Rapid global urbanization has drastically reshaped natural landscapes, particularly in plain river networks. These ecosystems face unprecedented challenges, ranging from habitat fragmentation to profound shifts in biological community structures. Critically, rivers are more than just water conduits; they sustain multiple processes, such as nutrient cycling and biomass production, collectively termed Ecosystem Multifunctionality (EMF).

However, while the positive link between biodiversity and EMF is well-documented in controlled experiments, significant knowledge gaps remain regarding how urbanization modulates EMF through biotic pathways in complex, highly human-impacted systems. The specific mechanisms by which urban stressors drive multifunctionality shifts in natural river networks have long remained elusive.

Recently, a research group led by Prof. XIE Zhicai and ZHANG Junqian from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences illustrated the mechanisms by which urbanization alters river ecosystem multifunctionality in highly human-impacted plain river networks, and identified that macroinvertebrate diversity, particularly taxonomic richness and rare species, plays a critical role in maintaining multiple ecosystem functions. This study was published in Limnology and Oceanography.

The research team conducted extensive field sampling at 83 sites across the Yangtze River Delta (YRD), using both on-site assessments and the global human footprint (HFP) map to quantify the urbanization gradient. They integrated six key functional indicators, including nutrient concentration, metabolism, producer biomass, consumer biomass, availability of photosynthetically active radiation (PAR), and habitat complexity, to calculate a comprehensive EMF index. The results demonstrated that urbanization significantly impairs river EMF.

The study further explored the biological mechanisms underlying this decline by comparing the mediating effects of macroinvertebrate taxonomic diversity (TD) and functional diversity (FD). Using PLS-PM, the team analyzed the pathways through which urban stressors reach ecosystem functions. The analysis revealed that the negative impact of urbanization on EMF is primarily mediated by the reduction in macroinvertebrate taxonomic richness. Notably, in these highly human-impacted plain river networks, taxonomic diversity emerged as a more robust predictor of multifunctionality than functional diversity, contrary to traditional ecological assumptions.

To gain a more granular understanding of community dynamics, the researchers distinguished between the ecological roles of common and rare macroinvertebrate species in sustaining EMF. They analyzed how species with different abundance levels contribute to the maintenance of multiple functions under urbanization pressure. The findings indicated that rare species exert a disproportionately strong positive influence on maintaining multifunctionality. Consequently, the loss of these sensitive species acts as a primary catalyst for the overall collapse of ecosystem functions.

The findings offer vital practical guidance for ecological management: effective river restoration must transcend simple water quality remediation. Conservation strategies should prioritize the protection of rare macroinvertebrate taxa and the rehabilitation of their specialized habitats. This study fills a gap in the mechanistic understanding of river ecosystem multifunctionality in plain river network under urbanization and informs strategies for sustainable urban development.

PLS-PM results showing direct and indirect effects of urbanization on EMF through multiple pathways. (Image by IHB)

(Editor: MA Yun)