Causal networks of phytoplankton diversity and biomass are modulated by environmental context

The authors present a global scale analysis deciphering causal interactions and feedbacks among phytoplankton diversity, biomass, and nutrients along environmental gradients of aquatic ecosystems. Their findings help to understand how ecosystems respond to changing environments in a network view.
Published in Ecology & Evolution
Causal networks of phytoplankton diversity and biomass are modulated by environmental context
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An international research team analyze aquatic ecosystems (ranging from lake, river to estuary and ocean) around the world to unveil key causal interactions and feedbacks between biodiversity, ecosystem functions, and various environmental factors. This study, published in Nature Communications (March 3, 2022), overcomes the long-lasting challenge in quantifying complex causal relationships and feedbacks based on observational data in natural systems. In addition, this study explores macroecological patterns, manifesting the variations of key causal interactions and feedbacks associated with biodiversity along global-scale environmental gradients. This innovative analytical framework brings integrated thinking to sustainable management of ecosystems.

Disentangling complex effects of biodiversity on ecosystem functioning, such as productivity, total biomass, and resource use efficiency, has long been a central topic in environmental and sustainability sciences. Over the past two decades, ecologists have used experimental systems to successfully demonstrate the critical roles of biodiversity in maintaining ecosystem functioning. However, in real-world ecosystems, not only biodiversity can affect ecosystem functioning, but ecosystem functioning and other environmental factors can mutually interact with biodiversity. Consequently, it remains difficult to decipher complex bidirectional interactions and more sophisticated feedback regulations in natural ecosystems. For example, the regulation of algal diversity on primary productivity might alter nutrient cycling, which eventually feedbacks to the algal diversity itself. In addition, the causal interactions and feedbacks are not set in stone, but easily change with fluctuating environmental conditions. These complexities make the conclusions obtained in experimental systems difficult to be generalized to real-world ecosystems; this hinders correct forecast on the ecological impacts caused by environmental changes and biodiversity loss.

To overcome the challenges in solving complex causal interactions and feedbacks associated with biodiversity, the research team analyzed long-term time series data (16-41 years) from 19 aquatic ecosystems around the world using a novel causality analysis, cross-convergent mapping (CCM). Their analysis based on CCM reconstructs the causal networks in natural aquatic ecosystems and quantifies key causal interactions among phytoplankton diversity, ecosystem function (total phytoplankton biomass as a proxy), and various environmental factors. Through a cross-system analysis of the networks, the study identifies the key factors affecting ecosystem functioning and biodiversity in natural aquatic ecosystems, and for the first time, presents how causal interactions and feedbacks vary with global-scale environmental gradients (i.e., macroecological patterns). For example, phytoplankton diversity effects are more important to phytoplankton biomass compared to other environmental factors and widely exists in various types of aquatic habitats; however, the strength of such effects is substantially weakened in warm and eutrophic environments. For more complex feedbacks, phytoplankton diversity tends to regulate phytoplankton biomass through nitrogen-based feedback regulations in warm and productive environments, while through phosphorous-based feedback in cold and less productive environments. These macroecological patterns improve our understanding on how ecosystems respond to global environmental change and provide a holistic network view, shedding light on revealing the complex causations between biodiversity, ecosystem functioning, and physicochemical environments in natural ecosystems. 

The novel framework proposed in this study shall allow to more correctly evaluate the impacts of environmental change and biodiversity loss on real ecosystems. Importantly, the findings provide a more comprehensive and integrated approach to managing key important ecosystem functioning and natural resources in various types of ecosystems.

Through the causality analysis based on analyzing long-term time series, the research team quantifies various causal links and complex feedback regulations among key ecosystem components, including biodiversity, ecosystem functioning, and environmental drivers. Their study presents a sophisticated analysis of long-term phytoplankton assemblage data from 19 aquatic ecosystems around the world, quantifying important causal interactions (e.g., biodiversity effects on ecosystem functioning) and feedback regulations (e.g., diversity-nutrient-biomass triangular feedback) and revealing how the strengths of these network modules vary with along large-scale environmental gradients in natural aquatic ecosystems.

Figure 1.  Through the causality analysis based on analyzing long-term time series, the research team quantifies various causal links and complex feedback regulations among key ecosystem components, including biodiversity, ecosystem functioning, and environmental drivers. Their study presents a sophisticated analysis of long-term phytoplankton assemblage data from 19 aquatic ecosystems around the world, quantifying important causal interactions (e.g., biodiversity effects on ecosystem functioning) and feedback regulations (e.g., diversity-nutrient-biomass triangular feedback) and revealing how the strengths of these network modules vary with along large-scale environmental gradients in natural aquatic ecosystems.

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