Water Hyacinth

January 27, 2010

Water hyacinth is a marine plant native to South America. Over the last few decades, however, it has infested Lake Victoria and made it hard for local fishermen to maintain their lifestyle. Large mats of water hyacinth have clogged waterways, reduced small local marine life, and blocked sunlight from reaching the lake bottom. Recently, there have been efforts to reduce the water hyacinth population, but whether they have been effective or not is another matter.

Williams et al. (2007) states that the waves caused by El Nino ungrounded water hyacinth plants and allowed for their destruction. Another proposed reason for the hyacinth’s disappearance is the fact that thick cloud cover due to the weather conditions blocked out sunlight, and led to decreased photosynthesis and fertility in established water hyacinth plants (Williams et al. 2007). In early 1998, the latter part of a particularly violent El Nino pattern, water hyacinth populations declined sharply.

Wilson et al. (2007), on the other hand, believes that the ultimate reason for the plant’s disappearance was the introduction of weevils as biocontrol – introducing one species to control another. Although the water hyacinth population did decline sharply in early 1998, plant populations steadied and rose again later that year. They didn’t begin to steadily decline until 1999, 4 years after the introduction of the weevils (Wilson et al. 2007), a typical time period as gathered from other weevil biocontrol situations. Wilson et. al (2007) suggests that El Nino harmed the weevil populations, and that the “new growth was able to proliferate in the absence of weevils”. Weevils were also introduced to other areas – West Africa and Papua New Guinea – and successfully reduced hyacinth populations where they were the only method of control being implemented (Wilson et al. 2007).

To me, it is clear that Wilson et al. (2007) presents a stronger argument. Although Williams et al. (2007) introduces many valid points, Wilson et al. (2007) presents its own evidence and nullifies several parts of Williams’ argument. Despite the fact that I believe Wilson’s argument holds more validity, both documents agree that both El Nino patterns and weevil influence helped decrease water hyacinth populations. As data shows, water hyacinth populations are rising again (NASA 2007). Instead of arguing over who’s more correct, I think the real issue here is to focus on the factors we can control (the weevils) while continuing to search for a more secure solution.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 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. 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.


SW2: Weather v. Weevils

January 27, 2010

There is much debate as to the efficiency of bio-control on Lake Victoria. Weevils were first introduced as a method to control water hyacinth in 1995. During the first half of 1998 there was a sharp decline in the population, coinciding with El Nino; however, shortly after there was in increase. In early 1999 a steady decline began and continued. There is argument as to whether biological control was the major factor or if El Nino associated weather patterns were the primary cause behind the decline.

Wilson et al. (2007) take the stance that bio-control was the main factor behind the water hyacinth’s decline, although the stormy weather associated with El Nino also played a part. They argue that bio-control typically takes four years to take full affect and this situation stands true in Lake Victoria. They state that biological control agents were the only control measures in place and they cite similar cases where bio-control was effective. They also argue the effects of El Nino explain the seeming resurgence of water hyacinth in certain areas. They conclude by saying that although another resurgence of the plant is unlikely there must be continued monitoring of both water hyacinth and weevil populations.

Williams et al. (2007) state that, while weevils did have a role, the reduction of water hyacinth in the second quarter of 1998 was the result of El Nino. They claim that it was not only due to a reduced light climate, but also water level, wave action, and other weather-related factors. They conclude by saying that they agree that bio-control should continue to be a factor on Lake Victoria, but its limits need to be considered and monitored.

I do believe that bio-control of the water hyacinth has been effective and played a major role in the reduction of water hyacinth on Lake Victoria. However, I agree with Williams et al. (2007) that the decline in 1998 was due to El Nino, but after that time I think that it was the weevils taking effect. However, I think that due to the resurgence proved by the images from NASA Earth Observatory 2007) that the weevils have not been as effective as Wilson et al. seems to think. I think that bio-control is a good option and will continue to be effective, but it definitely has flaws and the situation in Lake Victoria must continue to be monitored.

Tara Porter

NASA Earth Observatory.  2007  Water Hyacinth Re-Invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426.  Viewed 20 Jan 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. Aujuonu, 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 Biological Control in Lake Victoria: Was it Really that Effective?

January 27, 2010

Kevin Shia

SW 2: The Biological Control in Lake Victoria: Was it Really that Effective?

