But in a paper set to appear Friday (7/27) in the journal Science, 16 scientists and academicians from around the world argue that these trends were set into motion by a much older human transgression: overfishing.
Beginning long before Columbus and accelerating rapidly in Colonial and modern times, people have radically overfished marine mammals, large fishes and shellfish, according to the paper, whose co-authors include Karen Bjorndal, a zoology professor and director of the Archie Carr Center for Sea Turtle Research at the University of Florida. The reduction of these animals to a fraction of their historical abundance has caused ecological damage that remained hidden until recent decades, when other circumstances triggered its full effects, the scientists say.
"What we're finding is a number of the crises that our marine ecosystems are facing today can be traced back thousands of years in some cases, and hundreds of years in others, to when human beings first began affecting these ecosystems," Bjorndal says.
The paper is an unusual example of scientific literature, and not just because of the number and diversity of authors, who range from ecologists to paleobiologists and hail from institutions including the University of Chicago, Australia National University and the Smithsonian Tropical Research Institute.
In contrast to the narrow focus of many scientific papers, it discusses coastal ecosystems around the globe, hopscotching from the Chesapeake Bay to the Caribbean to Australia's coastal waters. And it steps outside the bounds of pure ecological science, drawing on a broad array of scientific literature, historical accounts and archaeological evidence of aboriginal fishing practices.
Among the interesting tidbits that emerge: While overfishing reached its zenith in Colonial and modern times, it was a problem in native societies long before Europeans arrived, "contrary to romantic notions of the supposedly superior ecological wisdom of non-Western and pre-Colonial societies," the paper states.
The authors tie several recent downturns in the world's coastal ecosystems to past overfishing. Examples include:
Declining underwater kelp forests -- Overfishing of sea otters in the Northern Pacific - starting with aboriginal Aleutian fishing 2,500 years ago -- has caused exploding sea urchin populations, the paper says. The plant-eating urchins have decimated the kelp forests, a crucial habitat for young fishes. Recent protection of otters has helped, but killer whales now are eating the otters. That's because people have overfished the fishes consumed by seals and sea lions - normally the whales' prey. This overfishing has in turn slashed numbers of seals and sea lions.
Shrinking seagrass beds -- Bjorndal's research ties overfishing of green sea turtles to the decline of turtlegrass in Florida Bay and in the Caribbean. The connection: The turtles, which eat only plants, prevent the turtlegrass from growing too long. In their absence, the longer grass shades the bottom, slows the current and decomposes on the sea bottom. This process both increases nutrients and encourages turtlegrass diseases. "One of our best estimates is that green sea turtle populations today are 5 to 10 percent of what they were when Columbus arrived," Bjorndal says. "The functional loss of this species has had a huge effect."
Declining shellfish beds -- The authors say overfishing of oysters in the Chesapeake Bay has contributed to low oxygen levels and other symptoms of nutrient pollution because the oysters no longer filter the bay's water. The resulting degradation of water quality has in turn prevented the oysters from regrowing. It has also spurred the outbreak of microbial diseases potentially harmful to sea life and people alike, such as Phisteria, a parasitic organism that has plagued North Carolina coastal systems.
Bjorndal says scientists have missed the connection between past overfishing and current ecological problems in part because they don't feel comfortable working with historical data, which may not conform to the rigors of scientific testing and experiment.
While it's true that past data may not be as complete as scientists would like, the resulting lack of historical perspective has often led to erroneous or incomplete conclusions about ecological problems, she says. For example, scientists attributed a massive die-off of coral reefs in the Caribbean in the 1980s to algae blooms spurred by the decline of a then-abundant plant-eating sea urchin. While technically accurate, she says, the bigger picture is the sea urchin was simply the last herbivore left after people had fished out most plant-eating fishes and turtles.
Indeed, data assembled by the scientists that compares historic populations of sea life to current populations paint a bleak picture.
A century ago, an estimated 104,000 dugongs, a type of sea cow, swam in the waters of Moreton Bay, Australia; now there are just 500. Where Atlantic cod in the Gulf of Maine once measured an average of about 3 feet, today's average size is about a foot. Where seagrass beds covered more than 76,000 acres in Tampa Bay less than 150 years ago, the grass now encompasses just 26,000 acres.
Despite such declines, the authors conclude on a cautiously optimistic note, saying most of the overfished species still survive in sufficient numbers to permit restoration. Recognition and study of the past richness of coastal ecosystems highlights "the extraordinary magnitude of economic resources that are retrievable" and is a first step toward rebuilding the systems to their past glory, they say.
"If we want to restore these ecosystems, we have to understand how they function, and just looking back a couple of decades isn't going to tell us," Bjorndal says.
A University of Florida biologist is among 16 authors of the paper in the journal Science.
Writer: Aaron Hoover