MOLLUSKS ARE TEXTBOOK EXAMPLE OF DECLINING BIODIVERSITY

by Whit Gibbons

January 30, 2011

Mollusks are arguably the most endangered major group of animals in the world. But few people realize the extent of their imperilment. The idea that mollusks are in severe trouble has reached a college level biology textbook so the word will now be spread more broadly.

Textbooks are critical sources of information, and the resources they use are key to their reliability. I feel confident about the section called "Molluscs: The Silent Extinction" in the ninth edition of Campbell Biology (Benjamin-Cummings Publishing Co., 2011). The textbook authors got their material from a scientific paper, "The Global Decline of Non-Marine Mollusks," by Charles Lydeard and others published in the journal BioScience. Considering the excellent scientific reputation of the authors and of the journal itself, I feel certain that the information is accurate.

Incidentally, the book refers to "molluscs," and the journal article calls them "mollusks." The first is the British spelling; the latter, the American version. Malacologists (scientists who study animals in the phylum Mollusca) debate about which is the proper spelling, but both mean the same thing. Being American, I will call them mollusks.

One fact stated in the textbook is that mollusks "account for a largely unheralded but sobering 40% of all documented extinctions of animal species." That is a remarkably high percentage of the animals that have gone extinct in historical times. According to the Lydeard article, 291 of the "693 recorded extinctions of animal species since the year 1500 are mollusks." Virtually all have been freshwater or terrestrial species. In the last five centuries, the number of mollusks disappearing from the world has been more than all the mammals, birds, reptiles, and amphibians combined that have gone extinct.

The facts presented in the original journal article are impressive in their broad coverage of the plight of mollusks of the world. Everyone is familiar with such marine mollusks as oysters, giant clams, and the chambered nautilus. Squids, too, are mollusks (though many people are unaware of that fact). But the article focuses on species like clams and mussels that live in freshwater aquatic habitats and land snails that inhabit terrestrial systems. The causes of extinctions are primarily a consequence of human activities, resulting mostly from "habitat loss, pollution, introduced species, and overharvesting."

Habitat loss can be particularly devastating to a land snail that lives on an island or a clam that inhabits a lake. Most mollusks cannot move rapidly away from environmental hazards and are more vulnerable than animals that can swim long distances, fly, or move rapidly overland. Freshwater mollusks living in lakes, streams, or rivers that are polluted or in which exotic competitive species have been introduced are, in essence, environmental prisoners unable to escape.

One of the most dramatic documented mollusk extinctions occurred in Alabama. In one journal article section titled "Silence of the Clams," the authors discuss the highly diverse and globally widespread group of freshwater mollusks known taxonomically as the unionoid mussels. These animals reach their greatest diversity of all in North America, and the record for species numbers is in the Tennessee River near Muscle Shoals, Alabama. Based on records from the early 1900s, 69 species of these mussels were once present. Only 32 of these species have been "recorded since the river was dramatically altered by the construction of a series of dams." This has a dramatic negative impact on regional biodiversity.

The textbook elegantly summarizes why the continued loss of freshwater and terrestrial mollusks to extinction should matter to us. Such losses "represent an irreversible loss of biological diversity. . . . Land snails, for example, play a key role in nutrient cycling, while the filtering activities of freshwater bivalves purify the waters of streams, rivers, and lakes." Another message worth noting is that when mollusks decline and ultimately disappear they serve as powerful indicators that ecosystem health itself is impaired. Like the canary in the mine that signals danger when it ceases to sing, mollusks are sending us a warning we should heed.

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