Host-plant richness and climate factors jointly shape the latitudinal diversity gradient of Psyllids

The latitudinal diversity gradient (LDG) is widespread, but the relative roles of climate and biotic resources remain poorly resolved for host-specialized insects. Here, we compiled distributional data for all recorded psyllid species (Hemiptera: Psylloidea) across China’s full latitudinal range and combined these with climatic, productivity, and host-plant diversity data to test five hypotheses concerning thermal energy, water availability, climatic seasonality, productivity, and host-plant richness. The results indicated that psyllid species richness declines with increasing latitude; host-plant richness is the strongest independent predictor in the variation partitioning and had the strongest direct association in the piecewise SEM, whereas water availability made the largest independent contribution in hypothesis-level hierarchical partitioning. Climatic seasonality showed a strong unimodal relationship with psyllid richness. At the individual-variable level, temperature annual range and winter cold severity exhibited the highest univariate explanatory power. Within-group hierarchical partitioning identified the mean temperature of the coldest quarter (BIO11) as the most important independent climatic variable. Piecewise structural equation modelling revealed that host-plant richness had the strongest direct positive association with psyllid richness; climate predicted productivity, but productivity did not significantly predict host-plant richness. Thus, biotic and abiotic pathways operated largely in parallel rather than through a serial climate–productivity–host-diversity pathway.