Aquatic Invasive Species

The National Research Council (2002) conducted scientific investigations of genetically engineered organisms to assess their environmental risk. After evaluating harm, hazard, and risk, William Muir of Purdue University has singled out three forms of GE modification which include inducing male-based sex ratios, releasing inducible fatality genes, and introducing non-disposable genetic loads to decrease exotic species populations. Scientists are reluctant to engage in the first two methods because they demand continuous reintroductions for effectiveness since the genes won’t spread unassisted. One disadvantageous effect associated with GE organisms, specifically non-disposable genetic loads, is the Trojan gene effect whereby subsequent development in the target species makes it more susceptible to transformation by the transgene into an even greater threat. However, this side effect may prove beneficial for the control of invasive species who up to now have only been approached via chemical, mechanical, and biological methods. Currently, implementation of selfish genes appears to hold the most promise. This alternative technology creates the spread of a ‘super-Mendelian’ gene that destroys a crucial gene function. It has a specific host (in this case, the sought-after invasive species) and employs faster eradication methods. A possible concern with this technique is the elimination of non-target species that possess similar qualities and are thus correspondingly affected.

Aquatic Sciences. DOI 10.1007/s00027-004-0721-x

Recent studies have discovered a higher alien richness in the Curonian lagoon than in the benthic zones of the Baltic Sea.  Through their research, scientists have postulated what allows an ecosystem to be invaded, and for that matter, why is the invader invading?

There are two general hypotheses concerning species diversity and the “invisibility” of a habitat. Stachowicz et al.(1999) argues that the diversity of an ecosystem  increases is resistance, amd protects it from foreign invasion Stohlgren et al.(2003), on the other hand, believes that invasive species are “invisible” and undetectable under such a huge lens of a marine ecosystem.

Research concluded that systems have already been modified by man, through dams or embankments, or have already been invaded by a species, such as the zebra mussel, are high susceptible to further invasion. For instance, when a concrete construction is made in the Klaipeda strait, it provides the zebra mussel a hard substrate to bind to. The mussels then provide foreign invaders with nutrients and thus create a positive feedback system that may never be stopped.  The correlation between debth range and inasice species richness is explained by the decline in oxygen and thus yielding more vigorous conditions (Leppakoski and Olenin 2000). Species tend to approach less diverse ecosystems that have less competition and more of their particular nutrients of interest. It’s important for biological control to focus on these hot spots of lagoons and swamps, even if they aren’t directly related to the fishing trade; an abundance of invasive species can clog or poison entire water channels that deposit  into our water reserves.

http://www.glerl.noaa.gov/pubs/fulltext/2006/20060037.pdf

Biol Invasions- DOI 10.1007

Melaleuca Quinquenervia, an invasive tree, has overrun much of South Florida’s wetlands.  The plant has incited and withstood multiple attempts at biological control from both animals and fungi.  A team from the Fort Lauderdale Research Center, headed by Dr. Rayachhetry, tested the effect of the fungus B. Ribis, a Florida native, on the invader.  Rayachhetry injected six strains of the fungus into foliated trees, defoliated trees, and tree stumps.  The fungus eats the tree from the inside, but the plant often forms cankers around infected areas and drives it out.  The foliated Melaleuca showed no increase in mortality, and only the fifth strain of fungus significantly increased mortality in the defoliated trees.  The Ribis infected tree stumps regrew at a slower rate, but not as slowly as those treated with the common herbicide imazapyr.  Given these results, B. Ribis unfortunately seems unable to control the Melaleuca infestation.

Weed Technology Vol 13: 59-64, 1999

The Essence of the Debate

In the early 2000’s, scientists noticed a significant decrease in the water hyacinth population of Lake Victoria in Africa. Two groups of scientist, Wilson et al. and Williams et al., have attributed various factors to this result. In a 2007 issue of Aquatic Botany, Wilson et al., which included Dr.James Ogwang, suggested that biological control by Neochetina spp, the South American weevil, was the major reason for the decline in water hyacinths on Lake Victoria. Williams et al. published an article in Aquatic Botany that same year challenging Wilson’s et al. claim. Williams et al instead argues that during 1997-1998 the climatic effects of an El Nino, cloud cover and increased wave action was responsible for the decline in water hyacinths. Specifically, Williams et al believed cloud cover decreased the growth rate of the water hyacinth, a photosynthesizing plant, and then increased wave action broke up, and drowned a majority of water hyacinths and weevils in process.

Method of Data Collection and Presentation of Data

Wilson et al utilized satellite images of Lake Victoria at various times to estimate the amount of hyacinth coverage for the entire lake.  Next they created a graph showing the amount of hectares covered by water hyacinths from 1996 to 2002. The graph also indicated the period when the weevils were introduced to three different areas by Dr. Ogwang, and the occurrence of the El Nino weather pattern. Williams et al. also used satellite images from various time periods to measure water hyacinth coverage on Lake Victoria. However unlike the other group, Williams et al estimated water hyacinth coverage separately for the Tanzanian, Ugandan and Kenyan (Winam Gulf) sides of the lake, the same three areas where biocontrol was implemented by Dr. Ogwang. They then produced three graphs showing the hyacinth coverage (hectares) between March 1994 and September 2001, and the time of biocontrol implementation for each region.

