Water hyacinth is an invasive plant that was first introduced to Lake Victoria in 1989 ( Wilson et al. 2007). The species spread quickly, infesting about 77 square miles of the lake by 1998 (NASA Earth Observatory 2007). This has become a huge problem, threatening the biodiversity of the lake, contaminating drinking water, and hampering transportation. In order to remove the aggressive species, biological control agents were released into different parts of the lake in 1995. There was also an El Niño weather pattern during 1997/1998, around the same time water hyacinth populations started to decline. Recently, there has been some debate over the cause of the decline of water hyacinth on Lake Victoria. Some researchers argue that without the presence of weevils, the control agents that eat water hyacinth, the population crash would not have occurred. Others disagree, saying that the wet and cloudy weather of el Niño played a major part by accelerating the decline through direct effects.
In an Aquatic Botany article, Wilson et al. (2007) claims that, although el Niño hastened the destruction of water hyacinth, the presence of weevils was the major contributor to the rapid decrease. Satellite images of water coverage were analyzed and then summarized on a graph, showing the trend of how lake-wide water hyacinth populations changed over time. They agree that increased wind and wave action from El Niño may have been a major stress to plants, but their data shows that the major turning point was in 1999, when the weevils finally became effective.
Williams et. al (2007) combats this opinion in another Aquatic Botany article, stating that el Niño was the major cause of the water hyacinth destruction. Wilson’s article is criticized for condensing the system into an “oversimplification of the spatial complexity”(Williams et. al 2007). They argue that the collapse could not be fully attributed to the weevils because they were introduced 4 years prior to the rapid reduction. There was also a concern of resurgence of the plant because weevils have never become established within the river systems around Lake Victoria. An infestation of weevils must occur before biocontrol can take affect.
Little did they know that in December 2006, satellite images revealed that water hyacinth was back (NASA Earth Observatory 2007). For Wilson, this must have been a hard thing to comprehend. The resurgence of water hyacinth, in my opinion, supports the Williams article. I also agree that Wilson oversimplified years worth of data onto one chart; I was skeptical that the analysis was accurate. Also, their approach was not restricted to dates where the whole lake was clear from cloud cover. Therefore, the Williams article provided the most convincing argument with the data.
In light of the MODIS satellite images that showed images of growing water hyacinth populations, it can be said that biocontrol in general does not always work, and can sometimes turn out for the worse. In this case, the weevils were not acclimated to the size and environment of Lake Victoria, and therefore did not thrive as well. In order for biocontrol to be effective, a large scale infestation must occur; this could take years. The biocontrol must also be regulated frequently; as water hyacinth starts to die off and rot, the weevils also die from rotting plants or lack of food. On broad terms, invasive species pose major threats to our society and environment, and biocontrol should not be taken for granted.
NASA Earth Observatory. 2007. Water Hyacinth Re-invades Lake Victoria. http://earthobservatory.nasa.gov/IOTD/view.php?i=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