Aquatic Invasive Species

The red lionfish (Pterois volitans), popular among many aquarium enthusiasts, is a large fish native to the indo-Pacific region of the Pacific ocean. It has few predators and is able to survive in a variety of conditions, making it a formidable invasive species. Originally introduced into the Atlantic Ocean along the coast of North America, it has since damaged Bahamian ecosystems and reef systems.

Red lionfish populations in the Bahamas have been observed to be at a density of 390 lionfish/hectare, a density high than recorded even in their native habitats. Its high rate of population growth is due to a lack of any predatory fish in the Atlantic Ocean. A lack of competition has enabled these invaders to decrease Atlantic fish recruitment in the Bahamas to coral reefs by 79%.

Rui Dai of Duke University, among numerous other researchers, postulates that not all hope is lost for the Bahamian coral reefs. In 2008, several Nassau and tiger grouper (native Bahamian grouper species) were found to have lionfish remains in their stomachs. Dai is interested in whether these Bahamian grouper, perhaps with the help of humans, may be able to serve as natural biocontrol for the red lionfish.

To test her theory, Dai plans to place several groups of different proportions of the grouper, the lionfish, and other fish species native to the Bahamas in commercial fish pens, which are used while still in the ocean. This type of fish pen will be used because it should enable salinity levels, etc., in the experiment to be identical to those in the Bahamas, the environment in which the results of this study are applicable.

Each test group will contain one Bahamian grouper (of either species) and two, four, or eight lionfish, as well as a variety of crustaceans, small fish, and squid found in the Bahamas. Dai will also test the grouper at different ages (five months old, one year old, and five years old) to see whether only the largest adult grouper could be successful biocontrol agents. Each morning, Dai plans to observe which species are still present in each pen. The entire study will last approximately three months, after which point Dai expects to have enough data to draw conclusions about the approximate success of Bahamian groupers as biocontrol agents against red lionfish.

Dai expects that large numbers of the adult grouper will be the most successful biocontrol agents, though human assistance will be imperative for their success. Once Dai has obtained results from her study, she hopes that others will continue research about how to best implement Bahamian grouper as biocontrol agents against the red lionfish.

Most invasive species pose no direct threat to and have no immediate impact on human life; however, since its introduction in the 1940s, Eurasian watermilfoil (Myriophyllum spicatum) has wreaked havoc on inland bodies of water throughout the United States.  The species has already invaded forty-four of the fifty states and its rapid proliferation causes swimming, boating, and fishing problems.

A proposed study at Duke University is seeking funding in order to investigate novel approaches to control of Eurasian watermilfoil.  Because previous studies have already shown that Eurasian watermilfoil is susceptible to biological control via the milfoil weevil (Euhrychiopsis lecontei), the aim of the proposed study is to use this weakness to find other methods to control the invading watermilfoil.

The study, suggested by Duke undergraduate Emily Chang, focuses on the relationship between the Eurasian watermilfoil and the E. lecontei weevils.  The weevils have been used to control the invading plant in other areas because they chew through the plant’s tissues, which results in the collapse, and death, of the plant.  The proposed study looks to analyze the composition of the plants in terms of chemical elements, organic functional groups, and damaged nonstructural hydrocarbons after the weevils have invaded a population of Eurasian watermilfoil.  Chang hopes that this information will provide insight into “the chemical nature of this biocontrol method” and will “allow scientists to improve the effects of biocontrol by manipulating the chemistry operating behind the watermilfoil.”

Chang states that “if milfoil can spread as much as it has, it is clearly very powerful and has the potential to spread everywhere.”  Thus, in hopes to combat the spread of this invasive species, Chang has proposed a study that hopes to advocate the use of biocontrol as a stepping block for the expansion of chemical control.  Currently, Chang feels that biocontrol is the only feasible method of dealing with Eurasian watermilfoil and notes that “mechanical control is bad because it is time consuming because each plant must be hand plucked and because milfoil is a resilient species.  Chemical control is also a poor option because it only offers temporary relief.”  However, Chang hopes to change this by using information from biological control to foster the development of chemical controls that will cause permanent effects.

