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Why tree diversity in the tropic matters
Author: Wiida Fourie-Basson
Published: 06/04/2021

What can the decomposed leaf litter from 40 streams on six continents tell us about the impact of biodiversity loss on global carbon fluxes?

According to a study published in Science Advances today (26 March 2021), there is reason to be concerned, especially at low latitudes where the rate of deforestation and conversion of forest to monoculture, plantations and agricultural land are already high.

The decomposition of plant litter in streams is a fundamental ecosystem service, as it mainly serves as a sink for carbon, thereby keeping the global carbon budget in balance. When the optimal functioning of that ecosystem service is disturbed, some of the world's rivers could become a source rather than a sink for carbon, creating potential feedbacks on climate.

Prof Cang Hui, a bio-mathematician from Stellenbosch University and a co-author of the article, says some broad-scale roles of biodiversity and the impacts of climate change only emerge and become detectable at the global scale. In this study, for example, the latitudinal gradient of instream decomposition only became clear over more than 100° of latitude, roughly around 10 000 km.

To conduct this global experiment, scientists from 67 institutions across six continents joined forces to coordinate the sampling of the rivers. From 2017 to 2019, they placed a total of 2 580 leave litter bags in 40 streams. Four rivers from Africa were sampled: the Lourens River near Somerset West in South Africa, the Njoro and the Ngetunyek rivers in Kenya and the Djigbè River in Guinea.

The bags were retrieved after 23 to 46 days, depending on the water temperature in each stream, put in individual ziplock bags and transported on ice to various laboratories. The aim of the study was to assess how the decomposition rates in streams vary from the lower to the higher latitudes, from which we can infer how biodiversity loss and climate change affect this ecosystem service.

The results demonstrated that plant diversity has a major impact on litter decomposition. The experiment was able to show that plant litter high in diversity (e.g., in toughness, nutrient content, presence of toxins) stimulates decomposition more at low latitudes than in cooler climates. This suggests that stream ecosystem functioning could be particularly vulnerable to forest practices that are detrimental to native tropical forests.

In other words, biodiversity loss in the lower latitudes will negatively affect tropical detritivores, as they require a diet high in diversity. If their ability to break down leaf litter is diminished, it will enhance the relative contribution of microbial decomposition in rivers to carbon fluxes.

Prof Hui, the only South African scientist to participate in the study, holds a South African Research Chair on mathematical biosciences in the Department of Mathematical Sciences at SU, and has been collaborating with the lead author of the study, Prof Luz Boyero from the University of the Basque Country in Spain, for the past ten years. She is also the coordinator of the GLoBe network, an international network of freshwater ecologists.

“For me, biodiversity is the most striking reality and puzzle of our biosphere. How biodiversity structures and functions across spatial and temporal scales is the focus of our research," he explains.

He believes we cannot enter an era of global change with the traditional view of a stable ecosystem: “Such novel ecosystems, constantly challenged by human activities and issues such as biological invasions and climate change, are in persistent transition. We are yet to develop a new set of theoretical frameworks to study such systems."

“Biomathematics, with its reaches into data science and informatics, provides a powerful set of tools from different areas of mathematics to (re)invent this theoretical framework for open adaptive systems. This era is not only an opportunity for global change biology, but also a chance for mathematics and physics to create new pathways," he concludes.

The article, “Latitude dictates plant diversity effects on instream decomposition" was published in Science Advances on 26 March 2021.

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Prof Cang Hui

South African Research Chair in Mathematical Biosciences, Department of Mathematical Sciences, Stellenbosch University, South Africa


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