In 1989, water hyacinth first appeared on Lake Victoria. Without natural enemies to control its spread, it quickly expanded and became a troublesome invader to the ecosystem. By 2000, the population had been brought back to a reasonable level, and there are two competing explanations for this decline: weevils and the El Niño weather pattern in 1997-1998.
According to Wilson et al. (2007), the main cause of the hyacinth decline was the introduction of weevils into the area, an insect that feeds on the plant, causing it considerable damage. The weevils were brought as a means of biocontrol, and Wilson et al. (2007) claim that they weakened the hyacinth, making it more susceptible to the inclement weather, and then finishing off the remaining hyacinth that surged in the aftermath of El Niño. They specifically cite the fact that the four years it took for the hyacinth population to be fully controlled is consistent with the time frames that occurred in weevil biocontrol of hyacinth in other areas, as well as the fact that the hyacinth population started to flourish again after El Niño, suggesting that the stormy weather alone did not cause its eventual downfall.
Williams et al. (2007), on the other hand, argue that it was in fact the El Niño weather pattern that played the biggest role. They point to Lake Victoria’s size and diverse range of habitats, claiming that Wilson et al. (2007) oversimplify the true nature of the hyacinth’s decline. By looking at the data for each section of the lake, Williams et al. (2007) show that nearly all hyacinth not in a sheltered gulf area was eliminated simultaneously in late 1997 to early 1998, the same time as the El Niño event. They acknowledge that weevils no doubt played a large role in weakening the plants, but assert that Wilson et al. (2007) give too much credit to the success of the biocontrol.
Ultimately, later photos revealed that water hyacinth has returned to Lake Victoria in extreme quantities in the sheltered Winam Gulf area. I believe that this suggests that Williams et al. were more correct in their analysis of the situation, as it appears that, in the area of Lake Victoria that was protected from the worst of the El Niño weather, the weevils alone were not enough to eliminate the hyacinth in the long term. Nonetheless, both sides emphasize the fact that the decline was due to a variety of factors, and neither can be considered completely correct, since water hyacinth never was truly eliminated. On a wider scale, this leads one to wonder how successful any biocontrol effort can ever be, and if perhaps all one can do is forestall the inevitable.
NASA Earth Observatory. 2007. Water Hyacinth Re-Invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/veiw.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