NASA Reveals a Startling Result 

 However while both group of scientists attributed different reasons for the decline in the hyacinth’s population, satellite imagery provided by NASA article in 2006 shows resurgence by the invaders. From the date of the image it appears odd and irrelevant for both groups to argue about the decline in the population when there’s clearly a problem. One must note however that these articles were initially submitted in 2005 and edited during early 2006. These articles were already in the publishing phase when the satellite image was presented by NASA.

Both groups attribute each other’s respective factor for playing a minor role in the decline of water hyacinths on Lake Victoria. However each group believes their respective factor was the major cause for the decline, and had strong arguments to support their claim. I agree with Williams’ et al. that “reduced incident light [caused by cloud cover] reduced the growth rate of a photosynthesizing plant [water hyacinth]”. However I do not believe that reduced incident light could have had that significant an impact. Fig. 1 in Williams’ et al.  article indicates that the El Nino was present over a course of approximately six months, but cloud cover couldn’t  have been absolutely continuous over the course of those six months. I also think that wave action might have contributed to the resurgence of water hyacinths for as mats are broken up water hyacinths along with their seeds spread to new areas of the lake. To me biocontrol seems the more plausible and relevant factor in the decline of water hyacinths in Lake Victoria.

References: 

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

In recent years, Lake Victoria has been struggling to cope with the introduction of the water hyacinth plant, which has been clogging the lake and destroying aquatic life there. To combat the invasive and aggressive hyacinth, ecologists introduced Neochetina, or weevils, in 1995,  to eat away at the hyacinth and thwart its expansion.  After years of studying the plant’s disappearance and reappearance, scientists debate over what caused the disappearance of the hyacinth; Was it the biological control through weevils? or, was it the rainy El NiNo weather  pattern that flushed the hyacinth from Lake Victoria?

Wilson’s article seems to conclude that although El Nino played a part in the disappearance of the hyacinth, the main driving force was the introduction of the foreign weevil species. Wilson’s data shows that there was a decrease in hyacinth due to El Nino, however, the hyacinth would come back and regroup. only in 1999, about 5 years after the weevils were  introduced, did the hyacinth population really begin to decline.  These 5 years were necessary for the weevil population to grow and damage the hyacinth.

Neochetica larvae do a great job of eating through the roots and petioles of the hyacinth, which in turn allows bacteria and fungi to invade the plant. Also, with increased holes in the roots and stems, the hyacinth mats becomes very water clogged and tend to sink to the bottom of the water, which allows it to be flushed out of the system easily.

Wilson’s main argument resides on satellite images taken by Albright et al, which show an increase in the hyacinth population through 1998, and then declined slightly in 1998 , only to increase more through 1999. Scientists thought the weevils weren’t doing their job, but starting in 2000, the hyacinth populations began to decrease greatly. Wilson argues had the weevils not weaken the plants, El Nino would have never been able to pull out the hyacinth.

Williams presents Albright’s data in a different way, on that seems to make more sense. Williams critiques Wilson by questioning his ability to measure the hyacinth population for the entire Lake Victoria, its simply too big. Instead, Williams charts show hyacinth growth in the Tanzanian,Kenyan, and Ugandan portions of Lake Victoria.

In the graphs, Williams’ takes note that regardless of when the weevils were introduced, the biggest drop in hyacinth was during the 97/98 El Nino event. He also makes mention of another factor that may have reduced hyacinth growth; light. El Nino is a very wet and cloudy weather pattern, thus denying the hyacinth sufficient light to perform photosynthesis and multiply. That, coupled with high tides and flooding, were the main factors flushing hyacinth out of Lake Victoria, according to Williams.

Given the evidence presented and the distinct introduction of weevils into Lake Victoria, I think it is impossible to claim that weevils weren’t the driving force behind the hyacinth disappearance. For one, El Nino had already stormed through the lake before weevil introduction, and hyacinth populations didn’t diminish, because the plant was strong and rooted. Weevils did their job, trying to destroy as many plants as possible, but failed to flush away the plants without El Nino patterns.

In reality, the biological control method only solved short term problems in reducing hyacinth numbers. Weevils can only try kill the plant, thus leaving mats of hyacinth plants on the ocean floor.  Hyacinth is too versatile of a plant; it can grow on any surface that the mats land on. El Nino was the best support in flushing away the mats out of the lake and shrinking hyacinth numbers, but that just raises questions about manual removal of hyacinth. Is it not possible to better regulate the removal of plant matter after weevils dissemble hyacinth from its roots?  Because  given the current trend, dead plant matter will continue to accumulate on the lake floor and continue to clog its waterways.

