How plant–pollinator network is linked to the biodiversity productivity relationship?

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George I. Memtsas, Maria Lazarina, Stefanos P. Sgardelis and Athanasios S. Kallimanis, Biology, Aristotle University of Thessaloniki, Thessaloniki, Greece, Theodora Petanidou, Geography, University of the Aegean, Mytilene, Greece/Lesvos, Greece

Background/Question/Methods
The Biodiversity – Ecosystem Functioning (B–EF) relationship has long been the focus of ecologists, with the majority of the relevant studies reporting that biodiversity promotes ecosystem functioning. Among the most common invoked hypotheses for the positive sign of the relationship are briefly, the sampling effect: the chance to sample a functional important species increases with sampling effort, the niche complementarity hypothesis: niche complementarity among species ensures effective resource partitioning and thus higher functioning, the insurance hypothesis: functional equivalent species replace each other in case of loss, thus ensuring maintenance of functioning. However, beyond species richness, the way species interact forming complex networks plays a significant role in ecosystem functioning. Yet, the link between network structure and ecosystem functioning is relatively neglected. Here, we explore the relationship between plant-pollinator network structure (plant and pollinator richness, nestedness, and specialization) and gross primary productivity (MODIS satellite data; five-year mean and inter-annual variability by Generalized Linear Model analysis using data of 80 local plant–pollinator networks of Aegean Islands, Greece, aiming to decipher which of the aforementioned hypotheses drive the relationship in space and time.
Results/Conclusions
We found a significant effect of pollinator species richness on mean value and inter-annual variability of GPP, with the relationship between them being U-shaped, i.e. productivity decreased up to approximately intermediate values of pollinator species richness and increased at higher levels of pollinator richness. Surprisingly, plant species richness did not exert a significant effect on GPP. Although, the relationship between richness and productivity is considered to be unimodal, negative or non-significant relationships have also been reported. Furthermore, we detected a marginally significant negative relationship between mean GPP and both nestedness and specialization and a significant negative relationship between nestedness values of plant–pollinator networks and GPP inter-annual variability. In nested networks, specialists interact only with a subset of generalist, implying a degree of functional redundancy, i.e. the loss of specialists can be compensated by the generalists. Therefore, the link between nested topology and temporal stability of gross primary productivity is in accordance with the prediction of insurance hypothesis.