TOEFL Integrated Writing Practice Test 040Zebra Mussels

The zebra mussel, a freshwater shellfish native to Eastern Europe, has been spreading beyond its original range and has now reached parts of North America. This invasion appears to be unstoppable, and it poses a serious threat to freshwater fish populations across the continent.

First, the history of the zebra mussel’s spread suggests that halting its invasion may be impossible. Its spread has been largely due to human transportation. From its native habitats in Eastern Europe, the zebra mussel spread through new canals connecting Europe’s waterways, often attaching to ships or surviving in the ballast water that ships carry for balance. By the early 19th century, the mussel had spread throughout Europe and later reached North America’s east coast in the ballast water of European ships. This pattern of spread indicates that zebra mussels are likely to colonize all of North America.

Secondly, zebra mussels tend to dominate new habitats once introduced. They are resilient, can thrive in a variety of conditions, and reproduce rapidly. Crucially, zebra mussels often encounter no natural predators in their new environments, allowing them to dominate ecosystems where they are introduced.

Finally, zebra mussels are likely to harm freshwater fish populations in the areas they invade. Since zebra mussels consume plankton, they compete for food with many fish species, potentially leading to a decline in fish populations where the mussels become dominant.