact of
agroforestry in tropical East Africa is largely unknown. Using common
beans (Phaseolus vulgaris L.) as the focal crop, this study tested whether
a tropical agroforestry system called Chagga home gardens supports
pollinator communities and improves pollination and crop yield. We
examined 16 agroforestry plots paired with 16 non-agroforestry plots
located along a gradient of woody (trees and shrubs) cover within a 1 km
radius to document the abundance, species richness and visitation rates of
pollinating insects on bean flowers and how they contributed to bean
yield. The beans planted in the agroforestry plots had almost twice the
abundance of insect pollinators, three times the richness of the species,
and almost twice the visitation rates than those planted in
non-agroforestry plots. We also found a significant positive effect of
woody cover in the surrounding landscape on insect pollinator abundances,
but not on species richness and visitation rates. Additionally, the
abundance and richness of insect pollinators increased significantly with
flower abundance, while the overall plant richness in a plot significantly
increased insect pollinator abundance and their visitation rate. The
difference in bean yield between unbagged flowers (in which insect
pollinators were allowed to access bean flowers) and bagged flowers and
the total yield were higher in agroforestry than in non-agroforestry
plots. Synthesis and Applications: This study showed that, compared to
monoculture, agroforestry generally promotes pollination services in a
tropical context. Furthermore, we found that agroforestry is likely to be
particularly helpful for pollinators when it increases flower abundance
and plant richness and if it is coordinated so that woody cover also
increases at the landscape scale.