Urbanisation strips São Paulo streams of food-web complexity and energy flow

Urban growth around São Paulo is dismantling the food webs of headwater streams, according to a study in Nature Cities. Across 30 micro-watersheds, estimated feeding interactions fell from 190 in fully forested catchments to just three in fully urbanised ones, and the energy moving through the community dropped by a factor of about 1,446.

Selective loss, not random decline

The team was led by Victor Saito of the Federal University of São Carlos (UFSCar) and Luis Schiesari of the University of São Paulo (USP), both affiliated with the Center for Research on Biodiversity Dynamics and Climate Change (CBioClima). Rather than simply counting species, they examined the body-size structure of stream communities.

The pattern of loss was not random. Urban conditions favoured small organisms, while very small, medium-sized and large ones disappeared. Because body size shapes what an animal needs to eat and what is likely to eat it, removing whole size classes disrupts the web of interactions as a whole.

How the streams were studied

The researchers sampled headwater streams, the small channels that feed larger rivers, in the Upper Tietê region. The 30 micro-watersheds covered the full range of urbanisation, from catchments entirely under Atlantic Forest to those entirely built over.

Fish and invertebrates were collected at each site. Using the body sizes and feeding preferences of the animals, the team reconstructed the food web and then estimated energy flow from the predator-prey relationships it implied. The interaction counts and energy figures are therefore model-based estimates built on field sampling, not direct measurements of every feeding link.

Degradation from the earliest stages

According to Schiesari, the collapse was already evident in micro-watersheds with only 12.5% urban cover and substantial forest remaining. This suggests that damage begins with the first stages of occupation rather than accumulating gradually as cities expand.

The authors link the simplification to ecosystem function. Complex food webs slow the movement of matter through a stream, allowing time for processing, including the breakdown of toxic pollutants. In simplified streams, material moves faster and is deposited only partly processed in low-lying areas. Reservoirs used for public supply sit in such areas; the Billings Reservoir, for example, receives water from several of the sampled micro-watersheds. The region has more than 22 million residents.

Implications for research and management

The study is notable for assessing biodiversity, body size, food-web structure and energy and carbon flux together, showing that urban change acts at several levels of ecological organisation at once. For researchers, it argues for metrics that go beyond species richness when judging the condition of urban waters.

The authors say that streams in good condition should be a conservation priority. For degraded ones, they point to wastewater treatment, better water quality and restoration of riverbank vegetation. Saito stresses that restoration should aim to bring back ecological processes, including interactions and energy flows, and not only improve appearance or water chemistry.

The source does not detail how far reconstructed food webs and energy estimates would recover after restoration, or whether the pattern holds in other tropical cities. Those questions remain open for follow-up work. The study also 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.

Source: Urban expansion causes ecological collapse in streams in the São Paulo Metropolitan Area (Phys.org, 15 November 2026).

Image: AI-generated illustration.