Aquatic Invasive Species

The Essence of the Debate

In the early 2000’s, scientists noticed a significant decrease in the water hyacinth population of Lake Victoria in Africa. Two groups of scientist, Wilson et al. and Williams et al., have attributed various factors to this result. In a 2007 issue of Aquatic Botany, Wilson et al., which included Dr.James Ogwang, suggested that biological control by Neochetina spp, the South American weevil, was the major reason for the decline in water hyacinths on Lake Victoria. Williams et al. published an article in Aquatic Botany that same year challenging Wilson’s et al. claim. Williams et al instead argues that during 1997-1998 the climatic effects of an El Nino, cloud cover and increased wave action was responsible for the decline in water hyacinths. Specifically, Williams et al believed cloud cover decreased the growth rate of the water hyacinth, a photosynthesizing plant, and then increased wave action broke up, and drowned a majority of water hyacinths and weevils in process.

Method of Data Collection and Presentation of Data

Wilson et al utilized satellite images of Lake Victoria at various times to estimate the amount of hyacinth coverage for the entire lake.  Next they created a graph showing the amount of hectares covered by water hyacinths from 1996 to 2002. The graph also indicated the period when the weevils were introduced to three different areas by Dr. Ogwang, and the occurrence of the El Nino weather pattern. Williams et al. also used satellite images from various time periods to measure water hyacinth coverage on Lake Victoria. However unlike the other group, Williams et al estimated water hyacinth coverage separately for the Tanzanian, Ugandan and Kenyan (Winam Gulf) sides of the lake, the same three areas where biocontrol was implemented by Dr. Ogwang. They then produced three graphs showing the hyacinth coverage (hectares) between March 1994 and September 2001, and the time of biocontrol implementation for each region.

NASA Reveals a Startling Result 

 However while both group of scientists attributed different reasons for the decline in the hyacinth’s population, satellite imagery provided by NASA article in 2006 shows resurgence by the invaders. From the date of the image it appears odd and irrelevant for both groups to argue about the decline in the population when there’s clearly a problem. One must note however that these articles were initially submitted in 2005 and edited during early 2006. These articles were already in the publishing phase when the satellite image was presented by NASA.

Both groups attribute each other’s respective factor for playing a minor role in the decline of water hyacinths on Lake Victoria. However each group believes their respective factor was the major cause for the decline, and had strong arguments to support their claim. I agree with Williams’ et al. that “reduced incident light [caused by cloud cover] reduced the growth rate of a photosynthesizing plant [water hyacinth]”. However I do not believe that reduced incident light could have had that significant an impact. Fig. 1 in Williams’ et al.  article indicates that the El Nino was present over a course of approximately six months, but cloud cover couldn’t  have been absolutely continuous over the course of those six months. I also think that wave action might have contributed to the resurgence of water hyacinths for as mats are broken up water hyacinths along with their seeds spread to new areas of the lake. To me biocontrol seems the more plausible and relevant factor in the decline of water hyacinths in Lake Victoria.

References: 

 NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?id=7426. Viewed 24 Jan 2011.

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.

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