São Paulo's Urban Sprawl Collapses Stream Food Webs, Study Finds

How early does the damage appear?

Strikingly, the collapse shows up in micro-basins with only 12.5% urban coverage and substantial forest remaining. Degradation kicks in right at the start of urbanization, not gradually as land conversion proceeds.

Saito offered a concrete image: "We can imagine a food web as a mesh: the more threads and connections it has, the more complex the possibilities of energy transfer. Urbanization removes these threads until only a small, simplified and potentially fragile network remains."

Why does this threaten drinking water?

Complex food webs slow the movement of matter through streams, allowing slow, complete processing that breaks down toxic pollutants. Simplified webs do the opposite.

"In streams with simple food webs, matter flows faster and is deposited, after incomplete processing, in the lowest points of the terrain," the researchers note. Those low points are exactly where public-supply reservoirs sit — including the Billings Reservoir, which receives water from several micro-basins sampled in the study.

"Each stream within a watershed contributes to the formation of major rivers and reservoirs that supply public water systems," Saito said. "When a stream stops processing organic matter and pollutants, those substances flow freely into larger systems, damaging the public use we depend on."

Can the trend be reversed?

The study stands out because it analyzes urbanization on several levels at once — species counts, organism sizes, food web structure, and energy and carbon flux. That combination shows transformation cascading through every layer of ecological organization.

The authors argue streams should be treated as complete ecosystems, not just channels running through cities. Preserving intact streams should come first, they say. For degraded ones, the prescription includes wastewater treatment, improved water quality, and restored vegetation along the banks.

Restoration, Saito stressed, must rebuild function rather than appearance: "Restoring a stream doesn't just mean improving its appearance or the quality of the water. It means restoring the conditions necessary for its biodiversity, ecological interactions and energy flows to function again."

Alongside UFSCar and USP, the study involved scientists from São Paulo State University (UNESP), the State University of Campinas (UNICAMP), the Federal University of ABC (UFABC), and the University of Canterbury in New Zealand. The paper appears as Victor S. Saito et al., Urban expansion and the collapse of tropical stream ecosystems, Nature Cities (2026), DOI: 10.1038/s44284-026-00495-z. The findings come from a single metropolitan region, so extrapolating to other cities warrants caution — but for São Paulo, the message is blunt: urban streams are losing their ecological machinery at the very first signs of concrete.