HOW DO WE COUNT ANIMALS WE CANNOT SEE? (Part 2 of 2)

by Whit Gibbons

November 4, 2002

Last week I wrote about the difficulties encountered by conservation biologists in establishing how many kinds of animals are present in the world. The beach mice of Alabama and Florida served as an example of an identifiable species of animal with variable habitat requirements and coat colors. Hence, the issue could become clouded as to how many kinds of animals, one or several, are actually represented. But even if taxonomists did accomplish the impossible task of naming and agreeing on all the species and subspecies of organisms, ecologists would be confronted with another problem. How can the number of different kinds of animals that live within a prescribed region be determined when individuals of the species are difficult to find?

Some sort of field sampling effort is necessary to characterize biodiversity of a prescribed area and is one of the first steps in conservation at the ecosystem level. For instance, knowing the species of plants and animals being protected by the country's extensive network of national parks and wildlife refuges can make us more respectful of our natural heritage. But to find out what we have, experts must do the counting, and for some groups of animals, the task can be more challenging than is at first apparent.

An unsettled question that remains for many major groups of animals is how closely the number of species that have been discovered and described in a group represent the actual number present. The issue can be particularly vexing for malacologists, scientists who study clams, mussels, and other mollusks. Mollusks have the highest known species diversity of any animal group native to the world's oceans. Mollusk biodiversity is especially high in the tropical Indo?Pacific region, but some malacologists think it is much higher than what has been described. For example, some estimates are that the number of coral reef species currently described represent as low as 1% of the species actually present. A recent study has put to the test the idea that what has already been described woefully underrepresents the actual numbers of mollusk species.

Philippe Bouchet of the Museum National d'Histoire Naturelle, France, and colleagues conducted an intensive survey of mollusk species at a site on the west coast of New Caledonia in the Coral Sea east of Australia. The investigators intensively sampled more than 100 square miles of coral reef lagoons, collecting every mollusk shell that could be found. The scientific sampling effort, more than 400 person?days was the most thorough ever made in determining species diversity of mollusks. Aside from ending up with the largest and most variable seashell collection ever made from one place, the scientists also uncovered the truth about how well sampling reflects reality for marine mollusks.

They collected more than 127,000 individual mollusks of 2,738 species, numbers that exceed any previous surveys of mollusks taken anywhere in the world for a comparable area of coral reef. Rare species, defined as ones that were represented by only a single specimen each, made up 20% of the sample. This is important to note, because when one?fifth of the species in a sample are represented by only one specimen, the likelihood increases that lots of other rare species were present but never found. When the investigators used a mathematical formula to estimate species numbers beyond their actual captures, they calculated that as many as 3,971 species were actually present in the region sampled. Thus, biodiversity was dramatically higher than ever before reported, confirming the suspicion that the number of species of mollusks present around that part of New Caledonia was much higher than could be determined by even the most intensive sampling.

The results from the New Caledonia study suggest that current estimates of global biodiversity of mollusks are greatly underestimated. The same is probably true of other groups of animals in many different regions of the world on land, in freshwater, and in the sea. Among the important lessons to be learned from that conclusion, consider this one: when we demolish natural habitats, we are destroying even more species than we realize.



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