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.
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.