Michael Di Nunzio
Water hyacinth has posed a problem for Lake Victoria since first being reported there in 1989. The plant forms dense mats of vegetation that inhibit the movements of fishermen, block sunlight to native plants, and obstruct irrigation systems. The invasive weeds can also deplete the water’s oxygen levels, suffocating the indigenous flora and fauna of the lake and in turn disrupting the local ecosystem. To control the hyacinth populations, invasive weevils (Neochetina spp.) were introduced with the intention of suppressing the noxious weed (Williams et al. 2007).
Using satellite image samples, Wilson et al. (2007) estimated the proliferation of water hyacinth over Lake Victoria and fluctuations in the plant’s presence over time, finally presenting their data in Aquatic Botany. However, Williams et al. (2007) warned with a rebutting article that this method of gathering data is oversimplified for such a complex environment. In spite of the dispute, both parties agreed that hyacinth levels dropped after the 1998 El Niño disturbed the lake. Following the initial decline came a steady rise until 1999 when hyacinth levels again began to decrease dramatically. From 2000 until 2002 hyacinth levels remained suppressed to under 5000 hectares of biomass over the lake’s entirety (Wilson et al. 2007).
Wilson et al. (2007) reasoned that the drop in 1999 was a result of the control weevils introduced in 1995 becoming effective after four years of relative dormancy. They also noted that the weevils control the weed by lowering its buoyancy and sinking it, and El Niño could have facilitated this process with wind and wave action. Because El Niño would inevitably blow some hyacinth into new areas, Wilson et al. (2007) suspected that local reports of hyacinth resurgences might have actually been false. Valid reports of resurgence may have resulted only if weevils died due to a lack of buoyant hyacinth, leaving the plant temporarily uncontrolled. According to Wilson et al. (2007), there was no substantive evidence to link low light levels with any of the withdrawals of hyacinth as Wilson et al. (2007) surmised.
Williams et al. (2007) placed less emphasis on the importance of the weevils in regards to water hyacinth control. Rather than biocontrol being a significant factor, they claimed El Niño more likely pulled hyacinth from the shoreline and destroyed it with wave action. This theory was both plausible and agreed seamlessly with the data showing a decline in 1998. Furthermore, the 2000 to 2002 nadir in hyacinth was thought to be a fleeting product of the weevil’s efficacy after 1999 and “suboptimal light” (Williams et al. 2007). Finally, Williams et al. (2007) pointed to the River Kagera as an ideal means of future resurgence, as hyacinth from this region is untainted with weevils and can float freely into the lake. This meant that there would be a delay before biomass control could take effect.
While Wilson et al. (2007) offered the more optimistic outlook on the data set, Williams et al. (2007) were unfortunately the most realistic. Williams et al. (2007) provided the most coherent argument, and aptly paralleled the situation in Lake Victoria with that of sub-tropical climates plagued by water hyacinth. They assumed that the lack of hyacinth is a part of a cyclic process involving a balance between weevils and weeds that will invariably lead to hyacinth resurgences. Wilson et al. (2007) tended to make unlikely excuses for all reported instances of resurgence, rather than offering any real insight into the possible validity of their reasoning. The satellite images from MODIS vindicate the argument of Williams et al. (2007), as resurgence obviously took place by 2006 (NASA 2007). Thus the relationship between adequate light, the presence of weevils, and the predominance of hyacinth must be a continued subject of study at Lake Victoria if definite conclusions about the hyacinth resurgence cycle are to be drawn. However, Williams et al (2007) seems to have lead us in the right general direction.
NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 12 Sep 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
The Water Hyacinth that invaded Lake Victoria in Western Africa was not welcomed. Scientists were looking for answers to rid their home state of this foreign invader. Bio control using Water hyacinth eating weevils were introduced to the lake as a promising way to rid the weed from the lake in December 1995. Along with bio control, El Niño a strong weather pattern was also given credit as a force of nature that helped reduce the water hyacinth by damaging and forcefully breaking up the plant. These two factors were major contributors to the hyacinths decline. Williams et. Al 2007 and Wilson et. Al 2007 are two articles that argue the problems and solutions to the water hyacinth. Williams argued that not only were the weevils responsible for the decline of the water hyacinth, but the weather pattern El Nino played more of a impacting role than the weevils did on the hyacinth. The unusual lack of sunlight from cloud coverage caused by El Nino would prevent the water hyacinth from photosynthesizing, thus preventing reproduction (Williams et. al 2007). Along with preventing reproduction, the unstable water and storm patterns caused plant mats to break up causing a less dense population in a certain vecinity. John R.U. Wilson author of the article elaborated on the problem that the main reason for the declination of the water hyacinth was because of the weevils. The weevils were introduced in December 1995 and the article explains how the weevils were such an important part of the destruction of the plant (Wilson et al 2007)“Neochetina larvae tunnel the petioles and the root-stock, thereby allowing bacteria and secondary fungi to enter the plant and cause severe damage (Wilson et. al 2007)”. Even if El Niño’s weather disrupted the plants, it was the destruction of the plant already by the weevil internally that would cause the plant to sink to the waters floor. Both arguments are sound when debating the depletion of the water hyacinth, but it seems that Wilson gives a better-rounded point. Yes, El Nino did play a major role in effecting the water hyacinth causing a lack of sunlight and hindering the spread of the hyacinth but it seemed that the weevils had a more long-term effect on the depletion. The weevils that been introduced had been eating away at the hyacinth for three years before El Nino occurred in 1998. Both factors contributed to the cause but I think that the weevil’s presence was more catastrophic over all the water hyacinth then El Nino’s occurrence.Bio control played a huge role in the battle. By introducing the weevils was an ingenious idea because this insect not only destroys the plant but also does not feed on other plants except for the water hyacinth. This was extremely successful because they did not affect the ecosystem around them when introduced while also devastating the hyacinth population. Bio control was an extremely effective in this situation. Along with El Nino, and the introduction of the weevils, lake Victoria saw a drastic decrease in this invader that was becoming a serious problem.
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.