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

Laura Hubbard

The water hyacinth is an invasive plant that grows in mats on the edge of lakes, blocking light from reaching the bottom of the lake, which eventually drives out or kills native plant species. Water hyacinth was introduced to Lake Victoria in 1989 and within a few years the plant had taken over the lake, and thousands of hectares of the lake were covered in the alien species (Wilson, et al. 2007).  In the late 1990s, scientists introduced the plant’s native herbivores, weevils of the genus Neochetina, in an attempt to reduce the population through biocontrol. The hyacinth population did decline, on average four years after the introduction of the weevil species to an area. Debate raged over the ultimate reason for the plant’s retreat. While researchers agreed on the influence of the weevils, different studies disagreed on the extent of their influence. Williams et al. argue that the majority of the decline was due to reduced light levels because of El Nino weather patterns, while a study released the same year written by Wilson et al. posits that the decline was mainly due to the introduction of the weevils.

Wilson et al. and Williams et al. use much of the same data analyzed in different ways to come to similar but slightly conflicting results. Wilson et al. contend that any affect El Nino had on hyacinth was minimal compared to the effect of the weevils, and that El Nino did not change the cloud cover and light availability in the Lake Victoria region significantly enough to cause any change in water hyacinth growth. The plants, they argue, thrive in otherwise shady regions whose light availability is not much different from the cloud cover caused by El Nino. Most of the effect of El Nino came through the movement of water hyacinth, which both spread the weevils and caused the amount of hyacinth in different parts of the lake to differ and confusing the study of the plants’ reduction. Conversely, Williams et al. state that El Nino had more of an impact than the weevils: the low light prevented growth, while variable waves caused masses of hyacinth to sink to the bottom after they had been attacked by the weevils. While the study does not deny the importance of the weevils, they do argue that to state that a single variable is responsible for the change or to represent the system with a single draft is “an over simplification of the special complexity” (Williams, et al, 2007) of the lake and the problem. Instead, they posit that the decline of the water hyacinth is due to many interconnected factors, including the weevils.

Wilson et al. and Williams et al. also disagree on the probability of the water hyacinth returning and on the stability of the weevil population. Both sets of authors agreed that the weevils had some importance to controlling the population. Williams et al. argued that since the host population for the weevils was variable and declining, this could increase the variability of the weevil population, and hyacinth could die before the weevils had a chance to reproduce successfully and infect other hyacinth. Wilson et al, however, argues that there is no reason that the weevil population should be inherently unstable, and that there are no signs of this instability. Also, since the two studies argue on the reason for the decline, they also disagree on the probability of the hyacinth reestablishing itself: Wilson, et al argues that this won’t happen since biocontrol has taken effect and the weevils are controlling the hyacinth population, but Williams et al states that biocontrol cannot be completely effective since the hyacinth coming in from the river has not been infected with weevils, and with the weevil population in an unsteady place, there is a possibility that the hyacinth will reestablish itself.

While Wilson et al’s argument is compelling, the argument put up in the Williams et al article is more realistic. As Williams et al state, nature is complicated, and it is dangerous to oversimplify such a complex system. The return of the hyacinth to the area further support their claims: it is possible that without the el Nino system and with a possibly unstable weevil population, the hyacinth reestablished itself.

In 1989, water hyacinth first appeared on Lake Victoria.  Without natural enemies to control its spread, it quickly expanded and became a troublesome invader to the ecosystem.  By 2000, the population had been brought back to a reasonable level, and there are two competing explanations for this decline: weevils and the El Niño weather pattern in 1997-1998.

According to Wilson et al. (2007), the main cause of the hyacinth decline was the introduction of weevils into the area, an insect that feeds on the plant, causing it considerable damage.  The weevils were brought as a means of biocontrol, and Wilson et al. (2007) claim that they weakened the hyacinth, making it more susceptible to the inclement weather, and then finishing off the remaining hyacinth that surged in the aftermath of El Niño.  They specifically cite the fact that the four years it took for the hyacinth population to be fully controlled is consistent with the time frames that occurred in weevil biocontrol of hyacinth in other areas, as well as the fact that the hyacinth population started to flourish again after El Niño, suggesting that the stormy weather alone did not cause its eventual downfall.

Williams et al. (2007), on the other hand, argue that it was in fact the El Niño weather pattern that played the biggest role.  They point to Lake Victoria’s size and diverse range of habitats, claiming that Wilson et al. (2007) oversimplify the true nature of the hyacinth’s decline.  By looking at the data for each section of the lake, Williams et al. (2007) show that nearly all hyacinth not in a sheltered gulf area was eliminated simultaneously in late 1997 to early 1998, the same time as the El Niño event.  They acknowledge that weevils no doubt played a large role in weakening the plants, but assert that Wilson et al. (2007) give too much credit to the success of the biocontrol.

