Aquatic Invasive Species

Challenges in Invasive Species Management

Managing invasive species in the large, complex ecosystem of Narragansett Bay is a difficult task, which is made even more challenging due to the many unknowns regarding these species and their interactions with the ecosystem. Human travel between Europe and Rhode Island goes back much farther than any comprehensive and detailed biological records of species living in the state. It is unclear whether a few species are native or introduced, which places them in the category of cryptogenic. The 2013 Rapid Assessment Survey identified 13 cryptogenic species in Rhode Island, such as the orange crust bryozoan and the sea vase tunicate.

The long-term presence of certain introduced species is another challenge to invasive species management. For example, the European green crab, which is not native to Rhode Island, was introduced over 200 years ago. We can observe their current impact on eelgrass beds and soft-shelled clams, but scientific data on the state of eelgrass and clams before the European green crab introduction are sparse.

For other species, the total impact on the ecosystem is unclear. One such species is the Asian shore crab, which, since its introduction in 1988, has significantly impacted some native crab species but not others. Asian shore crabs may pose a threat to lobsters as they eat larval lobsters. On the other hand, adult lobsters and some fish eat the Asian shore crabs. These lobster and fish populations are being impacted by several other factors, including harvesting, climate change, and pollution, so it is difficult to determine what effects can be attributed to the presence of Asian shore crabs.

Overall, it is difficult to predict the long-term relationship between the many different species,—native or not—that live in Narragansett Bay, but the conservative strategy is to prevent introduction of new species with the potential to disrupt the ecological balance, and monitoring is fundamental to prevention.

Freshwater Aquatic Plants in Rhode Island

Three invasive freshwater plant species are of greatest concern in Rhode Island: variable milfoil, fanwort, and water chestnut; variable milfoil and fanwort are the two most common species in Rhode Island freshwater. While water chestnut is currently only present in five Rhode Island lakes, it is defined as a demonstrated nuisance elsewhere in the northeastern United States. Between 1982 and 2011, federal, state, and non-governmental groups spent $9.6 million to control water chestnut in Lake Champlain alone.

Variable milfoil

Variable milfoil is a plant that lives entirely underwater. It is native to the Southeast and Midwest US, and has spread throughout the Northeast largely due to its ability to reproduce by fragmentation in which a single fragment of the plant can grow roots and become established in a new body of water. Milfoil spreads quickly and creates dense stands that crowd out native vegetation, interfere with recreational activities, and create areas well suited to mosquito breeding. Once established, variable milfoil is very difficult to control and virtually impossible to eradicate, which leads to the need for long-term lake management strategies. Handpulling and herbicides are both somewhat effective control measures, but each has its own downside: handpulling is very labor and time intensive with less than 100% effectiveness, while herbicides are less labor and time intensive but involve sprayed chemicals in the water that may have unintended consequences.

Fanwort

Fanwort is another plant that lives underwater, growing in shallow waters less than 30 feet deep. Like variable milfoil, fanwort is native to other parts of the US, but was introduced to the Northeast as an aquarium plant that was inadvertently dumped into ponds and lakes. Fragments can survive six to eight weeks out of the water, allowing easy transfer and establishment from one lake to another on boats and trailers. Fanwort is made more difficult to control because hand-pulling may be effective for small areas, but may also create more fragments that can drift into new areas, become established, and accelerate the plant’s spread. Herbicides can also be effective, but potential side effects need to be fully understood before implementing on a broad scale.

Water chestnut

Water chestnut was introduced to New York from Europe as an ornamental plant, and is also native to Asia and Africa. It typically spreads by dispersion of seeds on feathers of waterfowl, such as ducks and geese. Water chestnut grows in thick mats on the water’s surface thereby reducing available light for native plants growing under the surface. These mats impede recreation and can decay quickly reducing oxygen in the water, which may lead to fish kills. Water chestnut seeds remain viable for 12 years making control difficult; even if a population is eradicated, over a decade of monitoring is required to ensure that this invasive does not return. In Rhode Island, water chestnut was first documented in 2007, growing in Belleville Pond in North Kingstown. Though currently only established in five lakes in the state, the struggles of other states to control water chestnut demonstrates the importance of constant monitoring to prevent acceleration of this problem in our freshwater lakes and ponds.