Variation of stable carbon and nitrogen isotopes ratio in Ficus tikoua and their linkage to its specific pollinator

Abstract

The interactions between figs and their pollinators are among the most specialized examples of mutualism between plants and insects. Based on their life-histories and obligate mutualism, it is reasonable to expect an environmentally-dependent trophic cascade exists between figs and their pollinators. Understanding the mechanisms involved in soil-plant-pollinator trophic cascade relationships will advance our knowledge of how obligate mutualist systems respond to environmental change. We sampled rhizosphere soil, leaves, syconia, and fig wasps (Ceratosolen) of a dioecious prostrate shrub, Ficus tikoua from southwestern China, and measured their carbon (δ13C) and nitrogen (δ15N) stable isotope ratios. The mean leaves δ13C values of F. tikoua were -28.86 ± 1.24%, which suggests it is a typical C3 plant. We detected a significant difference for δ13C and δ15N values of F. tikoua among different populations related to environmental factors such as longitude, altitude, and annual average temperature. The δ13C values showed a significantly positive correlation between fig wasps and host. The δ15N in fig wasps were 3.4% higher than those in F. tikoua, and were positively correlated with δ15N in leaves. These results indicated that fig wasps were feeding on F. tikoua. Moreover, the δ15N values among soil, leaves, and syconia showed significantly positive correlations, indicating the trophic linkage among soil-plant-pollinators. In summary, we noted that environmental factors shaped population variations in stable isotopic patterns of F. tikoua, and such variations drove stable isotopic signatures of its specific pollinators by trophic cascade. Future environmental changes could potentially impact the maintenance and stability of this highly specific pollination system of figs and fig wasps.