Aquatic Invasive Species

Scott Rong

The Decline of Water Hyacinth on Lake Victoria

Scientific groups have recently debated the causes and factors behind water hyacinth decline on Lake Victoria. Evidence strongly supports the theory of biological control, South American weevils, as the primary reason for decline. However, scientists also argue that the El Nino weather patterns that occurred during the latter half of 1997 and the first half of 1998 have played a major role in destroying water hyacinth populations. Wilson et al. (2007) concludes that while the stormy weather and increased waves/currents on Lake Victoria associated with El Nino have contributed to vast damage of water hyacinth on Lake Victoria, foreign South American weevils were the ultimate factor in controlling this invasive species. However, on the other hand, Williams et al. (2005) asserts that although biological control played a large role in reducing water hyacinth populations, El Nino weather patterns deserve the most recognition for widespread population control. Thus, both articles and parties disagree on one main point: the degree of importance of each contributing factors.

Wilson et al. (2007) describes in depth the processes of using Neochetina weevils as biological control agents. Neochetina larvae tunnel into the water hyacinth through the root-stock and petioles. This allowed bacteria and fungi to infect the plant and cause massive damage. Flooding of larval tunnels and weakened tissue water-log, sink, and ultimately kill the water hyacinth mats. The increased wind and wave action due to El Nino further supported the destruction already attributed to the South American weevils. Therefore, the author stresses that without biological control agents, the huge problems associated with this invasive species would not have been resolved.

Williams et al. (2005) emphasizes that plant quality affects the efficiency of weevils. Plant deterioration and leaf damage kills eggs and young larvae, causing deterioration in weevil populations as well. Furthermore, the water hyacinth decline mimicked the widespread vegetation destruction caused by El Nino throughout eastern Africa. Thus, the power of nature trumped human intervention via biological control.

Perhaps the definitive answer to this debate will lie in the following solution: if authorities continue investing in the weevil populations and the water hyacinth population returns, Williams et al. (2005) will emerge as victor.

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.

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.

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.