Chang’s experiment will utilize four lakes that have substantial Eurasian watermilfoil populations.  Four 3×5 meter plots will be created for analysis; one plot will be a control containing just the watermilfoil, and the other three plots will contain the watermilfoil with varying numbers of weevil population densities.  The different plots will be monitored from May to October for three consecutives years.  Once a month, fifteen plants will be removed from each plot, frozen, then dried and crushed and analyzed for carbohydrate concentration.  Some methods of elemental analysis include the use of a CHN Analyzer, infrared spectroscopy (IR), and nuclear magnetic resonance spectroscopy (NMR).  The plant samples will be compared to the original composition of watermilfoil before the study.  On a side note, the contents of the weevils’ stomachs will also be analyzed in order to determine which tissues are targeted in feeding.

If specific carbohydrates are targeted by the milfoil weevils, the future of control could be very optimistic.  Scientists would be able to focus on attacking the specific weak carbohydrates and functional groups in order to take down the invading Eurasian watermilfoil populations.  Chang hopes to use chemical control to help thwart the dangers of watermilfoil and concludes that “if we can pinpoint which carbohydrates are favored, … we can apply certain chemical herbicides to cause plants to die and disappear.”

Increasing concern over the invasive threat of the red lionfish (Pterois volitans) has become ever present. The species has been observed to have spread ubiquitously throughout the Bahamas and along the east coast of North America. They disrupt coral reef by reducing indigenous Bahamian fish recruitment by over 79%. And because of the lionfish’s poisonous fins, there have been concern about whether its growth can be contained.

Maljkovic and Leeuwen (2008) of Simon Fraser University however reported anecdotal incidences of finding native Bahamian groupers (tiger grouper, Mycteroperca tigris and five other Nassau groupers) with partially digested red lionfish in their stomaches. This provides hope that there is a potential native biocontrol for the invasion of lionfish, and that the species will be safely integrated into Bahamian ecology without significant damage to the current ecological structure. However, the anecdotal evidence in this paper still needs to be empirically confirmed.

Coral Reefs (2008) 27:501 DOI 10.1007/s00338-008-0372-9

Emily Chang

Professor Sandra Cooke

Writing 20

24 March 2010

To Decline, or Not to Decline?

J. Aquat. Plant Manage 38: 105-111 (2000)

A study by Raymond M. Newman and David D. Biesboer (2000), both of the University of Minnesota, investigates whether there are possible relationships between the population of the milfoil weevil Euhrychiopsis lecontei and that of Eurasian watermilfoil (Myriophyllum spicatum). Control of the aquatic milfoil plant is important because it has harmed aquatic diversity, hindered human boating and recreation, and withstood both mechanical and chemical control methods. The researchers examined the Eurasian watermilfoil colonies and abiotic conditions, such as alkalinity, of the man-made Cenaiko Lake from 1996 to 1998. Samples of milfoil plants were collected from the lake and tested for carbohydrate analysis and weevil density determination after the plants were dried. The results of the study showed that declines in milfoil populations caused increases in the biomass of other plant species. Increases or decreases in weevil populations  paralleled those in watermilfoil. The researchers conclude that larger-scale experiments involving the watermilfoil, weevil, and their environment will help support these findings.

Oxygen Needed

By: David Lung

Giant salvinia is an invasive aquatic fern that quickly covers the water surface, outcompetes native aquatic plants, and kills other aquatic organisms due to a decrease of dissolved oxygen. Flores and Carlson (2006) worked with the salvinia weevil to figure out if there was a correlation between the control effects of the weevils on giant salvinia and also dissolved oxygen levels after the release of the weevils. Flores and Carlson (2006) reared the salvinia weevil in tanks with giant salvinia and released the weevils at 6 sites in Texas waterways. Insect population densities were sampled monthly to make sure that the insects were dispersing. Dissolved oxygen levels were then measured after a little over a year. Flores and Carlson (2006) observed a significant increase in dissolved oxygen levels following successful control. More research is needed to understand the relationship between the weevil and the fern in order to understand the difference in the amount of time needed for complete control.

