Aquatic Invasive Species

Though ballast water is the most widely recognized pathway for invasive species in the Great Lakes, the sale of live organisms may contribute significantly to the number of introduced non-native species. To assess the dangers associated with the sale of live food and aquarium organisms in the Great Lakes, Rixon et al. (2005) from the Great Lakes Institute for Environmental Research surveyed the surrounding regions for potentially invasive species sold in markets and pet stores. They indentified potential invaders by investigating each non-native species’ invasion history, frequency of sale, and thermal requirements, which they compared to local environmental data. The results of their survey showed five fish species and five plant species that had the potential for invasion, including two fish and one plant species that had already become established. This study suggests aquarium trade poses a significant risk to the Great Lakes ecosystem, and thus steps should be taken to minimize the introduction of live food and aquarium organisms.

Biodiversity and Conservation. doi: 10.1007/s10531-004-9663-9 (2005).

Aquarium and ornamental fish trade is one of the most dangerous and rapidly growing vectors for invasive species. Andrew Chang and his team from the Department of Environmental Science and Policy at the University of California examined the invasion risk posed by aquarium fish in the San Francisco Bay-Delta region by identifying potentially invasive species sold in local stores.

Chang’s team identified freshwater and saltwater fish species in pet stores around the Bay-Delta area and compared their temperature and salinity requirements to local environmental data. Under conservative estimates, the pet stores sold three potentially invasive freshwater species and two saltwater species. However, under less conservative estimates, which allowed more realistic temperature variations, they found nine freshwater and eighteen saltwater invaders. Additionally, in the researcher’s survey, most pet store employees agreed the potential invaders represent a serious threat to the Bay-Delta ecosystem. This study represents a step towards identifying species with high invasion risk and taking action against their introduction.

Biological Invasions. doi: 10.1007/s10530-008-9292-4 (2009).

There’s a new predator in the Western North Atlantic, and it’s taking over. Paula Whitfield and her colleagues at the NOAA Beaufort Laboratory examined the range and abundance of lionfish (Pterois volitans) on the North Carolina continental shelf and in potential Marine Protected Areas (MPA) from NC to Florida. Her team found well-established populations along the continental shelf that were as large as native grouper populations, a potential competitor. Grouper population size serves as an indicator of lionfish proliferation risk because overfishing could open a niche in the local marine ecosystem, increasing resources for the lionfish.

Lionfish have also been found from Long Island to the Bahamas and in Bermuda. In some areas, including in each MPA studied, lionfish were the second most abundant species after scamp. Because of the lionfish’s lack of predators, voracious appetite for local fish, and prolific reproduction strategies, it represents a substantial threat to the present biodiversity of marine ecosystems in the Atlantic.

Biol. Invasions. doi: 10.1007/s10530-006-9005-9 (2007)

Water hyacinth has been a menace to all who depend on Lake Victoria since its proliferation in the early 1990s. The aquatic plant forms a dense mat over the surface of the water, blocking sunlight and deoxygenating water, to the detriment of all native species. The vegetation stills water flow, which creates an ideal breeding ground for disease-carrying mosquitoes. (NASA Earth Observatory, 2007) Indeed, there has been a significant increase in the incidences of malaria, dysentery, schistosomiasis, and other diseases, according to the 2005 documentary series “Strange Days on Planet Earth.” Water hyacinth is also a dire concern for subsistence fishermen on the lake because the mats hinder boating. Finally, the dense growth has widespread consequences in the surrounding regions because it clogs intake valves in hydroelectric plants and fouls drinking water. Clearly, it was necessary for the people who depend on Lake Victoria to take action towards containment, if not eradication, of the water hyacinth.

Responding to the desperate situation, scientists implemented classical biocontrol methods in an effort to weaken and kill the population of water hyacinth. In biocontrol, scientists import natural enemies, in this case the weevils Neochetina bruchi and Neochetina eichhorniae, to harm the invasive plant, which usually lacks predators in its new location. Adult weevils damage water hyacinth by consuming its leaves, while larvae tunnel through its roots and stems. The combination opens the plants to secondary infection and causes them to sit lower in the water, according to Wilson et al. (2007), who claim the 2000 reduction in water hyacinth population was caused mainly by the weevils.