With the proliferation of the water hyacinth in Lake Victoria, it is necessary to discuss aspects of biological control. The debate is to determine if biological controls help reduce the population of the Water Hyacinth in Lake Victoria. The biological factor consists of importing Neochetina weevils into the environment (Williams et al. 2007). The opposing argument is that the effects of El Niño are more significant in reducing the population of the plant. Wilson et al. (2007) believe that the weevils were the main cause for the decrease in the water hyacinth, while Williams et al. (2007) believes that El Niño was the main cause of the decrease in the invasive plant.

Wilson et al. (2007) state that the reason why the Neochetina larvae are so effective in damaging the water hyacinth mats is that they tunnel into the roots and cause massive damage to the whole plant. Wilson et al. (2007) also discredit Williams et al. (2007) by explaining how unlikely it is that the cloudy weather that is associated with the El Niño that occurred during the years of 1997 and 1998 can still affect the decrease of the water hyacinth between 1999 to 2000. They also state the claim that the data of Williams et al. (2007) cannot truly prove the relationship between the low light levels due to El Niño and the mortality of the plants because plants can still grow in low light. Wilson et al. (2007) claim that because the Neochetina population is an unstable host population, it can potentially lead to uncontrollable plant populations. They also claim that the weevils can also cause potential harm to themselves. For example, if the weevils cause enough damage to the water hyacinth, they can make the mats sink lower into the water, which can drown the eggs and larvae and the death of the weevils can proliferate the growth of the invasive plants again (Williams et al. 2007).

Based on the given information from Williams et al. (2007) and Wilson et al. (2007), I believe that both biological controls and the effects of El Niño helped change the total amount of water hyacinths in Lake Victoria. Even though the environmental effects of El Niño assisted with the reduction of the water hyacinth, I believe that the biological controls of the Neochetina weevils were more influential in reducing the growth of the water hyacinth.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria, http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 24 Jan 2010.

Williams, A. E., R. E. Hecky, and H. C. Duthie. 2007. Wather 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.


Hyacinth Debate Water-logged in Technicality

January 27, 2010

Water hyacinth is an invasive species that harms the ecosystem of Lake Victoria in Africa by creating mats that block sunlight from reaching native species in the water (NASA Earth Observatory 2007). Insects called weevils were introduced as predators of the invasive weed in 1995 in an attempt to solve the problem through biological control (Wilson et al. 2007). A sharp decrease in water hyacinth population was seen by 2000, but the scientific community was not convinced that the weevils caused this change. Williams et al. (2007) released a subsequent paper suggesting that while weevil presence did help reduce hyacinth in Lake Victoria, el niño weather conditions at the time, were more significant factors. They alleged that wave conditions and limited sunlight had more significant effects, also stating that they believed the weevil population was unstable, and monitoring the population would be necessary in preventing a resurgence.
The arguments presented by Williams et al. (2007) seem to be more convincing. A major point made in the article is the synchronous nature of the disappearance of hyacinth. The population decreased uniformly across the lake, suggesting that a larger factor such as weather was responsible for the changes. While Wilson et al. (2007) states that reduced sunlight due to el niño would not kill the hyacinth, Williams et al. (2007) argues that while not fatal, such conditions would seriously impede growth. The Williams et al. article also appears to be more convincing in its logical claim that an unstable host population creates instability in the weevil population, and thus the weevils must be monitored to prevent hyacinth resurgence.
To me, the argument contained within the papers seems to be a moot point. Both sides agree that both factors have some level of influence, and the conclusion of Williams et al. (2007) supports continues use of weevils, fearing their disappearance at the hands of unstable conditions. Since we as humans are not presented with the ability to choose el niño conditions over the use of weevils, we should do what we can and use weevils to try and keep hyacinth populations low. Unfortunately, this did not happen. According to the NASA Earth Observatory (2007), hyacinth populations have returned in full force, once again covering Lake Victoria. Perhaps instead of arguing, both groups should have focused on maintaining the somewhat effective use of weevils and finding a more effective and proven solution.

