Urban expansion causes ecological collapse in streams in the São Paulo Metropolitan Area

The expansion of urban areas in the São Paulo Metropolitan Region of Brazil is causing the collapse of ecological systems in local streams, according to a study published in Nature Cities in August. The research, led by professors Victor Saito and Luis Schiesari from the Federal University of São Carlos and the University of São Paulo respectively, reveals that urbanization not only reduces species numbers but also fundamentally alters the functioning of these ecosystems.

The study found that urban environments predominantly favor small organisms, leading to the elimination of medium and large organisms. This selective extinction disrupts the entire ecosystem by eliminating species with varying size requirements and predation dynamics. As a result, the complexity of food webs diminishes, with connections falling from 190 in undisturbed forested areas to just three in fully urbanized streams.

This simplification dramatically reduces energy flow by over a thousandfold, impairing the streams' ability to break down and neutralize toxic pollutants. Consequently, the quality of water supplies for São Paulo's 22 million residents is compromised. The degradation begins even in the earliest stages of urban development, with complex food webs collapsing to just three interconnections in heavily urbanized areas.

The researchers liken urbanization to dismantling a complex network, leaving only a fragile and potentially unstable system. The loss of energy flows can lead to pollutants accumulating in low-lying areas, including public water reservoirs like the Billings Reservoir. The study underscores the necessity of viewing streams as integral ecosystems rather than isolated waterways, emphasizing the need for comprehensive ecological restoration practices that restore biodiversity, ecological interactions, and energy flows.