There has been recent controversy over whether the reduction of water hyacinth was due to biocontrol or natural weather events, such as the El Niño of 1997-1998. Wilson et al. (2007) argue that, while El Niño contributed to the water hyacinth decline, it can mostly be attributed to the weevils because they weakened the plant. They claim that the low light levels during El Niño did not harm the plants severely enough to cause their rapid decline. Williams et al. (2007) dispute this claim, saying prolonged suboptimal light would weaken the plants and make them vulnerable to other weather-related factors, such as wind, waves, and water quality. They also claim that the weevil population was too unstable after El Niño to cause a lake-wide reduction in the water hyacinth population, arguing that the severe weather patterns played a larger role in the water hyacinth decline. Both teams of scientists agree that both biological control and weather factors played a role in reducing water hyacinth prevalence, but they disagree as to their respective magnitude.

In my opinion, Williams et al. (2007) provide the more convincing argument. Wilson et al. (2007) maintain that the use of weevils in biological control usually takes 3-5 years on large bodies of water. The reduction of the water hyacinth population 3-4 years after the weevil was introduced would make sense in the absence of the El Niño event. However, they state that the weevil population was decimated when the large mats sank as a result of wind and wave action. It is doubtful the weevil population could recover fully enough after such a reduction in population size to eradicate the plant on their predicted schedule. Water hyacinth control could be more believably attributed to the weevil if the massive population reduction had occurred several years later. Furthermore, photos taken by NASA in 2007 showing a resurgence of water hyacinth may point to environmental factors. If the population of weevil had been large enough to cause the massive water hyacinth decline, they would have been able to maintain control. However, if environmental factors were the primary cause, the end of the weather event could allow a second invasion.

In reality, the initial decline of the water hyacinth population on Lake Victoria would have been less likely without both the weevil and the El Niño. For a permanent reduction in the water hyacinth population to take place, we cannot rely on unpredictable weather patterns for control. Biological control methods are necessary to keep the population in check. However, for biocontrol to be effective, especially in a body of water as large as Lake Victoria, all populations of weevils must be highly monitored and kept in good health.

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria.  http://earthobservatory.nasa.gov/IOTD/view.php?i=7426. Viewed 24 Jan 2011
Williams, A. E., R. E. Hecky, and H. C. Duthie. 2007. Water hyacinth decline across Lake Victoria – Was it caused by climatic perturbation or biological control? A reply. Aquatic Botany 87: 94-96
Wilson, J. R. U, O. Ajuonu, T. D. Center, M. P. Hill M. H. Julien, F. F. Katagira, P. Neuenschwander, S. W. Njoka, J. Ogwang, R. H. Reeder, and T. Van. 2007. The decline of water hyacinth on Lake Victoria was due to biological control by Neochetina spp. Aquatic Botany 87:90-93

Miconia calvescens, known to Hawaiian locals as the “green cancer” or “purple plague,” first came to the Hawaiian Islands in the 1960s as an ornamental plant for botanical gardens. Since its introduction, miconia has spread extensively through Maui, Big Island, and Oahu, causing major ecological damage wherever it invades. The South American plant can grow to heights of 50 ft and has large purple and green leaves. According to the HNIS Report for Miconia calvescens, the plant flowers and fruits simultaneously, usually several times each year. Miconia’s fruits are small, but they each can contain 50 to 200 seeds. It is estimated that mature trees can produce up to three million seeds each time they fruit. Non-native birds, such as the silvereye and red-vented bulbul, disperse the seeds far across the island when they consume the sweet fruit. Seeds may also spread via contaminated mud stuck to shoes, clothes, or vehicles. The seeds that fall from the trees accumulate in the soil, creating dense seeds banks that can remain dormant for years.