The water hyacinth was brought to Lake Victoria through a love story, or so it is thought. In 1989, a foreign engineer from South America fell in love and as an engagement present, gave his fiancé a water hyacinth. As their love grew, so did the plant. As it grew, pieces of the plant somehow escaped and found their way to Lake Victoria. The sad side of this love story, however, is that over the years, the water hyacinth transformed from a symbol of love to an invasive species in Lake Victoria.

A water hyacinth is a weed. By 1995, it covered 80% of Lake Victoria, which is the world’s second largest lake located in central Africa (Wilson et al 2007). The plants clog shorelines making it disastrous to people. Fisherman struggle with reduced numbers of fish, which in return threatens famine for the region. Civilians struggle with dirty drinking water caused by the water hyacinth’s rotting vegetation. Also, the weed creates breeding areas for mosquitoes and for parasites to lay eggs, thus further causing a bad situation for the lake’s human and animal inhabitants.

To deal with this increasing problem in Lake Victoria, Neochetina weevils, native to Brazil, were introduced through bio-control in the mid-1990′s. This process involved introducing a foreign organism, as an enemy of an existing organism, in hopes of getting rid of that existing organism. Weevils are natural enemies of the water hyacinth. These small insects chew holes in the leaves of the water hyacinth allowing bacteria to enter and cause severe damage to the plant.

Further, in 1997/1998 El Nino, a severe weather pattern, accompanied the water hyacinth reduction. This weather pattern brought storms with heavy rains, wind, and cloudy skies in the region. The severe weather change occurred around the same time that the weevil population was growing and the water hyacinth population was obviously decreasing.

In 1996, as the numbers of water hyacinth plants in Lake Victoria continued to decrease, scientists began to question what was actually responsible for the water hyacinth reduction.  Researchers debated whether it was the bio-control through the weevils or weather conditions through El Nino.

According to Wilson et al. (2007), who published his article in “Aquatic Botany”, a scientific journal, the weevils were the primary cause of reduction, however, El Nino hastened the process. After taking satellite images of Lake Victoria, Wilson concluded that bio-control was the only method that was utilized throughout the whole of Lake Victoria. Also, Wilson believed that the high winds and large waves caused by El Nino added to the stress of the already weakened water hyacinth causing the plants to separate from their pods and sink more quickly. Moreover, Wilson believed that the resurgence of water hyacinth would not happen.

On the opposite end of the spectrum, Williams et al. (2007), believed that although there was reduction in water hyacinth when the weevils were first introduced, the main reduction happened during El Nino, as stated in his article published in the “Aquatic Botany”. The greatest reduction happened during El Nino because the cloud coverage limited the amount of light the water hyacinth received reducing its levels of photosynthesis. Consequently, the plant suffered reduction in growth and reproduction rates. Thus, El Nino was the main cause of the water hyacinth destruction.

In conclusion, I believe that each scientist is correct in giving both the weevils and El Nino credit for the water hyacinth reduction. The weevils weakened the water hyacinth and El Nino provided the final “push” to destroy the water hyacinth. Without, El Nino I do not think that the weevils could have solely gotten rid of the water hyacinth and vice versa. Thus, I believe William’s argument over Wilson’s. I believe that the weevils did the hard part by weakening the water hyacinth and El Nino finalized the reduction. In regard to the resurgence, I believe that El Nino is a very rare occurrence and therefore, it makes sense to me that the water hyacinth could resurge. The weevils need the help of Mother Nature, as Mother Nature needed the help of the weevils.

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

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

The cane toad is an invasive aquatic species that was brought to Australia from Central and South America in an attempt to reduce the number of insect pests harming the sugar cane crops. These toads reproduced rapidly and overpowered native environments. Even local predators were dying after consuming the poison excreted by the toads. When I took AP environmental science in high school, I remember watching a video detailing the arrival and eventual harm caused by these cane toads. It is amazing that they have been negatively impacting our native environments since the 1930s!

Malakoff notifies us that recent studies by Sydney researchers have suggested the use of Australian frog tadpoles to diminish the intrusion by cane toads. A number of experiments were done in which cane toad tadpoles were placed in bins with a varied number of Australian frog tadpoles to see how this competition would impact the cane toads. Apparently, the presence of these native tadpoles prompted the cane toad tadpoles to become less reproductively fit, even causing a higher death rate. This biocontrol effort will hopefully prove to be as effective in the wild as it was in the ‘competition bins.’ I am both intrigued and impressed by this use of native species to combat the impact of the invasive cane toads.

Cane toads have been plaguing Australian natives for far too long. It is distressing that efforts to cease their negative impacts have not been successful. In fact, Professor Seebacher from the University of Sydney explained that cane toads may actually flourish under the conditions generated by global warming. This means that while other species (especially natives) suffer from the higher temperatures cane toads may actually become even more of a threat.

All in all, it seems as though scientists should have put a little more time and effort into reviewing the possible consequences of introducing this foreign species to Australia to solve their cane beetle woes.