Nonlinear survey-effort adjustment removes the reported bird-diversity effect

Canran Liu 

A recent study published in Science reported that greater photovoltaic policy stringency (PSI) was associated with lower Shannon diversity, lower Simpson diversity, lower species richness, and slightly higher species evenness in China. We exactly reproduced their published model and then evaluated seven alternative survey-effort specifications across Richness, Shannon diversity, Simpson diversity, and Evenness. Existing reanalyses incorporated additional measures of birdwatching effort but retained the original Duration variable, which is derived from total observation time and observer number. We found that once these two effort dimensions were represented explicitly, PSI became statistically indistinguishable from zero for all four biodiversity measures. Retaining Duration contributed little additional information and produced coefficient patterns that were difficult to interpret ecologically. Nonlinear representations of total observation time and observer participation substantially improved model fit, whereas additional nonlinear flexibility in environmental controls and PSI provided little benefit. A semi-spline specification containing nonlinear effort terms but linear PSI and environmental controls was consistently selected by the Bayesian Information Criterion (BIC) across all four biodiversity measures. Under this preferred specification, the reported PSI associations were no longer statistically detectable. These findings indicate that the reported biodiversity effect is highly sensitive to survey-effort specification and that nonlinear survey-effort adjustment provides a substantially better description of the observation process.