Ultimately, later photos revealed that water hyacinth has returned to Lake Victoria in extreme quantities in the sheltered Winam Gulf area.  I believe that this suggests that Williams et al. were more correct in their analysis of the situation, as it appears that, in the area of Lake Victoria that was protected from the worst of the El Niño weather, the weevils alone were not enough to eliminate the hyacinth in the long term.  Nonetheless, both sides emphasize the fact that the decline was due to a variety of factors, and neither can be considered completely correct, since water hyacinth never was truly eliminated.  On a wider scale, this leads one to wonder how successful any biocontrol effort can ever be, and if perhaps all one can do is forestall the inevitable.

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

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

Water hyacinth has long been an incredible pest in Lake Victoria since it was introduced. Many methods have been employed to control the spread of this nuisance, but few strategies have been successful. One technique has seemed to work better than the others though, and that is the use of bio-control. Specifically, Neochetina bruchi and Neochetina eichhornaie, also known as weevils, are used to control the plants.  These weevils are able to cause mayhem in the water hyacinth population through several methods. They feed on the leaves, causing the plant to lose buoyancy and sink lower into the lake. The weevil larvae burrow into the roots of the plant, weakening them and creating a path for bacteria to attack the plant. Since the weevil was introduced, the water hyacinth population of Lake Victoria has gone down tremendously. While there is no question that the weevils did damage the plants and lower the population, some argue that they did not play as big of a role as originally imagined. Wilson et al. (2007) argue that while the weather conditions did play a role in accelerating the reduction of water hyacinth biomass, they were only a crutch for the weevils to work their magic. They argue that the data gathered in weevil usage was the same as that of other countries and that seeing as how the weevils were the most prominent widespread control mechanism, they were the main contributor.  The strong waves and wind of El Nino did help reduce the hyacinth population, but only because the weevils weakened them first. The second article, by Williams et al. (2007), argues that the low light conditions caused by El Nino were the ultimate reason that the water hyacinth population was devastated. They say that El Nino interfered with shoreline hyacinth and caused the plants to dislodge, destroying the plants. They also debate that the low light levels created an environment where the plant was weakened and thus more susceptible to weevils and other enemies. I personally think that the weevils did play a major role in the destruction of the water hyacinth. While the conditions were optimal for hyacinth eradication, it was still the weevil that did all of the eradicating and thus should be given the most credit in the affair. Regardless of why the hyacinth population declined so rapidly, there is no argument as to whether or not bio-control works.

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

NASA Earth Observatory, 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426 Viewed 25 Jan 2010

Water Hyacinth is a thick invasive plant that grows in mat-like formation across the shallows of lakes. It invaded Lake Victoria to the extent that something had to be done, and bio-control was chosen as the method to re-open shorelines and fishing accessibility. Bio-control seeks to lessen or eliminate a problem species by introducing another species, such as a natural predator, into the environment. Weevils, Neochetina bruchi and Neochetina eichhorniae, were chosen to combat water hyacinth because they naturally feed on the plant, adults chewing holes in its leaves on the surface and larvae eating their way from underneath (Wilson et al. 2007).
Four years after their original introduction in 1995, water hyacinth was on a steady decline. Both Wilson et al. (2007) and Williams et al. (2007) acknowledge that weevils reduced the water hyacinth population, but they disagree on the magnitude of the weevil’s effect. According to Williams et al. 2007, the decline of water hyacinth was mainly attributable to the El Nino event. This extended heavy storm rained upon eastern Africa for the second half of 1997 and the first half of 1998, raising Lake Victoria’s water level by 1.7m and dislodging shoreline water hyacinth (Williams et al. 2007).
Wilson et al. (2007) also acknowledges the decline of water hyacinth in early 1998 due to the El Nino event, but cites the rapid growth during the latter half of 1998 as reason that the El Nino only temporarily reduced water hyacinth abundance. Then throughout 1999 the water hyacinth population steadily declined. Wilson et al. (2007) cites other countries that also experienced population declines four years after weevils had been introduced in his discussion, furthering his claim that the weevils were mainly responsible for the water hyacinth reduction.
Although both of these factors and others as well contributed to the reduction of water hyacinth, I believe Wilson makes the more convincing argument. According to the data, the El Nino event did not seem powerful enough to reduce water hyacinth on its own, yet I believe the weevils, unaided, could have lowered the water hyacinth populations, though the process may have been slightly longer. For this reason, bio-control remains to be an effective counter to invasive species, yet I agree with Williams et al. (2007) that bio-control must be monitored to prevent such return invasions.

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.

Since its introduction in the late twentieth century, water hyacinth has been a major nuisance in Lake Victoria.  Not only does it pollute the water by disrupting the lake’s flow, but because of the plant’s massive undergrowth, also reduces fishermen’s abilities to make a living and poses as a major threat to native species. To attempt to counteract this invasive species, in 1995, weevils, which are small herbivore beetles and natural predators of water hyacinth, were released into the water hyacinth clusters as biological control agents.

However, according to Williams et al. (2006), though the weevils aided in the reduction of water hyacinth in Lake Victoria from 1999 to 2000, they were not the main reason for the plant’s reduction. Another factor, the El Nino of 1997 – 1998, which had been the biggest ever recorded in the twentieth century, was, according to Williams et al. (2006), the most important contributing factor to the water hyacinth’s decline. Williams et al. (2006) argues that it was because of a combination of timing, reduced sunlight, and weevils, that the decline of water hyacinth in 1999/2000 was so dramatic. Therefore, Williams et al. (2006) warns that now the El Nino has calmed, the water hyacinth will resurge; the weevils will be unable to keep the plant population under control.

