Weather or Weevils?
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.