Most protected areas begin with a line drawn to keep people out. An extractive reserve starts from the opposite premise. It is public land where traditional communities hold the right to hunt, fish and gather wild produce, combined with subsistence farming and small-scale animal raising, and its stated objectives are to safeguard those livelihoods and that culture as much as the ecosystem underneath them. The IUCN files it under category VI — protected area with sustainable use of natural resources — which is bureaucratic language for a genuinely radical proposition: that the people who have extracted a living from a place for generations are, on balance, its most reliable custodians. On 10 November 2005 the reserve was formally recognised as supporting 2,000 families, qualifying them for the federal family-farming credit programme, PRONAF.

The reserve is divided between the municipality of Araioses in Maranhão and Ilha Grande in Piauí, which is fitting, since the Parnaíba spends its entire 1,400-kilometre course being the boundary between those two states before splitting into five arms and losing itself in the Atlantic. The delta it builds is a landscape of channels threaded between roughly seventy islands, with mangrove along the water, dune fields rising as high as forty metres behind the beaches, and sandbanks that rearrange with the season. There is no single shoreline here that could be drawn accurately twice. Boundaries in this kind of country are legal fictions of a particularly optimistic sort.

The second-largest island in the delta, Ilha das Canárias, sits inside the reserve on the Maranhão side and holds about three thousand residents across five settlements — Canárias, Passarinho, Caiçara, Torto and Morro do Meio. The work is tidal rather than clocked. Crab gatherers go out into the mangrove and come back muddy to the shoulders; fishers read the channels, which shift; the shellfish beds are known by people, not by charts. It is demanding, weather-dependent, poorly paid work, and it is also a body of practical knowledge about a complicated estuary that no survey has ever fully written down. That knowledge is, in a real sense, what the 2000 decree was drawn around.

Fishing and the harvesting of crabs, shellfish and molluscs support the reserve's communities, supplemented by small plots of cultivation. All of it depends on the mangrove staying intact — the nursery for the fish, the habitat for the crab, the wall that holds the sediment against the sea. Nothing about that dependence is abstract. Take the mangrove out and the fishery follows it, on a timescale measured in a few seasons rather than a few decades. The extractive reserve is the legal instrument meant to keep that from happening, and the argument it embodies is that the households working the channels have the strongest possible stake in the answer.

The marine reserve sits within a larger perimeter, the Delta do Parnaíba Environmental Protection Area, created on 28 August 1996 and covering 313,800 hectares across ten municipalities in Maranhão, Piauí and Ceará. That outer layer protects the deltas of the Parnaíba, Timonha and Ubatuba rivers together with their dune complex, and takes in a rare mix of habitats where coastal caatinga meets restinga, mangrove and floodplains of carnaúba palm. Among the species it shelters are the red-handed howler monkey — the northeastern subspecies Alouatta belzebul ululata, which is threatened — and the smalltooth sawfish, a creature now vanishingly rare across most of its former range. Both reserves are managed by the Chico Mendes Institute for Biodiversity Conservation, an agency named for a rubber tapper who was killed for arguing that forests and the people who work them are the same conservation problem.