CRF Blog

Unnatural Selection

by Bill Hayes

In Unnatural Selection for the New Yorker, Elizabeth Kolbert reports on the threats to the world’s coral reefs.

Coral reefs are often compared to cities, an analogy that captures both the variety and the density of life they support. The number of species that can be found on a small patch of healthy reef is probably greater than can be encountered in a similar amount of space anywhere else on Earth, including the Amazon rain forest. Researchers who once picked apart a single coral colony counted more than eight thousand burrowing creatures belonging to more than two hundred species. Using more sophisticated genetic-sequencing techniques, scientists recently looked to see how many species of crustaceans alone they could find. In one square metre at the northern end of the Great Barrier Reef, they came up with more than two hundred species — mostly crabs and shrimp — and in a similar-size stretch, at the southern end, they identified almost two hundred and thirty species. Extrapolating beyond crustaceans to fish and snails and sponges and octopuses and squid and sea squirts and on through the phyla, scientists estimate that reefs are home to at least a million and possibly as many as nine million species.

This diversity is even more remarkable in light of what might, to extend the urban metaphor, be called reefs’ environs. Tropical seas tend to be low in nutrients like nitrogen and phosphorus. Since most forms of life require nitrogen and phosphorus, tropical seas also tend to be barren; this explains why they’re often so marvellously clear. Ever since Darwin, scientists have been puzzled by how reefs support such richness under nutrient-poor conditions. The best explanation anyone has come up with is that on reefs — and here the metropolitan analogy starts to break down — all the residents enthusiastically recycle.

“In the coral city there is no waste,” Richard C. Murphy, a marine biologist who worked with Jacques Cousteau, has written. “The byproduct of every organism is a resource for another.” Corals are not only the architects of the system, they’re also the repairmen; without their ceaseless maintenance, there’s just “rapidly eroding rubble.” According to Roger Bradbury, an ecologist at Australia National University, if reefs were to disappear, the seas would look much as they did in Precambrian times, before fish had evolved. “It will be slimy,” he has observed. Worldwide, some five hundred million people rely on reefs for food, protection, income, or a combination of all three. Attaching a monetary value to these goods and services is difficult — some entire nations are composed of reefs — but estimates run as high as three hundred and seventy-five billion dollars a year. [more]