The Department of Land and Natural Resources of Hawaii warns Hawaiians of the dangers of miconia takeover with examples from Tahiti, which was also overrun with the plant. By the 1980s, miconia had invaded over 60 percent of the land. Currently, over a quarter of Tahiti’s native species are threatened with extinction as a direct result of the miconia invasion. Because the Hawaiian Islands are similar to Tahiti in climate and environment, miconia poses an equally large threat to Hawaiian ecosystems.

Miconia causes many problems for the ecosystem it has invaded. According to the Hawaiian Invasive Species Council, Miconia is on the Hawaiian State Noxious Weed List and has been designated one of Hawaii’s Most Invasive Horticultural Plants. It can harm the natural ecosystem in several ways. Miconia’s enormous leaves create large amounts of shade, killing all native species that require heavy sunlight for survival. Miconia also causes problems with erosion because its roots are not as deep as the roots of the native plants. As a result, there is less to hold the soil in place, which, in some cases, causes massive landslides, further harming the native ecosystem.

The film “Strange Days on Planet Earth” opens its segment on the Hawaiian miconia invasion with dramatic images of the massive amounts of erosion on the island. The film then shows the plant in the wild and gives background on the situation. Even more than the effects of myconia on the ecosystem, the film emphasizes the eradication efforts Hawaiians have taken against miconia.  The film shows high-tech removal operations that use aerial photography to find populations of miconia that are inaccessible by foot.  Those populations are then sprayed with an herbicide. People have also been uprooting the plants manually. However, miconia’s seeds, which fall into the soil in large quantities, pose a large problem to both eradication methods. Because they are difficult to kill with pesticides and can remain dormant for up to ten years, the populations of miconia must be monitored for years after the original plants are killed. New sprouts that spread their own seeds could reverse all of the previous eradication efforts. Removing the plants completely would require constant vigilance for several years. Five years after the film “Strange Days on Planet Earth” was filmed, it would be interesting to see if the eradication methods shown in the film were effective.

Hydrilla verticillata is a highly invasive aquatic plant that has seriously affected water flow and use in the United States. Brought from Asia to decorate aquariums, hydrilla was introduced when owners discarded their plants into canals in central Florida. Since its introduction in the 1950s, hydrilla has taken over costal waterways from Maine to Texas and lakes and reservoirs in several central states and California. The University of Florida and the Institute of Food and Agricultural Sciences (IFAS) Center for Aquatic and Invasive Plants made a short video demonstrating the pervasiveness of hydrilla.

Several characteristics of hydrilla make it the ideal aquatic invader. First, hydrilla reproduces by means of vegetative propagation. A new individual can grow from a small fragment of stem without the use of spores or seeds. Therefore, boat motors unintentionally contribute to its spread by chopping the plants into many pieces, which creates new individuals. Hydrilla can also spread from one body of water to another as stem fragments on boat trailers or motors. Second, hydrilla grows in deep water with little sunlight where few other plants can survive. The population then invades shallower water, blocking sunlight to native plants with its dense growth. Both characteristics make it extremely difficult to remove.

Many states have attempted to eradicate hydrilla using aquatic herbicides and predator species. However, deep tubers pose a large problem to removal, as they can remain dormant and undetected for years before sprouting. A 2003 New York Times article describes Texans’ attempts to gain approval for the use of grass carp, another potentially invasive species, to fight hydrilla. Thousands of sterile grass carp have already been released into Lake Austin.

Hydrilla overpopulation causes a variety of problems in a body of water. Its tangle of sprouts hinders boating, swimming, and fishing, and it outcompetes natural vegetation. The thick growth has even been blamed in drownings. An interesting question relates to why the plants have had their level of invasive success. Does hydrilla spread well due to its innate qualities, or was there a particular vulnerability in the invaded areas? Or can its success be mostly attributed to humans’ accidental help, such as when boaters break the stems into pieces and transport them to new bodies of water? Finding answers to these questions may be crucial in eradicating hydrilla.