Wilson et al. (2006), on the other hand, argues that the weevils were crucial, if not the only factor in the water hyacinth’s decline, and that the El Nino had little effect on the plant’s growth. This is because, according to Wilson et al. (2006), the weevils generally takes three to five years to fully reach their potential and the decline in water hyacinth fits that timeline perfectly in accordance with the three releases of weevil nests into the water hyacinth clusters. The El Nino, on the other hand, though would have moved already weevil-weakened water hyacinth mats around, could not have had reduced water hyacinth growth by increased cloud cover; there is no substantive link between low light and water hyacinth concentration. (Wilson et al., 2006)

In light of all the evidence, I think that Williams et al. (2006) gives a more convincing argument than Wilson et al. (2006), because the latter neglected many sides of the former’s argument, and did not even consider the possibility that water hyacinth might return, even if it weren’t because of the El Nino’s dispersion.

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.

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.

Jeremy Joven

There has been recent debate on the forces behind the decline of water hyacinth in Lake Victoria starting in 1999.  At it’s peak infestation of Lake Victoria in the late 1990s, according to Albright et al. (2004), the water hyacinth has had several negative impacts on the native ecosystem in which it has invaded.  They have hindered transportation on the lake, have reduced fishing, and have threatened the biodiversity of the lake (Wilson et al. (2007)).  Researchers are debating whether the introduction of the Neochetina (a type of weevil that eats the water hyacinth) into Lake Victoria or disturbances from El Niño weather patterns was responsible for the diminishing water hyacinth population.

The article by Wilson et al. (2007) mainly supports the belief that the spread of the weevil population was the main reason for the reduction of water hyacinth while the article by Williams et al. (2007) focuses on the chaos caused by El Niño as a contributing factor to the declining hyacinth numbers.  There is a reference in Wilson et al. (2007) from Williams et al. (2005) that describes how the weevil population is unstable and therefore could lead to a resurgence in the water hyacinth population.  In defense to this statement, Williams et al. (2007) clarifies that the unstable ecosystem due to El Niño may lead to “unstable controlling herbivore populations.”  In their argument, Williams et al. (2007) makes it known that they don’t believe El Niño to be the single influence behind the water hyacinth decline, but rather a compliment to the introduction of the weevils.  The main point behind the argument in Wilson et al. (2007) is that the opinions generated in an earlier article by Williams had no direct experimentation or data that supported the claim that El Niño contributed to the diminishing water hyacinth population.

After reading both articles, I felt that Williams et al. (2007) had a more convincing argument.  The article didn’t just specify what they believed, but it also acknowledged the opposing side’s (Wilson et al. 2007) opinion.  It merely combines both factors as the possible driving forces behind the dying hyacinth.  However, the data from Wilson et al. gave it’s argument a more solid stance from a scientific view.  In light of the MODIS satellite images, bio-control in general can work at times but also fail as well.  Bio-control is not the only answer to growing invasive species, but may aid to decline it.

References

NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria  http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 27 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 invasive Water hyacinth plant first appeared on Lake Victoria in 1989. Over the following years, the plant population grew so rapidly that it covered as much as 77 square miles (NASA Earth Observatory, 2007) of the lake by 1998, endangering native species and damaging the local economy. Following the introduction of the Neochetina weevil species to the lake as a biocontrol agent, the water hyacinth population declined drastically. There is, however, debate over the cause of the decline of the water hyacinth because of an El Nino event in 1997/1998. Wilson et al (2007) and Williams et al (2007) take differing viewpoints on this issue.

Wilson et al (2007) believe the reductions in water hyacinth were “ultimately caused by the widespread and significant damage to plants by Neochetina spp.,” classifying the effect of El Nino as secondary to the effect of weevil control. They argue that surface current and wave action from El Nino broke up mats and submerged damaged plants because they were already damaged due to insect feeding and secondary damage by bacteria and fungi.

Williams et al (2007), however, argue that weevil species had less relative impact on the hyacinth population than Wilson et al (2007) claims. They cite detailed population data from different areas of Lake Victoria from 1994-2001, concluding that “the synchronous, lake wide reduction of water hyacinth in 1998 was unequivocally a result of the El Nino”. Contrary to the opinion of Wilson et al (2007), Williams et al. predict that the hyacinth population will make a resurgence.

I believe that Williams et al (2007) makes a more convincing argument. While the Wilson et al (2007) article uses sound ecological theory to back up their argument (that weevils reversed, and currently prevent, increase in hyacinth population), theory often does not fully explain actuality. Williams et al (2007) provide a very convincing analysis with specific data – pointing out that population decline occurred simultaneously throughout the lake, even though weevils were introduced to different lake areas at different times. Given the return of the water hyacinth in Lake Victoria during 2006,as shown by satellite pictures by NASA (NASA Earth Observatory, 2007), it appears that the weevil has not been completely effective at bio-control as claimed by Wilson et al (2007).

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.