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How Can a Native Species Become Invasive?

Invasive species are usually non-native. Their ability to invade successfully begins when they gain access to new environments. However, some native species can become invasive when environmental conditions change.
These native species may respond by increasing their population size or by expanding their range. Two examples are the Sea Lamprey and Didymo, more commonly known as Rock Snot. Both are native to New York state but have expanded their range. It is unclear exactly what conditions changed to cause these population expansions. Their population expansions have been large enough that there are now concerns about how these native species affect other species and ecosystems.
LampreyUnited States Fish and Wildlife Service Archive, United States Fish and Wildlife Service,

Robert M. Bennett, Chancellor
Lee Emery, United States Fish and Wildlife Service,

Lamprey Victim
United States Fish and Wildlife Service Archive, U.S. Fish and Wildlife Service,

Sea Lamprey

Petromyzon marinus
Sea Lamprey is native to both coasts of the Atlantic Ocean, as well as to Lake Ontario and the Saint Lawrence River downstream and their tributaries. Sea Lamprey is also native to Lake Champlain. The removal of predators there may have enabled Sea Lamprey to increase its population and expand into new habitats within the lake. It has become a nuisance and an eradication program has been initiated.

In the early 1900s, the Welland Canal in Ontario was redesigned and the summit elevation was removed. This may have provided Sea Lamprey with direct access to the upper Great Lakes, where it is not native. The lamprey expansion may be an example of migration through manmade canals. Since the 1930s, the number of Sea Lamprey has exploded in the upper Great Lakes.

Sea Lamprey spends its juvenile years as a filter feeding ammocoetes in freshwater streams. After six to ten years, it transforms into an adult and heads out to sea to feed as a predator, most frequently on large fish. After about a year of feeding, adults usually return to a suitable natal stream, spawn, and die. In the Great Lakes, Finger Lakes, and Lake Champlain, Sea Lamprey does not migrate to the ocean. It spends its adult life in the lake instead.
Timothy Daley, Pennsylvania Department of Environmental Protection

Didymo, or Rock Snot

Didymosphenia geminata
Didymo, or Rock Snot, is a microscopic single-celled algae. It has sparse distribution among northern parts of the globe, where it often occurs in low-nutrient, high-elevation waters. In recent years, it has been expanding its range. It is unclear why.

However, Rock Snot does appear able to thrive in a much wider range of environments than previously thought. In freshwater streams, the original tiny organism can secrete massive amounts of strands. These strands can form thick mats covering the bottoms of streams. This disturbs habitat for invertebrates and fish, and affects dissolved oxygen levels vital for their survival. Rock Snot’s microscopic cells adhere easily to equipment, clothing, boats, or other damp items. This contributes to its current spread.

New York State Department of Environmental Conservation is currently tracking nuisance blooms in several recreational streams. These include the Batten Kill and the East and West Branches of the Delaware River. They are likely new extensions of the algae’s range. Its occurrence in these streams is of particular concern because of the renowned trout fisheries in both. In addition, Rock Snot may pose a threat to additional neighboring waters of the region.