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


SW2. Weevil vs. Weather: The Taming of the Water Hyacinth

January 26, 2010

SW2. Weevil vs. Weather: The Taming of the Water Hyacinth

By Emily Chang

When discussing the water hyacinth explosion in Lake Victoria, it is difficult to avoid mentioning the role of biocontrol. The water hyacinth originated in South America and arrived in Africa in 1879, engulfing about 77 square miles of Lake Victoria over a century later (NASA Earth Observatory 2007). Its extensive growth resulted in various detrimental consequences, such as “hampered transport links” (Wilson et al., 2007). Scientists have debated about the effects of the Neochetina spp. weevils on water hyacinths and whether El Niño could have influenced the plants. While both Wilson et al. (2007) and Williams et al. (2007) agree that the weevils have contributed to the decrease in water hyacinth in the lake, they disagree about the extent to which El Niño reduced water hyacinth growth.

Wilson et al. (2007) asserts that weevil introduction is the primary factor in the decrease in water hyacinths in Lake Victoria. According to Wilson et al. (2007), although both El Niño and a short-lived reduction in water hyacinth populations occurred there in 1998, this plant population increased in the latter half of that year and did not embark on a long-lasting decline until early 1999. It concludes that biocontrol via weevils released from 1995 through 1997 is the main cause in water hyacinth diminution and that abnormal weather only partially accounts for this reduction (Wilson et al., 2007). However, Williams et al. (2007) argues that only an “overriding metascale process” can alter the ecosystem of a lake as large as Lake Victoria and that the intensity of the 1998 El Niño makes it significant in the decrease in water hyacinths. They note that low sunlight levels can compound weather effects and that weevil populations could become unstable from damage to host water hyacinths (Williams et al., 2007). They conclude that the efficacy of biocontrol was aided by El Niño.

Because the factors mentioned in the two articles, such as light levels and abnormal weather, occurred around the same time, it is difficult to separate the effects of each. With such compounding processes happening, I believe that Williams et al. (2007) cannot draw sound conclusions from individual factors without further analysis. Wilson et al. (2007) deftly emphasizes the role of weevils and refutes Williams et al. (2007) with facts and statistics. The MODIS satellite images indicate that further study of the effects of these factors is necessary to improve water hyacinth control.

References:

NASA Earth Observatory. 2007 Water Hyacinth Re-Invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 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. Aujuonu, 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.


SW2- Weevil War III

January 26, 2010

By: David Lung

Bio-control has been an inconsistent method of controlling invasive species. Often it results in failure and may even exacerbate the situation. For controlling water hyacinth, the water hyacinth weevils were introduced to stop water hyacinth from disrupting Lake Victoria’s ecosystem any further.

Wilson et al. believed that the weevils were the greatest factor in reducing the water hyacinth population and the El Niño event being a contributing factor. They have stated that water hyacinth populations have not increased (after sufficient time allowed for weevil growth) since the introduction of the weevils in 1995 and also that the weevils’ actions were the main cause of the reduction and eventual control of the weed’s population and the El Niño’s weather patterns made the finishing blow. The weevils’ actions opportunistically allowed other factors to cause further damage to it. They also stated that the weevil “represents the only control method that was implemented across the whole of the lake…” The El Niño event, herbicides and other control measures were temporary or concentrated means of control on the lake.

Willams et al. focused on the El Niño event being the main cause of water hyacinth decline and the weevils being a later factor. They asserted El Niño event was a widespread factor whereas other factors were not. They stated that the El Niño event’s reduction of light slowed down plant growth, allowing other factors such as weather patterns and weevil activity to control the weeds’ population. After the El Niño event in 1998, there was a drastic population decrease in the weed’s population. Also, they noted that in 2000-2001, the water hyacinth population increased despite the weevils’ actions. Willams et al. also stated that the population density of the weevils was still low up to 2002, asserting they were merely a contributing factor to the El Niño event.

Both groups make valid statements on the major cause of the control of water hyacinth, but I agree with Wilson et al. They don’t counter Wilson et al.’s statement that significant plant growth can occur in low light and also neglects that there is a predator-prey sort of relationship between the weevil and weed and also the weevil’s long growth period in explaining the weevil’s fluctuating population. The satellite images show bio control works to an extent, but controlling the nutrient level and researching more into the issue is needed, which is supported by both groups.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/vi…. Viewed 20 Jan 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. 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.

(400 words without references)


Weather or Weevils?

