Hierarchy of the factors influencing the broad-scale waterbirds functional diversity gradients in temperate China

Geographical gradients in species diversity have long fascinated
biogeographers and ecologists. However, the extent and generality of the
positive/negative effects of the important factors governing functional
diversity (FD) patterns are still debated, especially for the freshwater
domain. We examined lake productivity and functional richness (FRic) of
waterbirds sampled from 35 lakes and reservoirs in northern China with a
geographic coverage of over 5 million km2. We used structural equation
modelling (SEM) to explore the causal relationships between geographic
position, climate, lake productivity and waterbirds FRic. We found
unambiguous altitudinal and longitudinal gradients in lake productivity
and waterbirds FD, which were strongly mediated by local environmental
factors. Specifically, we found 1) lake productivity increased northeast
but decreased with altitude, and the observed gradients were driven by
climate and nutrient availability, with 93% of variation explained in the
individual SEM; 2) waterbirds FD showed similar geographic and elevational
gradients.; the environmental factors which had direct and/or indirect
effects on these geographic and elevational gradients included climate,
lake productivity and morphology, which collectively explained more than
56% of the variation in waterbirds FD; and 3) a significant (P = 0.029)
causality between lake productivity and waterbirds FD was confirmed.
Nevertheless, the causality link was relatively weak in comparison with
climate and lake area (standardized path coefficient was 0.65, 0.21, and
0.17 for climate, area, and productivity, respectively). Through
articulating the dominant causality paths, our results could contribute to
the mechanistic explanations underlying the observed broad–scale
biodiversity gradients.