National Geographic : 2001 Jan
you drag an anchor across it." Just the same, I added genuine heroes to the list of life-forms I had encountered along the reef. That list, growing longer hourly, led me to James Cook University in Townsville, the main coastal city along the Great Barrier's Central section. I wanted a professional opinion as to why reef life was such a parade of forms and hues. Why 20 look-alike butterflyfish species instead of, say, two or three? David Bellwood, a leading marine ecologist, replied, "The answer is that we really don't know. But I can tell you that a lot of the diversity with in groups of tropical fish is a matter of historical accident. It may be that as sea levels fell during the ice ages, ocean basins became isolated, and their populations evolved along separate lines, which we now regard as separate species. Later, when sea levels rose again, many migrated to Australia and took up residence side by side." And the communities continue to flourish today. "The best management decision Austra lia ever made was to put its reef far offshore and along the northern end of the continent, which hardly has any people," Bellwood said jokingly. "We have some overfishing, but this is one of the few countries that has yet to make its big mistakes with coral ecosystems." On the other hand, the coral realm is not immune to the changes taking place in ecosys tems on land. Cane fields, other croplands, and development along Queensland's coastal plain have replaced many seaside wetlands, the natural filters for fresh water coming from the continent. Coupled with deforestation, over grazing by livestock, and runoff from towns, farms, and industries upstream, this sends more sediments and nutrients flowing out toward the Great Barrier. The total has qua drupled since colonial times. Corals can persist in surprisingly murky water as long as tides and currents periodically sweep the sediments questions," admits one marine biologist.One theory, though, posits that on a reef with nearly 2,000 species offish milling about, splashy, distinctive hues enable a fish to quickly recognize one of its own.