Aquatic Invasive Species

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.

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.

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.