Chapter 49. The Anthropocene: Humans as a Planetary Force

Overexploitation threatens species and disrupts ecological relationships within communities.

For thousands of years, humans have harvested protein from the sea, tapping a resource that long seemed both free and endless. Citizens of Newfoundland, however, know better. Journals from John Cabot’s voyage of 1498 tell of cod so abundant that a person “could walk across their backs,” and for two centuries cod supported the Newfoundland economy. The amount of cod caught each year increased markedly in the 1960s—surveys estimated that as much as 60% of adult cod was being harvested. By the late 1980s, however, harvests had plummeted (Fig. 49.21). By 1992, cod stocks were estimated at 1% of 1960 levels (which were themselves well below those in Cabot’s time) and so, with fish stocks on the verge of collapse, cod fishing was halted.

FIG. 49.21 Collapse of cod fishing in Newfoundland. Persistent harvesting of cod at rates higher than those at which stocks could be replenished led to the collapse of cod stocks. Data from Millennium Ecosystem Assessment.

Data from Millennium Ecosystem Assessment.

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VISUAL SYNTHESIS

VISUAL SYNTHESIS FIG. 49.18 Flow of Matter and Energy Through Ecosystems Integrating concepts from Chapters 47–49

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Unfortunately, the tragedy of cod in Newfoundland is not an isolated story. By 2003, nearly 30% of open ocean fisheries had declined by 90% or more, a percentage that will continue to rise unless sustainable fishing can be achieved. With collapsing fish stocks comes a loss in food production, economic decline, and, hidden beneath the surface of the sea, a shift in community structure, as species that compete with or depend on exploited fish species respond to changed conditions of natural selection.

Overexploitation occurs on land as well, perhaps most conspicuously affecting the majestic mammal species that have graced the African savanna for millennia. A global appetite for ivory has encouraged poaching, contributing to the reduction of elephant populations from an estimated 3–5 million in the 1930s to less than 10% of that today. Fueled by a (biologically unfounded) demand in parts of Asia for rhino horn as an aphrodisiac or folk medicine, poaching has similarly reduced populations of most rhinoceros species, limiting these once widespread animals to protected areas in both Africa and southeastern Asia. Note that habitat loss and overexploitation do not occur independently in these ecosystems; their effects are additive and, in some cases, synergistic, meaning that overexploitation exacerbates the harmful effects of habitat loss and vice versa.