Geoscience Reference
In-Depth Information
where cod numbers were depleted shrimp numbers rose, suggesting that there was top-down control of prey
numbers in these ocean systems.
One of the best examples of the cascading effects of top predator removal comes from the Mid-Atlantic.
Surveys carried out off North Carolina since 1972 reveal drastic declines in the number of great sharks, which
are targeted for their fins (a luxury item in Chinese cuisine) or reeled in as by-catch in the tuna and swordfish
fishery. Declines in seven great shark species range from 87 percent for sandbar sharks to 99 percent for bull
sharks and, in Chesapeake Bay, a 99 percent decline in tiger sharks between 1974 and 2004. These great sharks
prey on smaller sharks, rays, and skates. With these great predators out of the way, their prey proliferated. In
particular, cownose ray numbers jumped by an order of magnitude—to more than 40 million. As they migrated
south from northern estuaries each fall, they stopped over in North Carolina sounds, where they sought out
their preferred prey, the bay scallop. By 2004, cownose rays had almost wiped out the bay scallop population,
bringing to a close North Carolina's century-old bay scallop industry.
It was another example of the interconnectedness of marine ecosystems and a dramatic demonstration of
how the removal of one species echoes through the system with unexpected consequences—economic and
ecological—down the line. The top-down effects can ultimately impair the recovery of the predator population
through a predator-prey role reversal. This appears to have happened in the southern Gulf of St. Lawrence,
where the recovery of the collapsed cod population may be held in check by the predation of cod eggs and lar-
vae by the now more abundant prey species, herring and mackerel.
The human response to this disruption in the marine ecosystem has often been to continue fishing down
through the food web: first taking out the top predators, then concentrating on their prey, and finally harvesting
the plants such as rockweed that are at the base of the food-production pyramid. In the last two decades a eu-
phemism has emerged in bureaucratic circles to describe and justify this practice: the term is “underutilized
species,” which Ransom Myers, with typically trenchant humor, once ruefully characterized as government-
speak for “some left.”
Current policy is an endgame with potentially disastrous consequences for our ability to feed ourselves and
for the recovery of the marine environment to some semblance of health. In a now famous paper published in
the prestigious journal Science in November 2006, an international group of ecologists and economists, with
principal author Boris Worm, warned that if the stripping of the oceans continued at the same rate as over the
last fifty years, all species of wild seafood currently being exploited could collapse by the year 2048. Co-au-
thor Steve Palumbi, of Stanford University, observed that unless we begin to “manage all the oceans' species
together, as working ecosystems, then this century is the last century of wild seafood.”
It is a sobering wake-up call. Society will lose not only a vital source of food and pharmaceuticals but also
the other services the ocean provides, such as flood control and waste detoxification by oyster reefs, sea grass
beds, and coastal wetlands. The paper also poses potential solutions: the key to preserving these vital services
is to maintain biodiversity. Restoring biodiversity has been shown to increase productivity fourfold, and fish
diversity also aids the rate of recovery of collapsed populations. Evidence for the link between biodi-versity
and the productivity and resilience of marine ecosystems comes from examining fully protected marine re-
serves and large-scale fisheries closures. In these cases, species diversity of both target and nontarget species
has been boosted by nearly a quarter (23 percent), and these increases in biodiversity have translated into large
increases in fisheries productivity around reserves. The rebound time can be surprisingly quick—three to ten
years.
Historical Amnesia and Future Shock
University of British Columbia fisheries scientist Daniel Pauly has hypothesized that marine scientists gener-
ally suffer from a historical amnesia that has resulted in a misguided perception of what a productive marine
ecosystem actually looks like. Most often, he says, it is based on already degraded systems, and so our as-
