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The Human Exclusion Experiment

Experiment and Results

When Juan Carlos Castilla and Patricio Sánchez closed off the 1 km section of coastline and nearshore waters at ECIM in 1982, they began one of the first attempts in the world to scientifically study the effects of excluding humans from a coastal and marine ecosystem.

The results were startling. After just 2 years, scientists noticed inside the reserve an unexpectedly large increase in the biomass of “locos,” Concholepas concholepas, also known as the “Chilean abalone,” a carnivorous gastropod extensively harvested for food in Chile. The population of locos in the intertidal zone within the reserve had grown close to 10 times as large as the population outside. Furthermore, through this study, the loco was revealed to be a keystone species in Chilean nearshore communities. With the increased abundance of the carnivorous C. concholepas, populations of the space-dominating mussel Perumytilus purpuratus, the main intertidal prey of C. Concholepas, had drastically decreased. Where formerly this mussel had completely covered nearly all intertidal rocky platforms, it was now restricted to small patches, leaving large expanses of bare rock. Interestingly, because the matrices of this mussel provide habitat for other organisms, 60 to 70 other species that lived in the mussel matrices also greatly decreased in number.

Research by J.C. Castilla.

Thus the intertidal zone within the reserve, free from human intervention, took on a radically different appearance, species composition, and food web from that found outside the reserve. The common intertidal landscape of densely mussel-covered platforms and very few locos, which had thus far been considered the “normal and natural” state of the Chilean rocky intertidal, was revealed to be an ecosystem highly altered by human activities.

Impact of Findings

This seminal human exclusion experiment lead to a number of significant outcomes. On a global scale, the experiment was one of the first to demonstrate scientifically that human intervention can dramatically impact coastal and marine ecosystems and significantly affect populations of exploited species. These results countered widely held views of the time – that the ocean’s resources were inexhaustible, and that the large reproductive output and dispersal of marine organisms provided an ample buffer against overexploitation – showing these beliefs to be largely false. Thus it provided some of the first scientific bases for promoting the establishment of marine protected areas worldwide.

Concolepas Chart

C. concholepas extracted by the shellfishC. concholepas extracted by the shellfish

The experiment also demonstrated that in certain cases, at least, strongly human-impacted marine ecosystems and populations can recover quickly once protected from overexploitation, which suggested the possibility of developing sustainable management strategies for harvested marine resources. In fact, results from this experiment inspired a follow-up study in which ECIM scientist Juan Carlos Castilla convinced a group of Chilean artisanal shellfish harvesters to work with scientists to co-manage shellfish extraction in a 52-hectare area of rocky shore. The co-management experiment, initiated in 1988 through a research grant, resulted in both increased abundance of exploited species and improved economic output for the shellfish harvesters. This study and similar results gathered by scientists in southern Chile became Chilean shellfish organized the basis for part of Chile’s 1991 fisheries law, which created a pioneering system of Marine Resource Management Areas. These co-management areas, in which local artisanal fishermen work with marine biologists and fisheries experts to develop sustainable management plans within exclusive-access areas, now exist throughout Chile and supply most of the Chilean shellfish for local and export markets.