Why the extinction of Molluscs represents an irreversible loss. Discuss why the extinction of Molluscs or any other invertebrate represents an irreversible loss of biological diversity. Support your answers with the reading you have done for this unit’s topic.
Nonmarine mollusks belong to the second most diverse animal phylum in terms of numbers of described species. They include phylogenetically distinct lineages and assemblages, representing two molluscan classes: Bivalvia (clams and mussels) and Gastropoda (snails, limpets, and slugs). Global estimates of species richness for nonmarine mollusks, like estimates of total molluscan species richness, vary widely.
There are three major reasons for this variation: (1) the vast array of nominal taxa whose synonymy remains uncertain (they were described by early taxonomists using shell morphology alone, and may or may not reflect real biological taxa); (2) the vast regions of the world that remain unexplored, probably harboring many undiscovered and undescribed species; and (3) the lack of an adequate cadre of molluscan taxonomists to cover the breadth of molluscan diversity.
For a few taxonomic groups and geographical regions, diversity may have been overestimated because of the description of too many nominal species, but for most groups and areas, diversity is probably seriously underestimated.
Estimates of the total number of valid described and undescribed mollusk species range from 50,000 to 200,000 (van Bruggen 1995). However, most recent estimates tend to favor the higher end of the range (e.g., Stork 1999). Despite this uncertainty, and given the caveats mentioned above, we estimate that there are approximately 24,000 terrestrial and 7000 freshwater mollusk species for which valid descriptions exist. In addition, there are probably 11,000 to 40,000 undescribed terrestrial species and 3000 to 10,000 undescribed freshwater species (figure 1).
Mollusks have the dubious honor of having the highest number of documented extinctions of any major taxonomic group. A staggering 42% of the 693 recorded extinctions of animal species since the year 1500 are mollusks (260 gastropods and 31 bivalves); this is more than the total (231) of all tetrapod species that have gone extinct during the same period (figure 3).
Nonmarine species constitute 99% of all molluscan extinctions. Although terrestrial vertebrate extinctions are well documented, invertebrate extinctions often go unnoticed by the general public, by most biologists, and by many conservation agencies. Only a tiny fraction (< 2%) of known molluscan species have had their conservation status properly assessed.
Thus, the level of molluscan imperilment is poorly documented and is almost certainly underestimated. This view is supported by the continuing discovery of large numbers of small, narrow-range endemics, which occur especially in the tropical regions of the world, many of which are being rapidly deforested (e.g., Madagascar [Emberton 1995] and Tanzania [Emberton et al. 1997]).