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

Abstract

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.

Network structure of vertebrate scavenger assemblages at the global scale: drivers and ecosystem functioning implications

“…This suggests that both anthropogenic disturbance and environmental factors are important drivers of the network structure of vertebrate scavengers. There is growing evidence that human impact affects the structure of trophic networks, as already detected for several taxonomic groups, including terrestrial mammals (Mendoza and Araújo 2019), diatoms in lakes (Wang et al 2019) and reef fish (Ruppert et al 2018). Our study broadens this result to the functional group of scavengers, revealing detrimental effects for its functioning and subsequent ability to provision ecosystem services.…”

Late Quaternary chironomid community structure shaped by rate and magnitude of climate change

“…Four ecological metrics, taxon richness, beta diversity, compositional disorder and network skewness, were selected as indicators of ecosystem change for the temporal empirical chironomid assemblages. Beta diversity, compositional disorder, and network skewness reflect ecosystem structural change, through taxonomic turnover and taxon organisation (Baselga, 2010; Doncaster et al ., 2016; Wang et al ., 2019; Mayfield et al ., 2020). Palaeoecological datasets often have irregular time series (Glew et al ., 2001), including the records analysed here.…”

“…(2016) and Wang et al . (2019) explored the effect of taxon‐type losses from aquatic communities, suggesting that the early loss of less common, weakly interconnected taxa may leave a predominance of more strongly interconnected taxa defining a more rigid structure prone to abrupt collapse. The ability of a taxon to persist within an ecosystem relates to its tolerance levels, with greater magnitudes of stress often driving greater assemblage change (Cao and Hawkins, 2005).…”

Interannual stability of phytoplankton community composition in the North-East Atlantic

“…During periods of exogenous forcing, ecological theory predicts an initial rise of dominant competitors, which may take 'keystone' roles that impose structural rigidity on the community (Doncaster et al 2016). Ongoing exogenous forcing eventually disadvantages slow-replicating taxa, generally including the dominant competitors, tipping the community into a more fluid structure with prevalence of fast-replicating 'weedy' taxa (Wang et al 2019). Analysis of temporal variations in community composition may thus provide an early warning signal of exogenous forcing affecting the community (Beaugrand & Reid 2003, Edwards & Richardson 2004, Beaugrand et al 2008, Doncaster et al 2016.…”