Journal of Aquatic Plants Management. 44: 115-121

Ben Berg

Chemoecology 8.10.2009

Since their introduction to mainland Australia in 1935, the cane toad (Bufo marinus) has significantly decreased biodiversity amongst reptiles in Australia.  Scientists are now growing desperate in their attempts to limit the growth of this species.  Researcher Mattias Hagman conducted experiments involving biological control mechanisms to fight the toad invasion.

Working in conjunction with Team Bufo (a team of cane toad researchers) and receiving funding from the Australian government, Hagman tested lab results that suggested that a pheromone produced by larval toads lowered survival rates.  Toad larvae were placed into two groups in outdoor ponds, and the treatment group was exposed to the pheromone.  The pheromone, which is emitted by injured larvae to alert other individuals, caused the tadpoles to undergo accelerated metamorphosis, causing a smaller body size and reducing survivorship by 50%.  While this significantly effected cane toad populations, it appeared to have no effect on native species.  The authors have called for follow-up research on the topic.

J. Aquat. Plant Manage 38: 78-81 (2000)

A study led by Robert P. Creed, Jr., of Appalachian State University investigates the use of biological control to restrain the spread of Myriophyllum spicatum, better known as the Eurasian watermilfoil. This aquatic plant has invaded lakes across North America, and scientists are examining the effects of the North American weevil (Euhrychiopsis lecontei) on watermilfoil on 4 environmental levels ranging from the individual plant to entire geographic regions. On the smallest scale, that of an individual plant, weevil larvae damage meristems, which hinders stem growth, and both larvae and pupae injure vascular tissue, preventing roots from getting nonstructural carbohydrates. Also, scientists found that weevils can make watermilfoil beds collapse in lakes, but the precise weevil density to cause this is uncertain. More research concerning aquatic predators, the nutrient content in sediment, and the regional climate is necessary. Creed concludes that further investigation is crucial at all four spatial levels to determine the efficacy of weevil biocontrol on watermilfoil.

Journal of Aquatic Plant Management 44: 115-121 (2006)

Giant salvinia (Salvinia molesta) is a noxious aquatic fern native to Southern Brazil that has threatened many freshwater ecosystems. Giant salvinia reproduces rapidly by fragmenting part of their stems to create a new plant. Its overgrowing has replaced native vegetation, altering the food web of the aquatic ecosystems, and also reduced dissolved oxygen levels, which eventually asphyxiates all aquatic life. It hinders irrigation, clogs waterways and promotes diseases in the stagnant waters the fern creates.

Daniel Flores and J.W. Carlson of the USDA introduced the salvinia weevil (Cyrtobagous salviniae) to control the fern. Herbicides usually exacerbate the situation or are not effective. The places where the researchers introduced the weevils have a significant decrease of the fern and an increase in dissolved oxygen levels. The giant salvinia population has remained constant and the weevils have shown to only consume the fern and nothing else. The authors say more research is needed, but biocontrol has shown to be an effective option.

Emilia Rybak

Ever since water hyacinth was first reported on Lake Victoria in 1989, this invasive species has wreaked havoc on the lake and the valuable biodiversity that depend on it for survival. Although there is no disagreement regarding how much destruction water hyacinth has caused, there is an ongoing debate concerning the factors that brought about its decline in 1998.

Some scientists agree with the argument presented by Wilson et al. (2007) that wet and cloudy weather patterns caused by El Nino played a vital role in the water hyacinth’s decline in the second half of 1997 and the first half of 1998. However, others believe that, as Williams et al. (2005) argue, that the introduction of Neochetina spp., or weevils, in Lake Victoria as a form of bio-control was responsible for this drop.

