image courtesy of Univ. of Calif. Museum of Paleontology
Many species of molluscs are important to humans. Many varieties, such as snails, clams, and scallops, provide food. Oysters produce pearls, which are worth money. Some affect the lives of humans, carrying parasites like schistosomiasis.
Some molluscs are problematic to humans. Species may eat gardens and crops. Oyster drills prey on other molluscs that are important to humans.
Molluscs are both herbivores and predators, so they have a significant effect on the plant species present in an area and as predators, they have almost the same effect.
These species can also be considered prey. They provide food for large numbers of organisms, mostly vertebrates.
Molluscs have not only an economic effect, but also an ecological impact. Zebra mussels in the Great Lakes, feed by filtering particles in the water. The Great Lakes has now dramitically improved in clarity since zebra mussels first appeared. As for economic effects, zebra mussels are the base of a complicated food chain for other species including economically important game fishes.