History shows that Caribbean coastal ecosystems were severely degraded long before ecologists began to study them. Large vertebrates such as the green turtle, hawksbill turtle, manatee and extinct Caribbean monk seal were decimated by about 1800 in the central and northern Caribbean, and by 1990 elsewhere. Subsistence over-fishing subsequently decimated reef fish populations. Local fisheries accounted for a small fraction of the fish consumed on Caribbean islands by about the mid nineteenth century when human populations were less than one fifth their numbers today. Herbivores and predators were reduced to very small fishes and sea urchins by the 1950s when intensive scientific investigations began. These small consumers, most notably Diadema antillarum, were apparently always very abundant; contrary to speculation that their abundance had increased many-fold due to overfishing. Studying grazing and predation on reefs today is like trying to understand the ecology of the Serengeti by studying the termites and the locusts while ignoring the elephants and the wildebeeste. Green turtles, hawksbill turtles and manatees were almost certainly comparably important keystone species on reefs and seagrass beds. Small fishes and invertebrates feed very differently from turtles and manatees and could and can not compensate for their loss, despite their great abundance long before overfishing began. Loss of megavertebrates dramatically reduced and qualitatively changed grazing and excavation of seagrasses, predation on sponges, loss of production to adjacent ecosystems, and the structure of food chains. Megavertebrates are critical for reef conservation and, unlike land, there are no coral reef livestock to take their place.