Specifically, Wilson et al. state that the four-year gap between when weevils were introduced in Lake Victoria in 1995 and when they started to produce results is consistent with results of other bio-control agents in other countries. Thus, they argue that weevils were primarily accountable for the water hyacinth decline since their effects occurred in accordance with those of other species. On the other hand, Williams et al. assert that prolonged sub-optimal light will reduce growth and reproduction rates of plants while enhancing the results of other debilitating forces, including weevil herbivory. Therefore, the stormy weather in 1998 provided ideal conditions for impeding the spread of water hyacinth, and thus aided the weevils in declining the water hyacinth population.

I think that the argument of Williams et al. is more convincing since it acknowledges that El Nino weather patterns were not solely responsible for causing the decline in water hyacinth, but rather that the combined effects of El Nino and the weevils enabled the decline. Even if the weevil population did only begin to produce results after four years, it is undeniable that the El Nino patterns contributed to their efficacy.

The MODIS satellite images taken in 2005 and 2006 that display the resurgence of water hyacinth on Lake Victoria demonstrate that bio-control is not a fully reliable method of managing invasive species. They show that bio-control may sometimes be an effective strategy, yet its efficacy often falls short. Thus, scientists should not completely depend on this method to eradicate an invasive species, and both invasive species and bio-control agents should be regularly monitored to avoid resurgences.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-Invades Lake Victoria.
http://earthobservatory.nasa.gov/IOTD/veiw.php?id=7426. Viewed 27 January 2010

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. Katagaria, 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

Of the many methods used to control invasive species, the use of biocontrol, which often results only in the addition of another harmful invasive species to an ecosystem, is certainly one of the most controversial. Occasionally, however, this method successfully reduces the effects of the original invader.  Some, including Wilson et al. (2007), speculate that one such success story may have taken place in Lake Victoria, where Neochetina was used as biocontrol on water hyacinth, an invasive plant in many parts of the world.  This plant, which lived on Lake Victoria’s surface and ultimately altered its entire ecosystem, began to decline shortly after the introduction of the weevils.  This decline also coincided with changing climatic conditions caused by El Nino, leaving many uncertain of where credit for the plant’s decline belongs.

Examining the effects of weevils and El Nino’s weather conditions on water hyacinth in other areas with similar climates ecosystems may result in the most accurate determinations of why the dwindling of water hyacinth occurred. Wilson et al. (2007) have observed that, in areas with a climate similar to that of Kenya, such as West Africa and Papua New Guinea, the introduction of Neochetina has successfully slowed the invasion of water hyacinth, while low sunlight levels present because of El Nino did not prevent growth, leading us to believe that Neochetina caused the decline.  However, Williams et al. (2007) assert that low sunlight levels, flooding, and waves caused by El Nino caused the demise of water hyacinth in Lake Victoria.

I agree that “changes in plant quality [including those caused by climatic changes] can affect the efficiency of weevils and a rapid deterioration of plants can lead to an early decline in weevil populations … such that plants can recover” (Williams et al., 2007).  However, like Wilson et al. (2007), who acknowledge the effects of these natural Neochetina population fluctuations, I do not believe that the weevil population was disturbed severely enough to negate its effect on the water hyacinth.  The water hyacinth was likely overcome by the combined destructive forces of  Neochetina and El Nino.  Perhaps the recent reinvasion of water hyacinth in Lake Victoria, as shown by images from the NASA Earth Observatory (2007), will allow us to determine whether Neochetina can successfully slow water hyacinth invasion in as a large body of water as Lake Victoria without the weakening of the plant by El Nino.

Sources:

NASA Earth Observatory.  2007.  Water Hyacinth Re-invades Lake Victoria.http://earthobservatory.nasa.gov/IOTD/view.php?id=7426.  Viewed 27 January 2010.

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. Katagaria, 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.