What was natural in the coastal oceans?

Abstract

Humans transformed Western Atlantic coastal marine ecosystems before modern ecological investigations began. Paleoecological, archeological, and historical reconstructions demonstrate incredible losses of large vertebrates and oysters from the entire Atlantic coast. Untold millions of large fishes, sharks, sea turtles, and manatees were removed from the Caribbean in the 17th to 19th centuries. Recent collapses of reef corals and seagrasses are due ultimately to losses of these large consumers as much as to more recent changes in climate, eutrophication, or outbreaks of disease. Overfishing in the 19th century reduced vast beds of oysters in Chesapeake Bay and other estuaries to a few percent of pristine abundances and promoted eutrophication. Mechanized harvesting of bottom fishes like cod set off a series of trophic cascades that eliminated kelp forests and then brought them back again as fishers fished their way down food webs to small invertebrates. Lastly, but most pervasively, mechanized harvesting of the entire continental shelf decimated large, long-lived fishes and destroyed three-dimensional habitats built up by sessile corals, bryozoans, and sponges. The universal pattern of losses demonstrates that no coastal ecosystem is pristine and few wild fisheries are sustainable along the entire Western Atlantic coast. Reconstructions of ecosystems lost only a century or two ago demonstrate attainable goals of establishing large and effective marine reserves if society is willing to pay the costs. Historical reconstructions provide a new scientific framework for manipulative experiments at the ecosystem scale to explore the feasibility and benefits of protection of our living coastal resources.

Figure 1

Model of the consequences for reef corals of the increase in the sea urchin Diadema antillarum caused by overfishing of large predatory and herbivorous fishes and the subsequent mass mortality of Diadema caused by disease. Reproduced with permission from ref. (Copyright 1994, The Royal Society). Plane A, pristine condition, with high ratio of corals to macroalgae because of intense grazing of macroalgae by fishes. Plane B, abundant Diadema grazed macroalgae formerly consumed by herbivorous fishes so the ratio of corals to macroalgae remained high despite intensive fishing. Plane C, mass mortality of Diadema caused by infectious disease allowed macroalgae to proliferate and overgrow corals.

Figure 2

Model of the collapse of Western Atlantic coastal ecosystems caused by overfishing. Arrows indicate the three major ecological transitions discussed in the text.