January 26, 2010

What the invasive water hyacinth has done in colonizing Lake Victoria cannot be justified by the term invasion; rather, this nonnative species has implemented an all-out take over of the African lake.  Just as Alexander the Great expanded Hellenism and shaped the cultures of the areas he took over, the water hyacinth is dramatically affecting many aspects of life in Uganda, Kenya, and Tanzania including the economy and scientific research.  The water hyacinth grew at astonishing rates, doubling its area ever six to eighteen days (NASA Earth Observatory  2007).  This trend progressed until 1999 when Wilson et al. (2007) notes that the turning point where plant coverage began to decline is reached.

There is an extremely important debate among environmentalists and biologists as to exactly what biological factor caused sharp decline in coverage of the water hyacinth.  The two primary contributing factors that are being considered are the introduction of Neochetina weevils as biological control agents and environmental factors like what Williams et al. (2007) calls “the largest El Niño event recorded this century.”  The water hyacinth began its decline well after the weevils were introduced and very closely following the El Niño.  In order to understand how to combat invasive species in the future, it is crucial to discern whether it was the weather or the weevils that eliminated the invasive species.  According to Wilson et al. (2007), the weevils were the primary cause of the decline of water hyacinth.  It is argued that the weevil’s feeding weakens the plant and prevents root mats from developing, which damages plant survival; Wilson also notes  “the new growth [of the water hyacinth] was able to proliferate in the absence of weevils.  On the other hand, Williams et al. (2007) believes flooding and cloudy weather caused by El Niño played a major part in the decline of the water hyacinth.  Williams does not claim that weather was the only contributing factor, but that Wilson is wrong in claiming weevils were the only factor.  He pushes for a mutual effect by both factors; he states that cloudiness hinders growth and reproduction rates of plants and that deteriorating plant quality aids weevils’ eradication attempts.

I feel the disappearance of the water hyacinth can be contributed to both the effects of biological controls (weevils) and environmental changes (El Niño).  More research is clearly necessary, but the trends suggested by Wilson indicate no clear, sole cause.

References:

NASA Earth Observatory.  2007.  Water Hyacinth Reinvades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426.  Viewed 25 Jan 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. 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 Bio-Control: The Only Sustainable Solution

January 26, 2010

The recent resurgence on water hyacinth in Lake Victoria has brought into the light the debate regarding the role of bio-control in the elimination of invasive species. Water hyacinth, a weed native to South America, caused significant damage to Lake Victoria through the late 1990’s followed by a decline in 1997. Several factors have been considered in determining the cause of this decline. In December of 1995, weevils were steadily released into Lake Victoria to reduce the massive mats of the weed that covered Lake Victoria (Williams et. al). During the 1997 decline in water hyacinth, an El Nino weather pattern occurred (Williams et. al.). Until the water hyacinth population began to regain lost ground in 2007 (NASA Earth Observatory 2007) bio-control methods, namely the weevils, were considered the explanation for water hyacinth population decline. The El Nino weather pattern was recently suggested as a significant contributing factor to previous water hyacinth decline (Williams et. al.). Arguments made in favor of each explanation must be critically evaluated as both presentations were published before the resurgence of water hyacinth was recorded.

Williams et. al. proposes that the El Nino weather pattern, not the weevil introduction, is responsible for the reduction in water hyacinth during 1997/1998. Wave action and reduced light levels, a result of the wet and cloudy weather, are suggested as facilitating weed decline (Williams et. al.) Wilson et. al. counters this argument suggesting that bio-control remains the primary explanation for weed decline. Instead of directly causing plant decline, the effects of El Nino only exacerbated the damage caused to the weevil-weakened water hyacinth population. Citing the basic tenant of bio-control, “it is sustainable through population regulation,” (Wilson et. al.) a convincing argument is made in favor of bio-control’s successful implementation in Lake Victoria.

Clearly, neither presentation can be considered conclusive as both were written prior to the 2007 water hyacinth resurgence. However, the evidence reviewed in both articles clearly indicates that bio-control is the main action responsible for water hyacinth decline. The El Nino weather pattern is demonstrated as exacerbating weed death; however, Williams et. al. fails to demonstrate El Nino as the sole cause of weed decline. Thus, I believe bio-control is the most suitable and successful method of water hyacinth control. A new strategy for eliminating new water hyacinth growth is imperative, but bio-control must be a component of this plan.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 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. 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.


