Network parameters quantify loss of assemblage structure in human‐impacted lake ecosystems

• Published in: Global change biology 2019-11, Vol.25 (11), p.3871-3882
• Main Authors: Wang, Rong, Dearing, John A., Doncaster, C. Patrick, Yang, Xiangdong, Zhang, Enlou, Langdon, Peter G., Yang, Hui, Dong, Xuhui, Hu, Zhujun, Xu, Min, Zhao, Yanjie, Shen, Ji
• Format: Article
• Language: English
• Subjects: Aquatic ecosystems; Biodiversity; Catchment area; Catchments; China; Community structure; Deformation; diatom assemblages; Diatoms; ecological resilience; Ecosystem; Frequency distribution; Human influences; Humans; Lakes; network skewness; Nutrient loading; Parameters; Phosphorus; Regions; self‐organized system; Skewed distributions; Skewness; stability; Urbanization
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.14776

Lake biodiversity is an incomplete indicator of exogenous forcing insofar as it ignores underlying deformations of community structure. Here, we seek a proxy for deformation in a network of diatom assemblages comprising 452 species in 273 lakes across China. We test predictions from network theory that nodes of similar type will tend to self‐organize in an unstressed system to a positively skewed frequency distribution of nodal degree. The empirical data reveal shifts in the frequency distributions of species associations across regions, from positive skew in lakes in west China with a history of low human impacts, to predominantly negative skew amongst lakes in highly disturbed regions in east China. Skew values relate strongly to nutrient loading from agricultural activity and urbanization, as measured by total phosphorus in lake water. Reconstructions through time show that positive skew reduces with temporal intensification of human impacts in the lake and surrounding catchments, and rises as lakes recover from disturbance. Our study illustrates how network parameters can track the loss of aquatic assemblage structure in lakes associated with human pressures. We test a new proxy for ecological network deformation caused by external stress, using diatom (algal) assemblages across China. We link species through shared habitats (top‐left panel, upper part) to obtain a network of associations (top‐left, lower). We argue that undisturbed lakes will have positively skewed distributions in their degree of species associations (top‐right, upper), while disturbed lakes will shift towards negative skew (top‐right, lower). Field data show the frequency distributions of species associations that switch from positive to negative skew across a west–east gradient of increasing disturbance (bottom panel map) linked to human population density (bottom graph).