Aquatic Invasive Species

During the 1990’s, the water hyacinth became a major problem in the world’s second largest lake located in Africa, Lake Victoria. Water hyacinth a fast spreading plant causes major problems in the places where it invades. It hinders efficient transportation of fishing boats, and its exponential growth can be detrimental to many local species. In order to combat this dangerous plant, weevils were introduced into Lake Victoria in 1995 as a means of biological control. Two years after the introduction of the weevils, a strong storm, El Nino, hit the Lake Victoria area, and the population of water hyacinth quickly dipped. However, after the storm passed the invader quickly grew and it wasn’t until 1999 that the water hyacinth population was greatly reduced and remained steady. Discussions on how to prevent another influx of water hyacinth has led to a disagreement between two groups of scientists.

The Wilson et al. (2007) group strongly believes that the biological control put in place with the introduction of the weevil was the dominant force in reducing the water hyacinth population in Lake Victoria. They claim that El Nino only had a temporary effect on the water hyacinth population, citing the resurgence of hyacinth after the storm as evidence that El Nino did not have a lasting impact on the invader. In the paper, the group also describes how weevils are able to eat away at the plant. The damaged plant gains water, and then sinks to the bottom of the water. Using this line of reasoning the group explains that when the hyacinth mats sunk during El Nino, weevils were brought down and killed with the hyacinth. Therefore, after El Nino passed, the water hyacinth flourished because of the low levels of weevils.

The Williams et al. (2005) group has a different take on the cause of the decline of the water hyacinth. In their paper, they state that the El Nino caused a lack of light, which is critical to the growth and reproduction of water hyacinth. They do not deny the fact that the biological control also had an impact on the reduction of hyacinth, but that cloud cover that El Nino brought weakened the plants and was the main factor in their control.

I lean more towards the side of the Williams et al. (2005) group because they have a more moderate explanation for the cause of the population decline. They do not rule out the weevils as a factor that helped to reduce the population. I think it is likely that the storm could have done great permanent damage to the hyacinth, but the weevil and storm synergistic effect was needed to control the population and immediately following the end of the storm, the weevil population was very low, which allowed the hyacinth to increase in number for a short period of time.

Sources:

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

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.

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.

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

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.

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)

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.

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.