SW2: The Wilson-Williams Weevil War

January 26, 2010

The use of bio-control has always been a controversial topic. While it is more natural and eco-friendly compared to other methods of invasive species control, it has sometimes created unforeseen negative impacts, and its efficacy is disputed. For Africa’s water hyacinth-infested Lake Victoria, the weevil was introduced as a biological solution, but its long term impact is uncertain. A third party, the weather, has further complicated matters.

Both Wilson et al. (2007) and Williams et al. (2007) acknowledge that a combination of weevils and weather affected the water hyacinth population in the late nineties, but they offer their own theories as to how much each factor actually contributed.

Wilson et al. (2007) emphasize the weevils’ role. The beetles punch holes in the plants as they feed, allowing the plants to become waterlogged and slowly start sinking. Wilson et al. (2007) claim that the wave action associated with El Nino simply expedited this sinking process. The water hyacinth population declined drastically in 1999, four years after the weevils’ introduction. Other documented cases of bio-control also saw a 3-5 year latent period before any results were notable.

In contrast, Williams et al. (2007) claim that the weather served a more prominent role in the decline of the water hyacinth. According to their paper, constant low lighting in 1997/1998 weakened the plants, and the weevils’ effect on the water hyacinth might be overstated. Williams et al. (2007) also raise the issue that “unstable host populations may well lead to unstable controlling herbivore populations.” Because water hyacinths serve as food and shelter for weevils, reducing the water hyacinth population also reduces the lake’s carrying capacity for weevils.

Even though Wilson et al. (2007) might take a more general approach, I feel that they offer better evidence and stronger theories. They analyzed the opposing side’s data on light vs. plant mortality, deeming it inconclusive, and they explained unstable populations through “population regulation.” Wilson et al. (2007) suggest that a spike in hyacinth numbers is normal, that it does not necessarily mean weevil failure. This makes sense to me because populations are not static; they are rather dynamic, as satellite photos suggest (NASA Earth Observatory 2007).

Frank Chang

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 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. 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.


Where There’s A Weevil, There’s A Way

January 26, 2010

SW2: Where There’s A Weevil, There’s A Way

By: Evan Schwartz

By the late 1990’s, Water hyacinth became the most problematic invasive aquatic species in Africa’s Lake Victoria. As it spread, the plant began to clog waterways, impede fishers, destroy native flora and fauna, and turn the lake into a cesspool of stagnant water. However, after reaching unprecedented levels in 1998-2000 (Wilson 2007), the hyacinth population began to decrease rapidly. Two major explanations appear for this decrease.

Wilson et al. (2007) present the argument centering on the importance of Neochetina spp, or weevils. The weevil is a natural predator of the water hyacinth, and feeds primarily on the large, green leaves. The weevil then deposits its larvae inside the stalk of the plant, which they hollow out during their development. By attacking the plant from all angles, the weevil is clearly able to reduce the population of the hyacinth while increasing its own numbers for further control. Wilson et al. acknowledge that the El Nino conditions also play a role in the reduction of the hyacinth; however, this only served to weaken and redistribute the plants across the lake (Wilson et al. 2007).

The Williams et al. (2007) reply to the Wilson article centers on the importance of the El Nino weather system. They assume the position that while the weevil population clearly reduced the amount of water hyacinth in the lake, the weather system reduced the amount of available light to the area, significantly weakening the invasive plants (Williams et al. 2007). They also assume that a severe reduction in hyacinth severely weakens the stability of the weevil population, causing an unnatural flux in weevils, as well as an opportunity for the plants to recover (Williams et al. 2007).

After considering both arguments, I am under the impression that weevils are the major contributor to the reduction of the water hyacinth population. The stormy conditions caused by El Nino would attack anything within the region, and weaken both populations. Weather conditions can decimate populations for a period of time, but eventually the pre-disaster species will assume original levels. It is clear that the weevils are the major factor in hyacinth control, as they decimate the plant from all points of attack: the roots, leaves, stems, etc. After considering satellite imagery (NASA 2007), the weevil populations need to be regulated in high levels in order for the hyacinth to be kept in check.

References:

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 20 Jan 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. 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.


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